Chapter 2 Link-Editor

The link-editing process creates an output file from one or more input files. Output file creation is directed by the options that are supplied to the link-editor and the input sections provided by the input files.

All files are represented in the executable and linking format (ELF). For a complete description of the ELF format see Chapter 7, Object File Format. For this introduction, two ELF structures are introduced, sections and segments.

Sections are the smallest indivisible units that can be processed within an ELF file. Segments are a collection of sections that represent the smallest individual units that can be mapped to a memory image by exec(2) or by the runtime linker ld.so.1(1).

Although many types of ELF section exist, sections fall into two categories with respect to the link-editing phase.

• Sections that contain program data, whose interpretation is meaningful only to the application, such as the program instructions .text and the associated data .data and .bss.

• Sections that contain link-editing information, such as the symbol table information found from .symtab and .strtab, and relocation information such as .rela.text.

Basically, the link-editor concatenates the program data sections into the output file. The link-editing information sections are interpreted by the link-editor to modify other sections. The information sections are also used to generate new output information sections used in later processing of the output file.

The following simple breakdown of link-editor functionality introduces the topics that are covered in this chapter.

• The verification and consistency checking of all options provided.

• The concatenation of sections of the same characteristics from the input relocatable objects to form new sections within the output file. The concatenated sections can in turn be associated to output segments.

• The processing of symbol table information from both relocatable objects and shared objects to verify and unite references with definitions. The generation of a new symbol table, or tables, within the output file.

• The processing of relocation information from the input relocatable objects, and the application of this information to sections that compose the output file. In addition, output relocation sections might be generated for use by the runtime linker.

• The generation of program headers that describe all the segments that are created.

• The generation of dynamic linking information sections if necessary, which provide information such as shared object dependencies and symbol bindings to the runtime linker.

The process of concatenating like sections and associating sections to segments is carried out using default information within the link-editor. The default section and segment handling provided by the link-editor is usually sufficient for most link-edits. However, these defaults can be manipulated using the -M option with an associated mapfile. See Chapter 9, Mapfile Option.

Invoking the Link-Editor

You can either run the link-editor directly from the command line or have a compiler driver invoke the link-editor for you. In the following two sections the description of both methods are expanded. However, using the compiler driver is the preferred choice. The compilation environment is often the consequence of a complex and occasionally changing series of operations known only to compiler drivers.

Direct Invocation

When you invoke the link-editor directly, you have to supply every object file and library required to create the intended output. The link-editor makes no assumptions about the object modules or libraries that you meant to use in creating the output. For example, the following command instructs the link-editor to create a dynamic executable that is named a.out using only the input file test.o.

$ ld test.o

Typically, a dynamic executable requires specialized startup code and exit processing code. This code can be language or operating system specific, and is usually provided through files supplied by the compiler drivers.

Additionally, you can also supply your own initialization code and termination code. This code must be encapsulated and be labeled correctly for the code to be correctly recognized and made available to the runtime linker. This encapsulation and labeling can also be provided through files supplied by the compiler drivers.

When creating runtime objects such as executables and shared objects, you should use a compiler driver to invoke the link-editor. Direct invocation of the link-editor is recommended only when creating intermediate relocatable objects when using the -r option.

Using a Compiler Driver

The conventional way to use the link-editor is through a language-specific compiler driver. You supply the compiler driver, cc(1), CC(1), and so forth, with the input files that make up your application. The compiler driver adds additional files and default libraries to complete the link-edit. These additional files can be seen by expanding the compilation invocation.

$ cc -# -o prog main.o
/usr/ccs/bin/ld -dy /opt/COMPILER/crti.o /opt/COMPILER/crt1.o \
/usr/ccs/lib/values-Xt.o -o prog main.o \
-YP,/opt/COMPILER/lib:/usr/ccs/lib:/usr/lib -Qy -lc \
/opt/COMPILER/crtn.o

The actual files included by your compiler driver and the mechanism used to display the link-editor invocation might differ.

The 32–bit link-editor and 64–bit link-editor

The link-editor is provided as a 32–bit application and a 64–bit application. Each link-editor can operate on 32–bit objects and 64–bit objects. However, a link-edit can not contain a mix of 32–bit objects and 64–bit objects. Although a 32–bit link-editor can generate a 64–bit object, the size of the generated object, not including the .bss, is restricted to 2 Gbytes.

By default, the compiler drivers execute the 32–bit link-editor. This link-editor inspects the command line to determine whether the 64–bit link-editor should be executed to complete the link-edit.

Typically, no command line option is required to distinguish a 32–bit link-edit or 64–bit link-edit. The link-editor uses the ELF class of the first relocatable object on the command line to govern the mode in which to operate. Specialized link-edits, such as linking solely from a mapfile or an archive library, are uninfluenced by the command line object. These link-edits default to a 32–bit mode, and require a command line option to instigate a 64–bit link-edit.

The 64–bit link-editor is executed under one of the following conditions.

• The -64 option is used.

• The -z altexec64 option is used.

• The first relocatable object on the command line is 64–bit.

The creation of very large 32–bit objects can exhaust the virtual memory that is available to the 32–bit link-editor. The -z altexec64 option can be used to force the use of the associated 64–bit link-editor. The 64–bit link-editor provides a larger virtual address space for building 32–bit objects.

The LD_ALTEXEC environment variable can also be used to specify an alternative link-editor.

Specifying the Link-Editor Options

Most options to the link-editor can be passed through the compiler driver command line. For the most part, the compiler and the link-editor options do not conflict. Where a conflict arises, the compiler drivers usually provide a command line syntax that you can use to pass specific options to the link-editor. You can also provide options to the link-editor by setting the LD_OPTIONS environment variable.

$ LD_OPTIONS="-R /home/me/libs -L /home/me/libs" cc -o prog main.c -lfoo

The -R and -L options are interpreted by the link-editor. These options precede any command line options that are received from the compiler driver.

The link-editor parses the entire option list for any invalid options or any options with invalid associated arguments. When either of these cases are found, a suitable error message is generated. If the error is deemed fatal, the link-edit terminates. In the following example, the illegal option -X, and the illegal argument to the -z option, are caught by the link-editor's checking.

$ ld -X -z sillydefs main.o
ld: illegal option -- X
ld: fatal: option -z has illegal argument `sillydefs'

If an option that requires an associated argument is specified twice, the link-editor produces a suitable warning and continue with the link-edit.

$ ld -e foo ...... -e bar main.o
ld: warning: option -e appears more than once, first setting taken

The link-editor also checks the option list for any fatal inconsistencies.

$ ld -dy -a main.o
ld: fatal: option -dy and -a are incompatible

After processing all options, if no fatal error conditions have been detected, the link-editor proceeds to process the input files.

See Appendix A, Link-Editor Quick Reference for the most commonly used link-editor options, and ld(1) for a complete description of all link-editor options.

Input File Processing

The link-editor reads input files in the order in which the files appear on the command line. Each file is opened and inspected to determine the files ELF type, and therefore determine how the file must be processed. The file types that apply as input for the link-edit are determined by the binding mode of the link-edit, either static or dynamic.

Under static mode, the link-editor accepts only relocatable objects or archive libraries as input files. Under dynamic mode, the link-editor also accepts shared objects.

Relocatable objects represent the most basic input file type to the link-editing process. The program data sections within these files are concatenated into the output file image being generated. The link-edit information sections are organized for later use. Information sections do not become part of the output file image, as new information sections are generated to take their place. Symbols are gathered into an internal symbol table for verification and resolution. This table is then used to create one or more symbol tables in the output image.

Although input files can be specified directly on the link-edit command line, archive libraries and shared objects are commonly specified using the -l option. See Linking With Additional Libraries. During a link-edit, the interpretation of archive libraries and shared objects are quite different. The next two sections expand upon these differences.

Archive Processing

Archives are built using ar(1). Archives usually consist of a collection of relocatable objects with an archive symbol table. This symbol table provides an association of symbol definitions with the objects that supply these definitions. By default, the link-editor provides selective extraction of archive members. The link-editor uses unresolved symbolic references to select objects from the archive that are required to complete the binding process. You can also explicitly extract all members of an archive.

The link-editor extracts a relocatable object from an archive under the following conditions.

• The archive member contains a symbol definition that satisfies a symbol reference, presently held in the link-editor's internal symbol table. This reference is sometimes referred to as an undefined symbol.

• The archive member contains a data symbol definition that satisfies a tentative symbol definition presently held in the link-editor's internal symbol table. An example is a FORTRAN COMMON block definition, which causes the extraction of a relocatable object that defines the same DATA symbol.

• The archive member contains a symbol definition that matches a reference that requires hidden visibility or protected visibility. See Table 7–20.

• The link-editors -z allextract is in effect. This option suspends selective archive extraction and causes all archive members to be extracted from the archive being processed.

Under selective archive extraction, a weak symbol reference does not extract an object from an archive unless the -z weakextract option is in effect. See Simple Resolutions for more information.

The options -z weakextract, -z allextract, and -z defaultextract enable you to toggle the archive extraction mechanism among multiple archives.

With selective archive extraction, the link-editor makes multiple passes through an archive. Relocatable objects are extracted as needed to satisfy the symbol information being accumulated in the link-editor internal symbol table. After the link-editor has made a complete pass through the archive without extracting any relocatable objects, the next input file is processed.

By extracting only the relocatable objects needed when an archive is encountered, the position of the archive on the command line can be significant. See Position of an Archive on the Command Line.

Although the link-editor makes multiple passes through an archive to resolve symbols, this mechanism can be quite costly. Especially, for large archives that contain random organizations of relocatable objects. In these cases, you should use tools like lorder(1) and tsort(1) to order the relocatable objects within the archive. This ordering reduces the number of passes the link-editor must carry out.

Shared Object Processing

Shared objects are indivisible whole units that have been generated by a previous link-edit of one or more input files. When the link-editor processes a shared object, the entire contents of the shared object become a logical part of the resulting output file image. This logical inclusion means that all symbol entries defined in the shared object are made available to the link-editing process.

The shared object's program data sections and most of the link-editing information sections are unused by the link-editor. These sections are interpreted by the runtime linker when the shared object is bound to generate a runnable process. However, the occurrence of a shared object is remembered. Information is stored in the output file image to indicate that this object is a dependency that must be made available at runtime.

By default, all shared objects specified as part of a link-edit are recorded as dependencies in the object being built. This recording is made regardless of whether the object being built actually references symbols offered by the shared object. To minimize the overhead of runtime linking, only specify those dependencies that resolve symbol references from the object being built. The link-editor's debugging capabilities, and ldd(1) with the -u option, can be used to determine unused dependencies. The link-editor's -z ignore option can be used to suppress the dependency recording of any unused shared objects.

If a shared object has dependencies on other shared objects, these dependencies can also be processed. This processing occurs after all command line input files have been processed, to complete the symbol resolution process. However, the shared object names are not recorded as dependencies in the output file image being generated.

Although the position of a shared object on the command line has less significance than archive processing, the position can have a global effect. Multiple symbols of the same name are allowed to occur between relocatable objects and shared objects, and between multiple shared objects. See Symbol Resolution.

The order of shared objects processed by the link-editor is maintained in the dependency information that is stored in the output file image. In the absence of lazy loading, the runtime linker loads the specified shared objects in the same order. Therefore, the link-editor and the runtime linker select the first occurrence of a symbol of a multiply-defined series of symbols.

Multiple symbol definitions, are reported in the load map output generated using the -m option.

Linking With Additional Libraries

Although the compiler drivers often ensure that appropriate libraries are specified to the link-editor, frequently you must supply your own. Shared objects and archives can be specified by explicitly naming the input files required to the link-editor. However, a more common and more flexible method involves using the link-editor's -l option.

Library Naming Conventions

By convention, shared objects are usually designated by the prefix lib and the suffix .so. Archives are designated by the prefix lib and the suffix .a. For example, libfoo.so is the shared object version of the “foo” implementation that is made available to the compilation environment. libfoo.a is the library's archive version.

These conventions are recognized by the -l option of the link-editor. This option is commonly used to supply additional libraries to a link-edit. The following example directs the link-editor to search for libfoo.so. If the link-editor does not find libfoo.so, a search for libfoo.a is made before moving on to the next directory to be searched.

$ cc -o prog file1.c file2.c -lfoo

A naming convention exists regarding the compilation environment and the runtime environment use of shared objects. The compilation environment uses the simple .so suffix, whereas the runtime environment commonly uses the suffix with an additional version number. See Naming Conventions and Coordination of Versioned Filenames.

When link-editing in dynamic mode, you can choose to link with a mix of shared objects and archives. When link-editing in static mode, only archive libraries are acceptable for input.

In dynamic mode, when using the -l option, the link-editor first searches the given directory for a shared object that matches the specified name. If no match is found, the link-editor looks for an archive library in the same directory. In static mode, when using the -l option, only archive libraries are sought.

Linking With a Mix of Shared Objects and Archives

The library search mechanism in dynamic mode searches a given directory for a shared object, and then searches for an archive library. Finer control of the search is possible through the -B option.

By specifying the -B dynamic and -B static options on the command line, you can toggle the library search between shared objects or archives respectively. For example, to link an application with the archive libfoo.a and the shared object libbar.so, issue the following command.

$ cc -o prog main.o file1.c -Bstatic -lfoo -Bdynamic -lbar

The -B static and -B dynamic options are not exactly symmetrical. When you specify -B static, the link-editor does not accept shared objects as input until the next occurrence of -B dynamic. However, when you specify -B dynamic, the link-editor first looks for shared objects and then archive library's in any given directory.

The precise description of the previous example is that the link-editor first searches for libfoo.a. The link-editor then searches for libbar.so, and if that search fails, searches for libbar.a.

Position of an Archive on the Command Line

The position of an archive on the command line can affect the output file being produced. The link-editor searches an archive only to resolve undefined or tentative external references that have previously been encountered. After this search is completed and any required members have been extracted, the link-editor moves onto the next input file on the command line.

Therefore by default, the archive is not available to resolve any new references from the input files that follow the archive on the command line. For example, the following command directs the link-editor to search libfoo.a only to resolve symbol references that have been obtained from file1.c. The libfoo.a archive is not available to resolve symbol references from file2.c or file3.c.

$ cc -o prog file1.c -Bstatic -lfoo file2.c file3.c -Bdynamic

Interdependencies between archives can exist, such that the extraction of members from one archive must be resolved by extracting members from another archive. If these dependencies are cyclic, the archives must be specified repeatedly on the command line to satisfy previous references.

$ cc -o prog .... -lA -lB -lC -lA -lB -lC -lA

The determination, and maintenance, of repeated archive specifications can be tedious. The -z rescan option makes this process simpler. Following all input file processing, this option causes the entire archive list to be reprocessed. This processing attempts to locate additional archive members that resolve symbol references. This archive rescanning continues until a pass over the archive list occurs in which no new members are extracted. The previous example can be simplified as follows.

$ cc -o prog -z rescan .... -lA -lB -lC

You should specify any archives at the end of the command line unless multiple-definition conflicts require you to do otherwise.

Directories Searched by the Link-Editor

All previous examples assume the link-editor knows where to search for the libraries listed on the command line. By default, when linking 32–bit objects, the link-editor knows of only three standard directories in which to look for libraries, /usr/ccs/lib, followed by /lib, and finally /usr/lib. When linking 64–bit objects, only two standard directories are used, /lib/64 followed by /usr/lib/64. All other directories to be searched must be added to the link-editor's search path explicitly.

You can change the link-editor search path by using a command line option, or by using an environment variable.

Using a Command-Line Option

You can use the -L option to add a new path name to the library search path. This option alters the search path at the point the option is encountered on the command line. For example, the following command searches path1, followed by /usr/ccs/lib, /lib, and finally /usr/lib, to find libfoo. The command searches path1 and then path2, followed by /usr/ccs/lib, /lib, and /usr/lib, to find libbar.

$ cc -o prog main.o -Lpath1 file1.c -lfoo file2.c -Lpath2 -lbar

Path names that are defined by using the -L option are used only by the link-editor. These path names are not recorded in the output file image being created. Therefore, these path names are not available for use by the runtime linker.

You must specify -L if you want the link-editor to search for libraries in your current directory. You can use a period (.) to represent the current directory.

You can use the -Y option to change the default directories searched by the link-editor. The argument supplied with this option takes the form of a colon separated list of directories. For example, the following command searches for libfoo only in the directories /opt/COMPILER/lib and /home/me/lib.

$ cc -o prog main.c -YP,/opt/COMPILER/lib:/home/me/lib -lfoo

The directories that are specified by using the -Y option can be supplemented by using the -L option. Compiler drivers often use the -Y option to provide compiler specific search paths.

Using an Environment Variable

You can also use the environment variable LD_LIBRARY_PATH to add to the link-editor's library search path. Typically, LD_LIBRARY_PATH takes a colon-separated list of directories. In its most general form, LD_LIBRARY_PATH can also take two directory lists separated by a semicolon. These lists are searched before and after the -Y lists supplied on the command line.

The following example shows the combined effect of setting LD_LIBRARY_PATH and calling the link-editor with several -L occurrences.

$ LD_LIBRARY_PATH=dir1:dir2;dir3
$ export LD_LIBRARY_PATH
$ cc -o prog main.c -Lpath1 ... -Lpath2 ... -Lpathn -lfoo

The effective search path is dir1:dir2:path1:path2... pathn:dir3:/usr/ccs/lib:/lib:/usr/lib.

If no semicolon is specified as part of the LD_LIBRARY_PATH definition, the specified directory list is interpreted after any -L options. In the following example, the effective search path is path1:path2... pathn:dir1:dir2:/usr/ccs/lib:/lib:/usr/lib.

$ LD_LIBRARY_PATH=dir1:dir2
$ export LD_LIBRARY_PATH
$ cc -o prog main.c -Lpath1 ... -Lpath2 ... -Lpathn -lfoo

This environment variable can also be used to augment the search path of the runtime linker. See Directories Searched by the Runtime Linker. To prevent this environment variable from influencing the link-editor, use the -i option.

Directories Searched by the Runtime Linker

The runtime linker looks in two default locations for dependencies. When processing 32–bit objects, the default locations are /lib and /usr/lib. When processing 64–bit objects, the default locations are /lib/64 and /usr/lib/64. All other directories to be searched must be added to the runtime linker's search path explicitly.

When a dynamic executable or shared object is linked with additional shared objects, the shared objects are recorded as dependencies. These dependencies must be located during process execution by the runtime linker. When linking a dynamic object, one or more search paths can be recorded in the output file. These search paths are referred to as a runpath. The runtime linker uses the runpath of an object to locate the dependencies of that object.

Specialized objects can be built with the -z nodefaultlib option to suppress any search of the default location at runtime. Use of this option implies that all the dependencies of an object can be located using its runpaths. Without this option, no matter how you augment the runtime linker's search path, the last search paths used are always the default locations.

The default search path can be administrated by using a runtime configuration file. See Configuring the Default Search Paths. However, the creator of a dynamic object should not rely on the existence of this file. You should always ensure that an object can locate its dependencies with only its runpaths or the default locations.

You can use the -R option, which takes a colon-separated list of directories, to record a runpath in a dynamic executable or shared object. The following example records the runpath /home/me/lib:/home/you/lib in the dynamic executable prog.

$ cc -o prog main.c -R/home/me/lib:/home/you/lib -Lpath1 \ 
-Lpath2 file1.c file2.c -lfoo -lbar

The runtime linker uses these paths, followed by the default location, to obtain any shared object dependencies. In this case, this runpath is used to locate libfoo.so.1 and libbar.so.1.

The link-editor accepts multiple -R options. These multiple specifications are concatenate together, separated by a colon. Thus, the previous example can also be expressed as follows.

$ cc -o prog main.c -R/home/me/lib -Lpath1 -R/home/you/lib \ 
    -Lpath2 file1.c file2.c -lfoo -lbar

For objects that can be installed in various locations, the $ORIGIN dynamic string token provides a flexible means of recording a runpath. See Locating Associated Dependencies.

A historic alternative to specifying the -R option is to set the environment variable LD_RUN_PATH, and make this available to the link-editor. The scope and function of LD_RUN_PATH and -R are identical, but when both are specified, -R supersedes LD_RUN_PATH.

Initialization and Termination Sections

Dynamic objects can supply code that provides for runtime initialization and termination processing. The initialization code of a dynamic object is executed once each time the dynamic object is loaded in a process. The termination code of a dynamic object is executed once each time the dynamic object is unloaded from a process or at process termination. This code can be encapsulated in one of two section types, either an array of function pointers or a single code block. Each of these section types is built from a concatenation of like sections from the input relocatable objects.

The sections .preinitarray, .initarray and .finiarray provide arrays of runtime pre-initialization, initialization, and termination functions, respectively. When creating a dynamic object, the link-editor identifies these arrays with the .dynamic tag pairs DT_PREINIT_[ARRAY/ARRAYSZ], DT_INIT_[ARRAY/ARRAYSZ], and DT_FINI_[ARRAY/ARRAYSZ] accordingly. These tags identify the associated sections so that the sections can be called by the runtime linker. A pre-initialization array is applicable to dynamic executables only.

Functions that are assigned to these arrays must be provided from the object that is being built.

The sections .init and .fini provide a runtime initialization and termination code block, respectively. The compiler drivers typically supply .init and .fini sections with files they add to the beginning and end of your input file list. These compiler provided files have the effect of encapsulating the .init and .fini code from your relocatable objects into individual functions. These functions are identified by the reserved symbol names _init and _fini respectively. When creating a dynamic object, the link-editor identifies these symbols with the .dynamic tags DT_INIT and DT_FINI accordingly. These tags identify the associated sections so they can be called by the runtime linker.

For more information about the execution of initialization and termination code at runtime see Initialization and Termination Routines.

The registration of initialization and termination functions can be carried out directly by the link-editor by using the -z initarray and -z finiarray options. For example, the following command places the address of foo() in an .initarray element, and the address of bar() in a .finiarray element.

$ cat main.c
#include    <stdio.h>

void foo()
{
        (void) printf("initializing: foo()\n");
}

void bar()
{
        (void) printf("finalizing: bar()\n");
}

void main()
{
        (void) printf("main()\n");
}

$ cc -o main -zinitarray=foo -zfiniarray=bar main.c
$ main
initializing: foo()
main()
finalizing: bar()

The creation of initialization and termination sections can be carried out directly using an assembler. However, most compilers offer special primitives to simplify their declaration. For example, the previous code example can be rewritten using the following #pragma definitions. These definitions result in a call to foo() being placed in an .init section, and a call to bar() being placed in a .fini section.

$ cat main.c
#include    <stdio.h>

#pragma init (foo)
#pragma fini (bar)

.......
$ cc -o main main.c
$ main
initializing: foo()
main()
finalizing: bar()

Initialization and termination code, spread throughout several relocatable objects, can result in different behavior when included in an archive library or shared object. The link-edit of an application that uses this archive might extract only a fraction of the objects contained in the archive. These objects might provide only a portion of the initialization and termination code spread throughout the members of the archive. At runtime, only this portion of code is executed. The same application built against the shared object will have all the accumulated initialization and termination code executed when the dependency is loaded at runtime.

To determine the order of executing initialization and termination code within a process at runtime is a complex issue that involves dependency analysis. Limit the content of initialization and termination code to simplify this analysis. Simplified, self contained, initialization and termination code provides predictable runtime behavior. See Initialization and Termination Order for more details.

Data initialization should be independent if the initialization code is involved with a dynamic object whose memory can be dumped using dldump(3C).

Symbol Processing

During input file processing, all local symbols from the input relocatable objects are passed through to the output file image. All global symbols from the input relocatable objects, together with globals symbols from shared object dependencies, are accumulated internally within the link-editor.

Each global symbol supplied by an input file is searched for within this internal symbol table. If a symbol with the same name has already been encountered from a previous input file, a symbol resolution process is called. This resolution process determines which of two entries from relocatable objects are kept. This resolution process also determines how external references to shared object dependencies are established.

On completion of input file processing, and providing no fatal symbol resolution errors have occurred, the link-editor determines if any unresolved symbol references remain. Unresolved symbol references can cause the link-edit to terminate.

Finally, the link-editor's internal symbol table is added to the symbol tables of the image being created.

The following sections expand upon symbol resolution and undefined symbol processing.

Symbol Resolution

Symbol resolution runs the entire spectrum, from simple and intuitive to complex and perplexing. Most resolutions are carried out silently by the link-editor. However, some relocations can be accompanied by warning diagnostics, while others can result in a fatal error condition.

The most common simple resolutions involve binding symbol references from one object to symbol definitions within another object. This binding can occur between two relocatable objects, or between a relocatable object and the first definition found in a shared object dependency. Complex resolutions typically occur between two or more relocatable objects.

The resolution of two symbols depends on their attributes, the type of file that provides the symbol, and the type of file being generated. For a complete description of symbol attributes, see Symbol Table Section. For the following discussions, however, three basic symbol types are identified.

• Undefined – Symbols that have been referenced in a file but have not been assigned a storage address.

• Tentative – Symbols that have been created within a file but have not yet been sized, or allocated in storage. These symbols appear as uninitialized C symbols, or FORTRAN COMMON blocks within the file.

• Defined – Symbols that have been created, and assigned storage addresses and space within the file.

In its simplest form, symbol resolution involves the use of a precedence relationship. This relationship has defined symbols dominate tentative symbols, which in turn dominate undefined symbols.

The following example of C code shows how these symbol types can be generated. Undefined symbols are prefixed with u_. Tentative symbols are prefixed with t_. Defined symbols are prefixed with d_.

$ cat main.c
extern int      u_bar;
extern int      u_foo();

int             t_bar;
int             d_bar = 1;

int d_foo()
{
        return (u_foo(u_bar, t_bar, d_bar));
}
$ cc -o main.o -c main.c
$ nm -x main.o

[Index]   Value      Size      Type  Bind  Other Shndx   Name
...............
[8]     |0x00000000|0x00000000|NOTY |GLOB |0x0  |UNDEF  |u_foo
[9]     |0x00000000|0x00000040|FUNC |GLOB |0x0  |2      |d_foo
[10]    |0x00000004|0x00000004|OBJT |GLOB |0x0  |COMMON |t_bar
[11]    |0x00000000|0x00000000|NOTY |GLOB |0x0  |UNDEF  |u_bar
[12]    |0x00000000|0x00000004|OBJT |GLOB |0x0  |3      |d_bar

Simple Resolutions

Simple symbol resolutions are by far the most common. In this case, two symbols with similar characteristics are detected, with one symbol taking precedence over the other. This symbol resolution is carried out silently by the link-editor. For example, with symbols of the same binding, a symbol reference from one file is bound to a defined, or tentative symbol definition, from another file. Or, a tentative symbol definition from one file is bound to a defined symbol definition from another file. This resolution can occur between two relocatable objects, or between a relocatable object and the first definition found in a shared object dependency.

Symbols that undergo resolution can have either a global or weak binding. Within relocatable objects, weak bindings have lower precedence than global binding. Relocatable object symbols with different bindings are resolved according to a slight alteration of the basic rules.

Weak symbols can usually be defined through the compiler, either individually or as aliases to global symbols. One mechanism uses a #pragma definition.

$ cat main.c
#pragma weak    bar
#pragma weak    foo = _foo

int             bar = 1;

int _foo()
{
        return (bar);
}
$ cc -o main.o -c main.c
$ nm -x main.o
[Index]   Value      Size      Type  Bind  Other Shndx   Name
...............
[7]     |0x00000000|0x00000004|OBJT |WEAK |0x0  |3      |bar
[8]     |0x00000000|0x00000028|FUNC |WEAK |0x0  |2      |foo
[9]     |0x00000000|0x00000028|FUNC |GLOB |0x0  |2      |_foo

Notice that the weak alias foo is assigned the same attributes as the global symbol _foo. This relationship is maintained by the link-editor and results in the symbols being assigned the same value in the output image. In symbol resolution, weak defined symbols are silently overridden by any global definition of the same name.

Another form of simple symbol resolution, interposition, occurs between relocatable objects and shared objects, or between multiple shared objects. In these cases, when a symbol is multiply-defined, the relocatable object, or the first definition between multiple shared objects, is silently taken by the link-editor. The relocatable object's definition, or the first shared object's definition, is said to interpose on all other definitions. This interposition can be used to override the functionality provided by another shared object. Multiply-defined symbols that occur between relocatable objects and shared objects, or between multiple shared objects, are treated identically. A symbols weak binding or global binding is irrelevant. By resolving to the first definition, regardless of the symbols binding, both the link-editor and runtime linker behave consistently.

The combination of weak symbols defined within a shared object together with symbol interposition over the same shared object, can provide a useful programming technique. For example, the standard C library provides several services that you are allowed to redefine. However, ANSI C defines a set of standard services that must be present on the system. These services cannot be replaced in a strictly conforming program.

The function fread(3C), for example, is an ANSI C library function. The system function read(2) is not an ANSI C library function. A conforming ANSI C program must be able to redefine read(2) and still use fread(3C) in a predictable way.

The problem here is that read(2) underlies the fread(3C) implementation in the standard C library. Therefore, a program that redefines read(2) might confuse the fread(3C) implementation. To guard against this occurrence, ANSI C states that an implementation cannot use a name that is not reserved for the implementation. Use the following #pragma directive to define just such a reserved name. Use this name to generate an alias for the function read(2).

#pragma weak read = _read

Thus, you can quite freely define your own read() function without compromising the fread(3C) implementation, which in turn is implemented to use the _read() function.

The link-editor has no difficulty with this redefinition of read(), either when linking against the shared object or archive version of the standard C library. In the former case, interposition takes its course. In the latter case, the fact that the C library's definition of read(2) is weak allows that definition to be quietly overridden.

Use the link-editor's -m option to write a list of all interposed symbol references, along with section load address information, to the standard output.

Complex Resolutions

Complex resolutions occur when two symbols of the same name are found with differing attributes. In these cases, the link-editor generates a warning message, while selecting the most appropriate symbol. This message indicates the symbol, the attributes that conflict, and the identity of the file from which the symbol definition is taken. In the following example, two files with a definition of the data item array have different size requirements.

$ cat foo.c
int array[1];

$ cat bar.c
int array[2] = { 1, 2 };

$ cc -dn -r -o temp.o foo.c bar.c
ld: warning: symbol `array' has differing sizes:
        (file foo.o value=0x4; file bar.o value=0x8);
        bar.o definition taken

A similar diagnostic is produced if the symbol's alignment requirements differ. In both of these cases, the diagnostic can be suppressed by using the link-editor's -t option.

Another form of attribute difference is the symbol's type. In the following example, the symbol bar() has been defined as both a data item and a function.

$ cat foo.c
int bar()
{
        return (0);
}
$ cc -o libfoo.so -G -K pic foo.c
$ cat main.c
int     bar = 1;

int main()
{
        return (bar);
}
$ cc -o main main.c -L. -lfoo
ld: warning: symbol `bar' has differing types:
        (file main.o type=OBJT; file ./libfoo.so type=FUNC);
        main.o definition taken

Symbol types in this context are classifications that can be expressed in ELF. These symbol types are not related to the data types as employed by the programming language, except in the crudest fashion.

In cases like the previous example, the relocatable object definition is taken when the resolution occurs between a relocatable object and a shared object. Or, the first definition is taken when the resolution occurs between two shared objects. When such resolutions occur between symbols of weak or global binding, a warning is also produced.

Inconsistencies between symbol types are not suppressed by the link-editor's -t option.

Fatal Resolutions

Symbol conflicts that cannot be resolved result in a fatal error condition and an appropriate error message. This message indicates the symbol name together with the names of the files that provided the symbols. No output file is generated. Although the fatal condition is sufficient to terminate the link-edit, all input file processing is first completed. In this manner, all fatal resolution errors can be identified.

The most common fatal error condition exists when two relocatable objects both define non-weak symbols of the same name.

$ cat foo.c
int bar = 1;

$ cat bar.c
int bar()
{ 
        return (0);
}

$ cc -dn -r -o temp.o foo.c bar.c
ld: fatal: symbol `bar' is multiply-defined:
        (file foo.o and file bar.o);
ld: fatal: File processing errors. No output written to int.o

foo.c and bar.c have conflicting definitions for the symbol bar. Because the link-editor cannot determine which should dominate, the link-edit usually terminates with an error message. You can use the link-editor's -z muldefs option to suppress this error condition. This option allows the first symbol definition to be taken.

Undefined Symbols

After all of the input files have been read and all symbol resolution is complete, the link-editor searches the internal symbol table for any symbol references that have not been bound to symbol definitions. These symbol references are referred to as undefined symbols. Undefined symbols can affect the link-edit process according to the type of symbol, together with the type of output file being generated.

Generating an Executable Output File

When generating an executable output file, the link-editor's default behavior is to terminate with an appropriate error message should any symbols remain undefined. A symbol remains undefined when a symbol reference in a relocatable object is never matched to a symbol definition.

$ cat main.c
extern int foo();

int main()
{
        return (foo());
}
$ cc -o prog main.c
Undefined           first referenced
 symbol                 in file
foo                     main.o
ld: fatal: Symbol referencing errors. No output written to prog

Similarly, if a shared object is used to create a dynamic executable and leaves an unresolved symbol definition, an undefined symbol error results.

$ cat foo.c
extern int bar;
int foo()
{
        return (bar);
}

$ cc -o libfoo.so -G -K pic foo.c
$ cc -o prog main.c -L. -lfoo
Undefined           first referenced
 symbol                 in file
bar                     ./libfoo.so
ld: fatal: Symbol referencing errors. No output written to prog

To allow undefined symbols, as in the previous example, use the link-editor's -z nodefs option to suppress the default error condition.

Take care when using the -z nodefs option. If an unavailable symbol reference is required during the execution of a process, a fatal runtime relocation error occurs. This error might be detected during the initial execution and testing of an application. However, more complex execution paths can result in this error condition taking much longer to detect, which can be time consuming and costly.

Symbols can also remain undefined when a symbol reference in a relocatable object is bound to a symbol definition in an implicitly defined shared object. For example, continuing with the files main.c and foo.c used in the previous example.

$ cat bar.c
int bar = 1;

$ cc -o libbar.so -R. -G -K pic bar.c -L. -lfoo
$ ldd libbar.so
        libfoo.so =>     ./libfoo.so

$ cc -o prog main.c -L. -lbar
Undefined           first referenced
 symbol                 in file
foo                     main.o  (symbol belongs to implicit \
                        dependency ./libfoo.so)
ld: fatal: Symbol referencing errors. No output written to prog

prog is built with an explicit reference to libbar.so. libbar.so has a dependency on libfoo.so. Therefore, an implicit reference to libfoo.so from prog is established.

Because main.c made a specific reference to the interface provided by libfoo.so, prog really has a dependency on libfoo.so. However, only explicit shared object dependencies are recorded in the output file being generated. Thus, prog fails to run if a new version of libbar.so is developed that no longer has a dependency on libfoo.so.

For this reason, bindings of this type are deemed fatal. The implicit reference must be made explicit by referencing the library directly during the link-edit of prog. The required reference is hinted at in the fatal error message that is shown in the preceding example.

Generating a Shared Object Output File

When the link-editor is generating a shared object output file, undefined symbols are allowed to remain at the end of the link-edit. This default behavior allows the shared object to import symbols from a dynamic executable that defines the shared object as a dependency.

The link-editor's -z defs option can be used to force a fatal error if any undefined symbols remain. This option is recommended when creating any shared objects. Shared objects that reference symbols from an application can use the -z defs option, together with defining the symbols by using an extern mapfile directive. See Defining Additional Symbols with a mapfile.

A self-contained shared object, in which all references to external symbols are satisfied by named dependencies, provides maximum flexibility. The shared object can be employed by many users without those users having to determine and establish dependencies to satisfy the shared object's requirements.

Weak Symbols

Weak symbol references that remain unresolved, do not result in a fatal error condition, no matter what output file type is being generated.

If a static executable is being generated, the symbol is converted to an absolute symbol with an assigned value of zero.

If a dynamic executable or shared object is being produced, the symbol is left as an undefined weak reference with an assigned value of zero. During process execution, the runtime linker searches for this symbol. If the runtime linker does not find a match, the reference is bound to an address of zero instead of generating a fatal relocation error.

Historically, these undefined weak referenced symbols have been employed as a mechanism to test for the existence of functionality. For example, the following C code fragment might have been used in the shared object libfoo.so.1.

#pragma weak    foo

extern  void    foo(char *);

void bar(char * path)
{
        void (* fptr)(char *);

        if ((fptr = foo) != 0)
                (* fptr)(path);
}

When building an application that references libfoo.so.1, the link-edit completes successfully regardless of whether a definition for the symbol foo is found. If during execution of the application the function address tests nonzero, the function is called. However, if the symbol definition is not found, the function address tests zero and therefore is not called.

Compilation systems view this address comparison technique as having undefined semantics, which can result in the test statement being removed under optimization. In addition, the runtime symbol binding mechanism places other restrictions on the use of this technique. These restrictions prevent a consistent model from being made available for all dynamic objects.

Undefined weak references in this manner are discouraged. Instead, you should use dlsym(3C) with the RTLD_DEFAULT, or RTLD_PROBE handles as a means of testing for a symbol's existence. See Testing for Functionality.

Tentative Symbol Order Within the Output File

Contributions from input files usually appear in the output file in the order of their contribution. An exception occurs when processing tentative symbols and their associated storage. These symbols are not fully defined until their resolution is complete. The resolution of a defined symbol from a relocatable object, results in the order of appearance of the symbol following the order of the definition.

If you need to control the ordering of a group of symbols, then any tentative definition should be redefined to a zero-initialized data item. For example, the following tentative definitions result in a reordering of the data items within the output file, as compared to the original order described in the source file foo.c.

$ cat foo.c
char A_array[0x10];
char B_array[0x20];
char C_array[0x30];

$ cc -o prog main.c foo.c
$ nm -vx prog | grep array
[32]    |0x00020754|0x00000010|OBJT |GLOB |0x0  |15  |A_array
[34]    |0x00020764|0x00000030|OBJT |GLOB |0x0  |15  |C_array
[42]    |0x00020794|0x00000020|OBJT |GLOB |0x0  |15  |B_array

By defining these symbols as initialized data items, the relative ordering of these symbols within the input file is carried over to the output file.

$ cat foo.c
char A_array[0x10] = { 0 };
char B_array[0x20] = { 0 };
char C_array[0x30] = { 0 };

$ cc -o prog main.c foo.c
$ nm -vx prog | grep array
[32]    |0x000206bc|0x00000010|OBJT |GLOB |0x0  |12  |A_array
[42]    |0x000206cc|0x00000020|OBJT |GLOB |0x0  |12  |B_array
[34]    |0x000206ec|0x00000030|OBJT |GLOB |0x0  |12  |C_array

Defining Additional Symbols

Besides the symbols provided from input files, you can supply additional global symbol references or global symbol definitions to a link-edit. In the simplest form, symbol references can be generated using the link-editor's -u option. Greater flexibility is provided with the link-editor's -M option and an associated mapfile. This mapfile enables you to define global symbol references and a variety of global symbol definitions.

Defining Additional Symbols with the u option

The -u option provides a mechanism for generating a global symbol reference from the link-edit command line. This option can be used to perform a link-edit entirely from archives. This option can also provide additional flexibility in selecting the objects to extract from multiple archives. See section Archive Processing for an overview of archive extraction.

For example, perhaps you want to generate a dynamic executable from the relocatable object main.o, which refers to the symbols foo and bar. You want to obtain the symbol definition foo from the relocatable object foo.o contained in lib1.a, and the symbol definition bar from the relocatable object bar.o, contained in lib2.a.

However, the archive lib1.a also contains a relocatable object that defines the symbol bar. This relocatable object is presumably of differing functionality to the relocatable object that is provided in lib2.a. To specify the required archive extraction, you can use the following link-edit.

$ cc -o prog -L. -u foo -l1 main.o -l2

The -u option generates a reference to the symbol foo. This reference causes extraction of the relocatable object foo.o from the archive lib1.a. The first reference to the symbol bar occurs in main.o, which is encountered after lib1.a has been processed. Therefore, the relocatable object bar.o is obtained from the archive lib2.a.

This simple example assumes that the relocatable object foo.o from lib1.a does not directly or indirectly reference the symbol bar. If lib1.a does reference bar, then the relocatable object bar.o is also extracted from lib1.a during its processing. See Archive Processing for a discussion of the link-editor's multi-pass processing of an archive.

Defining Additional Symbols with a mapfile

An extensive set of global symbol definitions can be provided by using the link-editor's -M option and an associated mapfile. Symbol definition mapfile entries have the following syntax.

[ name ] {
      scope:
            symbol [ = [ type ] [ value ] [ size ] [ information ] ];
} [ dependency ];

name

A label for this set of symbol definitions, if present, identifies a version definition within the image. See Chapter 5, Application Binary Interfaces and Versioning.

scope

Indicates the visibility of the symbols' binding within the output file being generated. All symbols defined with a mapfile are treated as global in scope during the link-edit process. These symbols are resolved against any other global symbols of the same name that are obtained from any of the input files. The following definitions, and aliases, define a symbols' visibility in the object being created.

default / global

Global symbols of this scope are visible to all external objects. References to such symbols from within the object are bound at runtime, thus allowing interposition to take place. This visibility scope provides a default, that can be demoted, or eliminated by other symbol visibility techniques. This scope definition has the same affect as a symbol with STV_DEFAULT visibility. See Table 7–20.

protected / symbolic

Global symbols of this scope are visible to all external objects. References to these symbols from within the object are bound at link-edit, thus preventing runtime interposition. This visibility scope can be demoted, or eliminated by other symbol visibility techniques. This scope definition has the same affect as a symbol with STV_PROTECTED visibility. See Table 7–20.

If an object is created with a single symbolic scope, all relocations within the object are bound to the object at link-edit. With this single scope, even reserved symbols are reduced to symbolic scope. See Generating the Output File for a list of reserved symbol names.

hidden / local

Global symbols of this scope are reduced to symbols with a local binding. Symbols of this scope are not visible to other external objects. This scope definition has the same affect as a symbol with STV_HIDDEN visibility. See Table 7–20.

eliminate

Global symbols of this scope are hidden. Their symbol table entries are eliminated. Note that local symbols can also be eliminated by using the link-editor -z redlocsym option.

The STV_ symbol visibility attributes, originate from symbol declarations that are embedded in source code that are processed by the compilers.

symbol

A symbol name. This name can result in a symbol definition, or a symbol reference, depending on any qualifying attributes. In the simplest form, without any qualifying attributes, a symbol reference is created. This reference is exactly the same as would be generated using the -u option discussed in Defining Additional Symbols with the u option. Typically, if the symbol name is followed by any qualifying attributes, then a symbol definition is generated using the associated attributes.

When a local scope is defined, the symbol name can be defined as the special auto-reduction directive “*”. Symbols that have no explicitly defined visibility are demoted to a local binding within the dynamic object being generated. Explicit visibility definitions originate from mapfile definitions, or visibility definitions that are encapsulated within relocatable objects.

Similarly, when an eliminate scope is defined, the symbol name can be defined as the special auto-elimination directive “*”. Symbols that have no explicitly defined visibility are eliminated from the dynamic object being generated.

type

Indicates the symbol type attribute. This attribute can be either COMMON, data, or function. The COMMON attribute results in a tentative symbol definition. The data and function attributes result in a section symbol definition or an absolute symbol definition. See Symbol Table Section.

A data attribute results in the creation of an OBJT symbol. A data attribute that is accompanied with a size, but no value creates a section symbol by associating the symbol with an ELF section. This section is filled with zeros.

A function attribute results in the creation of an FUNC symbol. A function attribute that is accompanied with a size, but no value creates a section symbol by associating the symbol with an ELF section. This section is assigned a void function return, void (*)(void).

A data or function attribute that is accompanied with a value results in the appropriate symbol type together with an absolute, ABS, section index.

The creation of a section data symbol is useful for the creation of filters. External references to a section data symbol of a filter from an executable result in the appropriate copy relocation being generated. See Copy Relocations.

value

Indicates the value attribute. This attribute takes the form of Vnumber. This attribute results in the creation of a symbol definition.

size

Indicates the size attribute. This attribute takes the form of Snumber. This attribute results in the creation of a symbol definition.

information

This keyword provides additional information for the symbol.

AUXILIARY name

Indicates that this symbol is an auxiliary filter on the shared object name. See Generating Auxiliary Filters.

DIRECT

Indicates that this symbol should be directly bound to. When used with a symbol definition, this keyword results in any reference from within the object being built to be directly bound to the definition. When used with a symbol reference, this keyword results in a direct binding to the dependency that provides the definition. See Direct Bindings. This keyword can also be used with the PARENT keyword to establish a direct binding to any parent at runtime.

EXTERN

Indicates the symbol is defined externally to the object being created. This keyword is typically defined to label callback routines. Undefined symbols that would be flagged with the -z defs option are suppressed with this keyword.

This keyword is only meaningful when generating a symbol reference. Should a definition for this symbol occur within the objects combined at link-edit, then the keyword is silently ignored.

FILTER name

Indicates that this symbol is a filter on the shared object name. See Generating Standard Filters. Filter symbols do not require any backing implementation to be provided from an input relocatable object. Therefore, use this directive together with defining the symbol's type, to create an absolute symbol table entry.

NODIRECT

Indicates that this symbol should not be directly bound to. This state applies to references from within the object being created and from external references. See Direct Bindings. This keyword can also be used with the PARENT keyword to prevent a direct binding to any parent at runtime.

PARENT

Indicates the symbol is defined in the parent of the object being created. A parent is an object that references this object at runtime as an explicit dependency. A parent can also reference this object at runtime using dlopen(3C). This keyword is typically defined to label callback routines. This keyword can be used with the DIRECT or NODIRECT keywords to establish individual direct, or no-direct references to the parent. Undefined symbols that would be flagged with the -z defs option are suppressed with this keyword.

This keyword is only meaningful when generating a symbol reference. Should a definition for this symbol occur within the objects combined at link-edit, then the keyword is silently ignored.

dependency

Represents a version definition that is inherited by this definition. See Chapter 5, Application Binary Interfaces and Versioning.

If either a version definition or the auto-reduction directive is specified, then versioning information is recorded in the image created. If this image is an executable or shared object, then any symbol reduction is also applied.

If the image being created is a relocatable object, then by default, no symbol reduction is applied. In this case, any symbol reductions are recorded as part of the versioning information. These reductions are applied when the relocatable object is finally used to generate an executable or shared object. The link-editor's -B reduce option can be used to force symbol reduction when generating a relocatable object.

A more detailed description of the versioning information is provided in Chapter 5, Application Binary Interfaces and Versioning.

To ensure interface definition stability, no wildcard expansion is provided for defining symbol names.

The following sections presents several examples of using the mapfile syntax.

Defining Symbol References

The following example shows how three symbol references can be defined. These references are then used to extract members of an archive. Although this archive extraction can be achieved by specifying multiple -u options to the link-edit, this example also shows how the eventual scope of a symbol can be reduced to local.

$ cat foo.c
void foo()
{
        (void) printf("foo: called from lib.a\n");
}
$ cat bar.c
void bar()
{
        (void) printf("bar: called from lib.a\n");
}
$ cat main.c
extern  void    foo(), bar();

void main()
{
        foo();
        bar();
}
$ ar -rc lib.a foo.o bar.o main.o
$ cat mapfile
{
        local:
                foo;
                bar;
        global:
                main;
};
$ cc -o prog -M mapfile lib.a
$ prog
foo: called from lib.a
bar: called from lib.a
$ nm -x prog | egrep "main$|foo$|bar$"
[28]    |0x00010604|0x00000024|FUNC |LOCL |0x0  |7      |foo
[30]    |0x00010628|0x00000024|FUNC |LOCL |0x0  |7      |bar
[49]    |0x0001064c|0x00000024|FUNC |GLOB |0x0  |7      |main

The significance of reducing symbol scope from global to local is covered in more detail in the section Reducing Symbol Scope.

Defining Absolute Symbols

The following example shows how two absolute symbol definitions can be defined. These definitions are then used to resolve the references from the input file main.c.

$ cat main.c
extern  int     foo();
extern  int     bar;

void main()
{
        (void) printf("&foo = %x\n", &foo);
        (void) printf("&bar = %x\n", &bar);
}
$ cat mapfile
{
        global:
                foo = FUNCTION V0x400;
                bar = DATA V0x800;
};
$ cc -o prog -M mapfile main.c
$ prog
&foo = 400
&bar = 800
$ nm -x prog | egrep "foo$|bar$"
[37]    |0x00000800|0x00000000|OBJT |GLOB |0x0  |ABS    |bar
[42]    |0x00000400|0x00000000|FUNC |GLOB |0x0  |ABS    |foo

When obtained from an input file, symbol definitions for functions or data items are usually associated with elements of data storage. A mapfile definition is insufficient to be able to construct this data storage, so these symbols must remain as absolute values. A simple mapfile definition that is associated with a size, but no value results in the creation of data storage. In this case, the symbol definition is accompanied with a section index. However, a mapfile definition that is accompanied with a value results in the creation of an absolute symbol. If a symbol is defined in a shared object, an absolute definition should be avoided. See Augmenting a Symbol Definition.

Defining Tentative Symbols

A mapfile can also be used to define a COMMON, or tentative, symbol. Unlike other types of symbol definition, tentative symbols do not occupy storage within a file, but define storage that must be allocated at runtime. Therefore, symbol definitions of this kind can contribute to the storage allocation of the output file being generated.

A feature of tentative symbols that differs from other symbol types is that their value attribute indicates their alignment requirement. A mapfile definition can therefore be used to realign tentative definitions that are obtained from the input files of a link-edit.

The following example shows the definition of two tentative symbols. The symbol foo defines a new storage region whereas the symbol bar is actually used to change the alignment of the same tentative definition within the file main.c.

$ cat main.c
extern  int     foo;
int             bar[0x10];

void main()
{
        (void) printf("&foo = %x\n", &foo);
        (void) printf("&bar = %x\n", &bar);
}
$ cat mapfile
{
        global:
                foo = COMMON V0x4 S0x200;
                bar = COMMON V0x100 S0x40;
};
$ cc -o prog -M mapfile main.c
ld: warning: symbol `bar' has differing alignments:
        (file mapfile value=0x100; file main.o value=0x4);
        largest value applied
$ prog
&foo = 20940
&bar = 20900
$ nm -x prog | egrep "foo$|bar$"
[37]    |0x00020900|0x00000040|OBJT |GLOB |0x0  |16     |bar
[42]    |0x00020940|0x00000200|OBJT |GLOB |0x0  |16     |foo

This symbol resolution diagnostic can be suppressed by using the link-editor's -t option.

Augmenting a Symbol Definition

The creation of an absolute data symbol within a shared object should be avoided. An external reference from a dynamic executable to a data item within a shared object typically requires the creation of a copy relocation. See Copy Relocations. To provide for this relocation, the data item should be associated with data storage. This association can be produced by defining the symbol within a relocatable object file. This association can also be produced by defining the symbol within a mapfile together with a size declaration and no value declaration. See Defining Additional Symbols with a mapfile.

A data symbol can be filtered. See Shared Objects as Filters. To provide this filtering, an object file definition can be augmented with a mapfile definition. The following example creates a filter containing a function and data definition.

$ cat mapfile
{
        global:
                foo = FUNCTION FILTER filtee.so.1;
                bar = DATA S0x4 FILTER filtee.so.1;
        local:
                *;
};
$ cc -o filter.so.1 -G -Kpic -h filter.so.1 -M mapfile -R.
$ nm -x filter.so.1 | egrep "foo|bar"
[39]    |0x000102b0|0x00000004|OBJT |GLOB |0    |12     |bar
[45]    |0x00000000|0x00000000|FUNC |GLOB |0    |ABS    |foo
$ elfdump -y filter.so.1 | egrep "foo|bar"
       [1]  F        [0] filtee.so.1        bar
       [7]  F        [0] filtee.so.1        foo

At runtime, a reference from an external object to either of these symbols is resolved to the definition within the filtee.

Reducing Symbol Scope

Symbol definitions that are defined to have local scope within a mapfile can be used to reduce the symbol's eventual binding. This mechanism removes the symbol's visibility to future link-edits which use the generated file as part of their input. In fact, this mechanism can provide for the precise definition of a file's interface, and so restrict the functionality made available to others.

For example, say you want to generate a simple shared object from the files foo.c and bar.c. The file foo.c contains the global symbol foo, which provides the service that you want to make available to others. The file bar.c contains the symbols bar and str, which provide the underlying implementation of the shared object. A shared object created with these files, typically results in the creation of three symbols with global scope.

$ cat foo.c
extern  const char *    bar();

const char * foo()
{
        return (bar());
}
$ cat bar.c
const char * str = "returned from bar.c";

const char * bar()
{
        return (str);
}
$ cc -o libfoo.so.1 -G foo.c bar.c
$ nm -x libfoo.so.1 | egrep "foo$|bar$|str$"
[29]    |0x000104d0|0x00000004|OBJT |GLOB |0x0  |12     |str
[32]    |0x00000418|0x00000028|FUNC |GLOB |0x0  |6      |bar
[33]    |0x000003f0|0x00000028|FUNC |GLOB |0x0  |6      |foo

You can now use the functionality offered by libfoo.so.1 as part of the link-edit of another application. References to the symbol foo are bound to the implementation provided by the shared object.

Because of their global binding, direct reference to the symbols bar and str is also possible. This visibility can have dangerous consequences, as you might later change the implementation that underlies the function foo. In so doing, you could unintentionally cause an existing application that had bound to bar or str to fail or misbehave.

Another consequence of the global binding of the symbols bar and str is that these symbols can be interposed upon by symbols of the same name. The interposition of symbols within shared objects is covered in section Simple Resolutions. This interposition can be intentional and be used as a means of circumventing the intended functionality offered by the shared object. On the other hand, this interposition can be unintentional, the result of the same common symbol name used for both the application and the shared object.

When developing the shared object, you can protect against these scenarios by reducing the scope of the symbols bar and str to a local binding. In the following example, the symbols bar and str are no longer available as part of the shared object's interface. Thus, these symbols cannot be referenced, or interposed upon, by an external object. You have effectively defined an interface for the shared object. This interface can be managed while hiding the details of the underlying implementation.

$ cat mapfile
{
        local:
                bar;
                str;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
$ nm -x libfoo.so.1 | egrep "foo$|bar$|str$"
[27]    |0x000003dc|0x00000028|FUNC |LOCL |0x0  |6      |bar
[28]    |0x00010494|0x00000004|OBJT |LOCL |0x0  |12     |str
[33]    |0x000003b4|0x00000028|FUNC |GLOB |0x0  |6      |foo

This symbol scope reduction has an additional performance advantage. The symbolic relocations against the symbols bar and str that would have been necessary at runtime are now reduced to relative relocations. See When Relocations are Performed for details of symbolic relocation overhead.

As the number of symbols that are processed during a link-edit increases, defining local scope reduction within a mapfile becomes harder to maintain. An alternative and more flexible mechanism enables you to define the shared object's interface in terms of the global symbols that should be maintained. Global symbol definitions allow the link-editor to reduce all other symbols to local binding. This mechanism is achieved using the special auto-reduction directive “*”. For example, the previous mapfile definition can be rewritten to define foo as the only global symbol required in the output file generated.

$ cat mapfile
ISV_1.1 {
        global:
                foo;
        local:
                *;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
$ nm -x libfoo.so.1 | egrep "foo$|bar$|str$"
[30]    |0x00000370|0x00000028|FUNC |LOCL |0x0  |6      |bar
[31]    |0x00010428|0x00000004|OBJT |LOCL |0x0  |12     |str
[35]    |0x00000348|0x00000028|FUNC |GLOB |0x0  |6      |foo

This example also defines a version name, libfoo.so.1.1, as part of the mapfile directive. This version name establishes an internal version definition that defines the file's symbolic interface. The creation of a version definition is recommended. The definition forms the foundation of an internal versioning mechanism that can be used throughout the evolution of the file. See Chapter 5, Application Binary Interfaces and Versioning.

If a version name is not supplied, the output file name is used to label the version definition. The versioning information that is created within the output file can be suppressed using the link-editor's -z noversion option.

Whenever a version name is specified, all global symbols must be assigned to a version definition. If any global symbols remain unassigned to a version definition, the link-editor generates a fatal error condition.

$ cat mapfile
ISV_1.1 {
        global:
                foo;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
Undefined           first referenced
 symbol                 in file
str                     bar.o  (symbol has no version assigned)
bar                     bar.o  (symbol has no version assigned)
ld: fatal: Symbol referencing errors. No output written to libfoo.so.1

The -B local option can be used to assert the auto-reduction directive “*” from the command line. The previous example an be compiled successfully as follows.

$ cc -o libfoo.so.1 -M mapfile -B local -G foo.c bar.c

When generating an executable or shared object, any symbol reduction results in the recording of version definitions within the output image. When generating a relocatable object, the version definitions are created but the symbol reductions are not processed. The result is that the symbol entries for any symbol reductions still remain global. For example, using the previous mapfile with the auto-reduction directive and associated relocatable objects, an intermediate relocatable object is created with no symbol reduction.

$ cat mapfile
ISV_1.1 {
        global:
                foo;
        local:
                *;
};
$ ld -o libfoo.o -M mapfile -r foo.o bar.o
$ nm -x libfoo.o | egrep "foo$|bar$|str$"
[17]    |0x00000000|0x00000004|OBJT |GLOB |0x0  |3      |str
[19]    |0x00000028|0x00000028|FUNC |GLOB |0x0  |1      |bar
[20]    |0x00000000|0x00000028|FUNC |GLOB |0x0  |1      |foo

The version definitions created within this image show that symbol reductions are required. When the relocatable object is used eventually to generate an executable or shared object, the symbol reductions occur. In other words, the link-editor reads and interprets symbol reduction information that is contained in the relocatable objects in the same manner as versioning data is processed from a mapfile.

Thus, the intermediate relocatable object produced in the previous example can now be used to generate a shared object.

$ cc -o libfoo.so.1 -G libfoo.o
$ nm -x libfoo.so.1 | egrep "foo$|bar$|str$"
[22]    |0x000104a4|0x00000004|OBJT |LOCL |0x0  |14     |str
[24]    |0x000003dc|0x00000028|FUNC |LOCL |0x0  |8      |bar
[36]    |0x000003b4|0x00000028|FUNC |GLOB |0x0  |8      |foo

Symbol reduction at the point at which an executable or shared object is created is typically the most common requirement. However, symbol reductions can be forced to occur when creating a relocatable object by using the link-editor's -B reduce option.

$ ld -o libfoo.o -M mapfile -B reduce -r foo.o bar.o
$ nm -x libfoo.o | egrep "foo$|bar$|str$"
[15]    |0x00000000|0x00000004|OBJT |LOCL |0x0  |3      |str
[16]    |0x00000028|0x00000028|FUNC |LOCL |0x0  |1      |bar
[20]    |0x00000000|0x00000028|FUNC |GLOB |0x0  |1      |foo

Symbol Elimination

An extension to symbol reduction is the elimination of a symbol entry from an object's symbol table. Local symbols are only maintained in an object's .symtab symbol table. This entire table can be removed from the object by using the link-editor's -s option, or strip(1). On occasion, you might want to maintain the .symtab symbol table but remove selected local symbol definitions.

Symbol elimination can be carried out using the mapfile keyword ELIMINATE. As with the local directive, symbols can be individually defined, or the symbol name can be defined as the special auto-elimination directive “*”. The following example shows the elimination of the symbol bar for the previous symbol reduction example.

$ cat mapfile
ISV_1.1 {
        global:
                foo;
        local:
                str;
        eliminate:
                *;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
$ nm -x libfoo.so.1 | egrep "foo$|bar$|str$"
[31]    |0x00010428|0x00000004|OBJT |LOCL |0x0  |12     |str
[35]    |0x00000348|0x00000028|FUNC |GLOB |0x0  |6      |foo

The -B eliminate option can be used to assert the auto-elimination directive “*” from the command line.

External Bindings

When a symbol reference from the object being created is satisfied by a definition within a shared object, the symbol remains undefined. The relocation information that is associated with the symbol provides for its lookup at runtime. The shared object that provided the definition typically becomes a dependency.

The runtime linker employs a default search model to locate this definition at runtime. Typically, each object is searched, starting with the dynamic executable, and progressing through each dependency in the same order in which the objects were loaded.

Objects can also be created to use direct bindings. With this technique, the relationship between the symbol reference and the object that provides the symbol definition is maintained within the object being created. The runtime linker uses this information to directly bind the reference to the object that defines the symbol, thus bypassing the default symbol search model. See Direct Bindings.

String Table Compression

String tables are compressed by the link-editor by removing duplicate entries, together with tail substrings. This compression can significantly reduce the size of any string tables. For example, a compressed .dynstr table results in a smaller text segment and hence reduced runtime paging activity. Because of these benefits, string table compression is enabled by default.

Objects that contribute a very large number of symbols can increase the link-edit time due to the string table compression. To avoid this cost during development use the link-editors -z nocompstrtab option. Any string table compression performed during a link-edit can be displayed using the link-editors debugging tokens -D strtab,detail.

Generating the Output File

After input file processing and symbol resolution has completed with no fatal errors, the link-editor generates the output file. The link-editor first generates the additional sections necessary to complete the output file. These sections include the symbol tables, which contain local symbol definitions together with resolved global symbol and weak symbol information, from all the input files.

Also included are any output relocation and dynamic information sections required by the runtime linker. After all the output section information has been established, the total output file size is calculated. The output file image is then created accordingly.

When creating a dynamic executable or shared object, two symbol tables are usually generated. The .dynsym table and its associated string table .dynstr contain register, global, weak, and section symbols. These sections become part of the text segment that is mapped as part of the process image at runtime. See mmap(2). This mapping enables the runtime linker to read these sections to perform any necessary relocations.

The .symtab table, and its associated string table .strtab contain all the symbols collected from the input file processing. These sections are not mapped as part of the process image. These sections can even be stripped from the image by using the link-editor's -s option, or after the link-edit by using strip(1).

During the generation of the symbol tables, reserved symbols are created. These symbols have special meaning to the linking process. These symbols should not be defined in your code.

_etext

The first location after all read-only information, typically referred to as the text segment.

_edata

The first location after initialized data.

_end

The first location after all data.

_DYNAMIC

The address of the .dynamic information section.

_END_

The same as _end. The symbol has local scope and, together with the _START_ symbol, provides a simple means of establishing an object's address range.

_GLOBAL_OFFSET_TABLE_

The position-independent reference to a link-editor supplied table of addresses, the .got section. This table is constructed from position-independent data references that occur in objects that have been compiled with the -K pic option. See Position-Independent Code.

_PROCEDURE_LINKAGE_TABLE_

The position-independent reference to a link-editor supplied table of addresses, the .plt section. This table is constructed from position-independent function references that occur in objects that have been compiled with the -K pic option. See Position-Independent Code.

_START_

The first location within the text segment. The symbol has local scope and, together with the _END_ symbol, provides a simple means of establishing an object's address range.

When generating an executable, the link-editor looks for additional symbols to define the executable's entry point. If a symbol was specified using the link-editor's -e option, that symbol is used. Otherwise the link-editor looks for the reserved symbol names _start, and then main.

Identifying Hardware and Software Capabilities

The hardware and software capabilities of a relocatable object are typically recorded at compile time. The link-editor combines the capabilities of any input relocatable objects to create a final capabilities section for the output file. See Hardware and Software Capabilities Section.

In addition, capabilities can be defined when the link-editor creates an output file. These capabilities are identified using a mapfile and the link-editor's -M option. Capabilities that are defined by using a mapfile can augment, or override, the capabilities that are supplied from input relocatable objects.

The following sections describe how capabilities can be defined using a mapfile.

Identifying Hardware Capabilities

The hardware capabilities of an object identify the hardware requirements of a platform necessary for the object to execute correctly. An example of this requirement might be the identification of code that requires the MMX or SSE features that are available on some x86 architectures.

Hardware capability requirements can be identified using the following mapfile syntax.

hwcap_1 = TOKEN | Vval [ OVERRIDE ];

The hwcap_1 declaration is qualified with one or more tokens, which are symbolic representations of hardware capabilities. In addition, or alternatively, a numeric value representing one of more capabilities can be supplied by prefixing the value with a V. For SPARC platforms, hardware capabilities are defined as AV_ values in sys/auxv_SPARC.h. For x86 platforms, hardware capabilities are defined as AV_ values in sys/auxv_386.h.

The following x86 example shows the declaration of MMX and SSE as hardware capabilities required by the object foo.so.1.

$ egrep "MMX|SSE" /usr/include/sys/auxv_386.h
#define AV_386_MMX    0x0040
#define AV_386_SSE    0x0800
$ cat mapfile
hwcap_1 = SSE MMX;
$ cc -o foo.so.1 -G -K pic -Mmapfile foo.c -lc
$ elfdump -H foo.so.1

Hardware/Software Capabilities Section:  .SUNW_cap
     index  tag               value
       [0]  CA_SUNW_HW_1     0x840  [ SSE  MMX ]

Relocatable objects can contain hardware capabilities values. The link-editor combines any hardware capabilities values from multiple input relocatable objects. The resulting CA_SUNW_HW_1 value is a bitwise-inclusive OR of the associated input values. By default, these values are combined with the hardware capabilities specified by a mapfile.

The hardware capability requirements of the output file can be controlled explicitly from a mapfile by using the OVERRIDE keyword. An OVERRIDE keyword, together with a hardware capability value of 0, effectively removes any hardware capabilities requirement from the object being built.

$ elfdump -H foo.o

Hardware/Software Capabilities Section:  .SUNW_cap
     index  tag               value
       [0]  CA_SUNW_HW_1     0x840  [ SSE  MMX ]
$ cat mapfile
hwcap_1 = V0x0 OVERRIDE;
$ cc -o bar.o -r -Mmapfile foo.o
$ elfdump -H bar.o
$ 

Any hardware capability requirements defined by an object are validated by the runtime linker against the hardware capabilities that are available to the process. If any of the hardware capability requirements can not be satisfied, the object is not loaded at runtime. For example, if the SSE feature is not available to a process, ldd(1) indicates the following error.

$ ldd prog
        foo.so.1 =>      ./foo.so.1  - hardware capability unsupported: \
                         0x800 [ SSE ]
        ....

Dynamic objects that exploit different hardware capabilities can provide a flexible runtime environment using filters. See Hardware Capability Specific Shared Objects.

Identifying Software Capabilities

The software capabilities of an object identify characteristics of the software that might be important for debugging or monitoring processes. Presently, the only software capabilities that are recognized relate to frame pointer usage by the object.

Objects can indicate that their frame pointer use is known. This state is then qualified by declaring the frame pointer as being used or not.

Software capabilities flags are defined in sys/elf.h.

#define  SF1_SUNW_FPKNWN    0x001
#define  SF1_SUNW_FPUSED    0x002

These software capability requirements can be identified using the following mapfile syntax.

sfcap_1 = TOKEN | Vval [ OVERRIDE ];

The sfcap_1 declaration can be qualified with the tokens FPKNWN and FPUSED. Or, alternatively with a numeric value that represents these states.

Relocatable objects can contain software capabilities values. The link-editor combines the software capabilities values from multiple input relocatable objects. Software capabilities can also be supplied with a mapfile. By default, any mapfile values are combined with the values supplied by relocatable objects.

The software capability requirements of the output file can be controlled explicitly from a mapfile by using the OVERRIDE keyword. An OVERRIDE keyword, together with a software capability value of 0, effectively removes any software capabilities requirement from the object being built.

$ elfdump -H foo.o

Hardware/Software Capabilities Section:  .SUNW_cap
     index  tag               value
       [0]  CA_SUNW_SF_1     0x3  [ SF1_SUNW_FPKNWN  SF1_SUNW_FPUSED ]
$ cat mapfile
sfcap_1 = V0x0 OVERRIDE;
$ cc -o bar.o -r -Mmapfile foo.o
$ elfdump -H bar.o
$ 

Software Capability Frame Pointer Processing

The computation of a CA_SUNW_SF_1 value from two frame pointer input values is as follows.

This computation is applied to each relocatable object value and mapfile value. The frame pointer software capabilities of an object are unknown if no .SUNW_cap section exists, or if the section contains no CA_SUNW_SF_1 value, or if neither the SF1_SUNW_FPKNW or SF1_SUNW_FPUSED flags are set.

Relocation Processing

After you have created the output file, all data sections from the input files are copied to the new image. Any relocations specified by the input files are applied to the output image. Any additional relocation information that must be generated is also written to the new image.

Relocation processing is normally uneventful, although error conditions might arise that are accompanied by specific error messages. Two conditions are worth more discussion. The first condition involves text relocations that result from position-dependent code. This condition is covered in more detail in Position-Independent Code. The second condition can arise from displacement relocations, which is described more fully in the next section.

Displacement Relocations

Error conditions might occur if displacement relocations are applied to a data item, which can be used in a copy relocation. The details of copy relocations are covered in Copy Relocations.

A displacement relocation remains valid when both the relocated offset and the relocation target remain separated by the same displacement. A copy relocation is where a global data item within a shared object is copied to the .bss of an executable. This copy preserves the executable's read-only text segment. If the copied data has a displacement relocation applied to the data, or an external relocation is a displacement into the copied data, the displacement relocation becomes invalidated.

Two areas of validation attempt to catch displacement relocation problems.

• The first occurs when generating a shared object. Any potential copy relocatable data items that can be problematic if the copied data is involved in a displacement relocation are flagged. During construction of a shared object, the link-editor has no knowledge of what external references might be made to a data item. Thus, all that can be flagged are potential problems.

• The second occurs when generating an executable. The creation of a copy relocation whose data is known to be involved in a displacement relocation is flagged.

  However, displacement relocations applied to a shared object might be completed during the shared objects creation at link-edit time. These displacement relocations might not have been flagged. The link-edit of an executable that references an unflagged shared object has no knowledge of a displacement being in effect in any copy-relocated data.

To help diagnose these problem areas, the link-editor indicates the displacement relocation use of a dynamic object with one or more dynamic DT_FLAGS_1 flags, as shown in Table 7–34. In addition, the link-editor's -z verbose option can be used to display suspicious relocations.

For example, say you create a shared object with a global data item, bar[], to which a displacement relocation is applied. This item could be copy-relocated if referenced from a dynamic executable. The link-editor warns of this condition.

$ cc -G -o libfoo.so.1 -z verbose -K pic foo.o
ld: warning: relocation warning: R_SPARC_DISP32: file foo.o: symbol foo: \
    displacement relocation to be applied to the symbol bar: at 0x194: \
    displacement relocation will be visible in output image

If you now create an application that references the data item bar[], a copy relocation is created. This copy results in the displacement relocation being invalidated. Because the link-editor can explicitly discover this situation, an error message is generated regardless of the use of the -z verbose option.

$ cc -o prog prog.o -L. -lfoo
ld: warning: relocation error: R_SPARC_DISP32: file foo.so: symbol foo: \
    displacement relocation applied to the symbol bar at: 0x194: \
    the symbol bar is a copy relocated symbol

ldd(1), when used with either the -d or -r options, uses the displacement dynamic flags to generate similar relocation warnings.

These error conditions can be avoided by ensuring that the symbol definition being relocated (offset) and the symbol target of the relocation are both local. Use static definitions or the link-editor's scoping technology. See Reducing Symbol Scope. Relocation problems of this type can be avoided by accessing data within shared objects by using functional interfaces.

Debugging Aids

A debugging library is provided with the Solaris OS link editors. This library enables you to trace the link-editing process in more detail. This library can help you understand and debug the link-edit of your applications and libraries. The type of information that is displayed by using this library is expected to remain constant. However, the exact format of the information might change slightly from release to release.

Some of the debugging output might be unfamiliar if you do not have an intimate knowledge of the ELF format. However, many aspects might be of general interest to you.

Debugging is enabled by using the -D option. All output that is produced is directed to the standard error. This option must be augmented with one or more tokens to indicate the type of debugging that is required. The tokens available can be displayed by typing -D help at the command line.

$ ld -Dhelp
............
debug: files      display input file processing (files and libraries)
............

Most compiler drivers interpret the -D option during their preprocessing phase. Therefore, the LD_OPTIONS environment variable is a suitable mechanism for passing this option to the link-editor.

The following example shows how input files can be traced. This syntax can be useful to determine what libraries are used as part of a link-edit. Objects that are extracted from an archive are also displayed with this syntax.

$ LD_OPTIONS=-Dfiles cc -o prog main.o -L. -lfoo
............
debug: file=main.o  [ ET_REL ]
debug: file=./libfoo.a  [ archive ]
debug: file=./libfoo.a(foo.o)  [ ET_REL ]
debug: file=./libfoo.a  [ archive ] (again)
............

Here, the member foo.o is extracted from the archive library libfoo.a to satisfy the link-edit of prog. Notice that the archive is searched twice to verify that the extraction of foo.o did not warrant the extraction of additional relocatable objects. Multiple “(again)” diagnostics indicates that the archive is a candidate for ordering using lorder(1) and tsort(1).

By using the symbols token, you can determine which symbol caused an archive member to be extracted, and which object made the initial symbol reference.

$ LD_OPTIONS=-Dsymbols cc -o prog main.o -L. -lfoo
............
debug: symbol table processing; input file=main.o  [ ET_REL ]
............
debug: symbol[7]=foo  (global); adding
debug:
debug: symbol table processing; input file=./libfoo.a  [ archive ]
debug: archive[0]=bar
debug: archive[1]=foo  (foo.o) resolves undefined or tentative symbol
debug:
debug: symbol table processing; input file=./libfoo(foo.o)  [ ET_REL ]
.............

The symbol foo is referenced by main.o. This symbol is added to the link-editor's internal symbol table. This symbol reference causes the extraction of the relocatable object foo.o from the archive libfoo.a.

This output has been simplified for this document.

By using the detail token together with the symbols token, the details of symbol resolution during input file processing can be observed.

$ LD_OPTIONS=-Dsymbols,detail cc -o prog main.o -L. -lfoo
............
debug: symbol table processing; input file=main.o  [ ET_REL ]
............
debug: symbol[7]=foo  (global); adding
debug:   entered  0x000000 0x000000 NOTY GLOB  UNDEF REF_REL_NEED
debug:
debug: symbol table processing; input file=./libfoo.a  [ archive ]
debug: archive[0]=bar
debug: archive[1]=foo  (foo.o) resolves undefined or tentative symbol
debug:
debug: symbol table processing; input file=./libfoo.a(foo.o)  [ ET_REL ]
debug: symbol[1]=foo.c
.............
debug: symbol[7]=bar  (global); adding
debug:   entered  0x000000 0x000004 OBJT GLOB  3     REF_REL_NEED
debug: symbol[8]=foo  (global); resolving [7][0]
debug:       old  0x000000 0x000000 NOTY GLOB  UNDEF main.o
debug:       new  0x000000 0x000024 FUNC GLOB  2     ./libfoo.a(foo.o)
debug:  resolved  0x000000 0x000024 FUNC GLOB  2     REF_REL_NEED
............

The original undefined symbol foo from main.o has been overridden with the symbol definition from the extracted archive member foo.o. The detailed symbol information reflects the attributes of each symbol.

In the previous example, you can see that using some of the debugging tokens can produce a wealth of output. To monitor the activity around a subset of the input files, place the -D option directly in the link-edit command line. This option can be toggled on and toggled off. In the following example, the display of symbol processing is switched on only during the processing of the library libbar.

$ ld .... -o prog main.o -L. -Dsymbols -lbar -D!symbols ....

To obtain the link-edit command line, you might have to expand the compilation line from any driver being used. See Using a Compiler Driver.