test

Linker and Libraries Guide

Shared Object Dependencies

When the runtime linker creates the memory segments for a program, the dependencies tell what shared objects are needed to supply the program's services. By repeatedly connecting referenced shared objects and their dependencies, the runtime linker generates a complete process image.

Note –

Even when a shared object is referenced multiple times in the dependency list, the runtime linker connects the object only once to the process.

Locating Shared Object Dependencies

During the link-edit of a dynamic executable, one or more shared objects are explicitly referenced. These objects are recorded as dependencies within the dynamic executable.

The runtime linker uses this dependency information to locate, and load, the associated objects. These dependencies are processed in the same order as they were referenced during the link-edit of the executable.

Once all the dynamic executable's dependencies are loaded, they too are inspected, in the order they are loaded, to locate any additional dependencies. This process continues until all dependencies are located and loaded. This technique results in a breadth-first ordering of all dependencies.

Directories Searched by the Runtime Linker

The runtime linker looks in one default location for dependencies. This location is /usr/lib when processing 32–bit objects, and /usr/lib/64 when processing 64–bit objects. Any dependency specified as a simple file name is prefixed with this default directory name. The resulting path name is used to locate the actual file.

The dependencies of a dynamic executable or shared object can be displayed using ldd(1). For example, the file /usr/bin/cat has the following dependencies: 


$ ldd /usr/bin/cat
        libc.so.1 =>     /usr/lib/libc.so.1
        libdl.so.1 =>    /usr/lib/libdl.so.1

The file /usr/bin/cat has a dependency, or needs, the files libc.so.1 and libdl.so.1.

The dependencies recorded in an object can be inspected using dump(1). Use this command to display the file's .dynamic section, and look for entries that have a NEEDED tag. In the following example, the dependency libdl.so.1, displayed in the previous ldd(1) example, is not recorded in the file /usr/bin/cat. ldd(1) shows the total dependencies of the specified file, and libdl.so.1 is actually a dependency of /usr/lib/libc.so.1. 


$ dump -Lvp /usr/bin/cat
 
/usr/bin/cat:
[INDEX] Tag      Value
[1]     NEEDED   libc.so.1
.........

In the previous dump(1) example, the dependencies are expressed as simple file names. In other words, there is no `/' in the name. The use of a simple file name requires the runtime linker to generate the path name from a set of rules. File names that contain an embedded `/', are used as provided.

The simple file name recording is the standard, most flexible mechanism of recording dependencies. The -h option of the link-editor records a simple name within the dependency. See Naming Conventions and Recording a Shared Object Name.

Frequently, dependencies are distributed in directories other than /usr/lib or /usr/lib/64. If a dynamic executable or shared object needs to locate dependencies in another directory, the runtime linker must explicitly be told to search this directory.

You can specify additional search path, on a per-object basis, by recording a runpath during the link-edit of an object. See Directories Searched by the Runtime Linker for details on recording this information.

Any runpath recording can be displayed using dump(1). Reference the .dynamic entry that has the RUNPATH tag. In the following example, prog has a dependency on libfoo.so.1. The runtime linker must search directories /home/me/lib and /home/you/lib before it looks in the default location. 


$ dump -Lvp prog
 
prog:
[INDEX] Tag      Value
[1]     NEEDED   libfoo.so.1
[2]     NEEDED   libc.so.1
[3]     RUNPATH  /home/me/lib:/home/you/lib
.........

Another way to add to the runtime linker's search path is to set the environment variable LD_LIBRARY_PATH. This environment variable, which is analyzed once at process startup, can be set to a colon-separated list of directories. These directories are searched by the runtime linker before any runpath specification or default directory.

These environment variables are well suited to debugging purposes, such as forcing an application to bind to a local dependency. In the following example, the file prog from the previous example is bound to libfoo.so.1, found in the present working directory.


$ LD_LIBRARY_PATH=. prog

Although useful as a temporary mechanism of influencing the runtime linker's search path, the use of LD_LIBRARY_PATH is strongly discouraged in production software. Any dynamic executables that can reference this environment variable will have their search paths augmented. This augmentation can result in an overall degradation in performance. Also, as pointed out in Using an Environment Variable and Directories Searched by the Runtime Linker, LD_LIBRARY_PATH affects the link-editor.

Environmental search paths can result in a 64–bit executable searching a path that contains a 32–bit library matching the name being looked for. Or, vice versa. The runtime linker rejects the mismatched 32–bit library and continues down its search path looking for a valid 64–bit match. If no match is found, an error message is generated. This can be observed in detail by setting the LD_DEBUG environment variable to include the files token. See Debugging Library.


$ LD_LIBRARY_PATH=/usr/bin/64 LD_DEBUG=files /usr/bin/ls
...
00283: file=libc.so.1;  needed by /usr/bin/ls
00283: 
00283: file=/usr/lib/64/libc.so.1  rejected: ELF class mismatch: \
00283:                                  32-bit/64-bit
00283: 
00283: file=/usr/lib/libc.so.1  [ ELF ]; generating link map
00283:     dynamic:  0xef631180  base:  0xef580000  size:      0xb8000
00283:     entry:    0xef5a1240  phdr:  0xef580034  phnum:           3
00283:      lmid:           0x0
00283: 
00283: file=/usr/lib/libc.so.1;  analyzing  [ RTLD_GLOBAL  RTLD_LAZY ]
...

If a dependency cannot be located, ldd(1) indicates that the object cannot be found. Any attempt to execute the application results in an appropriate error message from the runtime linker: 


$ ldd prog
        libfoo.so.1 =>   (file not found)
        libc.so.1 =>     /usr/lib/libc.so.1
        libdl.so.1 =>    /usr/lib/libdl.so.1
$ prog
ld.so.1: prog: fatal: libfoo.so.1: open failed: No such file or directory

Configuring the Default Search Paths

The default search paths used by the runtime linker are /usr/lib or /usr/lib/64. These search paths can be administered using a runtime configuration file created by the crle(1) utility. This file is often a useful aid for establishing search paths for applications that have not been built with the correct runpaths.

A configuration file can be constructed in the default location /var/ld/ld.config, for 32–bit applications, or /var/ld/64/ld.config, for 64–bit applications. This file affects all applications of the respective type on a system. Configuration files can also be created in other locations, and the runtime linker's LD_CONFIG environment variable used to select these files. This latter method is useful for testing a configuration file before installing it in the default location.

Dynamic String Tokens

The runtime linker allows for the expansion of various dynamic string tokens. These tokens may be applicable for filter, runpath and dependency definitions.