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, but 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, and archives are designated by the prefix lib and the suffix .a. For example, libc.so is the shared object version of the standard C library made available to the compilation environment, and libc.a is its 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:

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

directs the link-editor to search for libfoo.so, and if it does not find it, to search for libfoo.a, before moving on to the next directory to be searched.

There is a naming convention 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" for more details.

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.

When in dynamic mode and using the -l option to enable a library search, the link-editor will first search in a given directory for a shared object that matches the specified name. If no match is found, the link-editor will then look for an archive library in the same directory. When in static mode and using the -l option, only archive libraries will be sought.

Linking With a Mix of Shared Objects and Archives

Although the library search mechanism, in dynamic mode, searches a given directory for a shared object, then an archive library, finer control of the type of search required can be achieved using the -B option.

By specifying the -Bdynamic and -Bstatic options on the command-line, as many times as required, the library search can be toggled 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 -Bstatic and -Bdynamic keywords are not exactly symmetrical. When you specify -Bstatic, the link-editor does not accept shared objects as input until the next occurrence of -Bdynamic. However, when you specify -Bdynamic, the link-editor first looks for shared objects and then archives in any given directory.

In the previous example it is more precise to say that the link-editor first searches for libfoo.a, and then for libbar.so, and if that fails, for libbar.a. Finally, it will search for libc.so, and if that fails, libc.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 it has previously seen. After this search is completed and the required relocatable objects have been extracted, the archive is not available to resolve any new symbols obtained from the input files that follow the archive on the command-line. For example, the command

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

directs the link-editor to search libfoo.a only to resolved symbol references that have been obtained from file1.c. libfoo.a is not available to resolve symbol references from file2.c or file3.c.

As a rule, it is best to 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 two standard directories to look for libraries, /usr/ccs/lib and /usr/lib. When linking 64-bit objects only one standard directory is used /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 in two ways: using a command-line option, or using an environment variable.

Using a Command-Line Option

The -L option can be used to add a new pathname to the library search path. This option affects the search path at the point it is encountered on the command-line. For example, the command

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

searches path1 (then /usr/ccs/lib and /usr/lib) to find libfoo, but searches path1 and then path2 (and then /usr/ccs/lib and /usr/lib) to find libbar.

Pathnames defined using the -L option are used only by the link-editor. They are not recorded in the output file image created 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.

The -Y option can be used 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 command

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

searches for libfoo only in the directories /opt/COMPILER/lib and /home/me/lib. The directories specified using the -Y option can be supplemented by using the -L option.

Using an Environment Variable

You can also use the environment variable LD_LIBRARY_PATH, which takes a colon-separated list of directories, to add to the link-editor's library search path. In its most general form, LD_LIBRARY_PATH takes two directory lists separated by a semicolon. The first list is searched before the list(s) supplied on the command-line, and the second list is searched after.

Here is 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 will be dir1:dir2:path1:path2... pathn:dir3:/usr/ccs/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. For example:

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

Here the effective search path will be path1:path2... pathn:dir1:dir2:/usr/ccs/lib:/usr/lib.

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

Directories Searched by the Runtime Linker

By default, the runtime linker knows of only one standard place to look for libraries, /usr/lib when processing 32-bit objects, and /usr/lib/64 when processing 64-bit objects. 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, these shared objects are recorded as dependencies that must be located again during process execution by the runtime linker. During the link-edit, one or more pathnames can be recorded in the output file. These pathnames are used by the runtime linker to search for any shared object dependencies. These recorded pathnames are referred to as a runpath.

Objects may be built with the -znodefaultlib option to suppress any search of the standard locations (/usr/lib or /usr/lib/64) at runtime. Use of this option implies that all the dependencies of an object can be located using its runpaths. Without this option, which is the most common case, no matter how you modify the runtime linker's library search path, its last element is always /usr/lib for 32-bit objects and /usr/lib/64 for 64-bit objects.

Default search paths can be administrated using a runtime configuration file (see "Configuring the Default Search Paths"). However the creator of an object should not rely on the existence of this file, and should always insure their object can locate its dependencies with only its runpaths or standard system defaults.

The -R option, which takes a colon-separated list of directories, can be used to record a runpath in a dynamic executable or shared library. For example:

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

will record the runpath /home/me/lib:/home/you/lib in the dynamic executable prog. The runtime linker uses these paths, then the default location /usr/lib, to locate 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 and concatenates each of these specifications, separated by a colon. Thus, the previous example can also be expressed as:

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

For objects that may 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.