Neither the link-editor nor the runtime linker interprets any file by virtue of its file name. All files are inspected to determine their ELF type (see ELF Header). This information enables the link-editor to deduce the processing requirements of the file. However, shared objects usually follow one of two naming conventions, depending on whether they are being used as part of the compilation environment or the runtime environment.
When used as part of the compilation environment, shared objects are read and processed by the link-editor. Although these shared objects can be specified by explicit file names as part of the command passed to the link-editor, the –l option is usually used to take advantage of the link-editor's library search facilities. See Shared Object Processing.
A shared object that is applicable to this link-editor processing, should be designated with the prefix lib and the suffix .so. For example, /lib/libc.so is the shared object representation of the standard C library made available to the compilation environment. By convention, 64-bit shared objects are placed in a subdirectory of the lib directory called 64. For example, the 64-bit counterpart of /lib/libc.so.1, is /lib/64/libc.so.1.
When used as part of the runtime environment, shared objects are read and processed by the runtime linker. To allow for change in the exported interface of the shared object over a series of software releases, provide the shared object as a versioned file name.
A versioned file name commonly takes the form of a .so suffix followed by a version number. For example, /lib/libc.so.1 is the shared object representation of version one of the standard C library made available to the runtime environment.
If a shared object is never intended for use within a compilation environment, its name might drop the conventional lib prefix. Examples of shared objects that fall into this category are those used solely with dlopen(3C). A suffix of .so is still recommended to indicate the actual file type. In addition, a version number is strongly recommended to provide for the correct binding of the shared object across a series of software releases. Interfaces and Versioning describes versioning in more detail.
The recording of a dependency in a dynamic object will, by default, be the file name of the associated shared object as it is referenced by the link-editor. For example, the following dynamic executables, that are built against the same shared object libfoo.so, result in different interpretations of the same dependency.
$ cc -o ../tmp/libfoo.so -G foo.o $ cc -o prog main.o -L../tmp -lfoo $ elfdump -d prog | grep NEEDED  NEEDED 0x123 libfoo.so.1 $ cc -o prog main.o ../tmp/libfoo.so $ elfdump -d prog | grep NEEDED  NEEDED 0x123 ../tmp/libfoo.so $ cc -o prog main.o /usr/tmp/libfoo.so $ elfdump -d prog | grep NEEDED  NEEDED 0x123 /usr/tmp/libfoo.so
As these examples show, this mechanism of recording dependencies can result in inconsistencies due to different compilation techniques. Also, the location of a shared object as referenced during the link-edit might differ from the eventual location of the shared object on an installed system. To provide a more consistent means of specifying dependencies, shared objects can record within themselves the file name by which they should be referenced at runtime.
During the link-edit of a shared object, its runtime name can be recorded within the shared object itself by using the –h option. In the following example, the shared object's runtime name libfoo.so.1, is recorded within the file itself. This identification is known as an soname.
$ cc -o ../tmp/libfoo.so -G -K pic -h libfoo.so.1 foo.c
The following example shows how the soname recording can be displayed using elfdump(1) and referring to the entry that has the SONAME tag.
$ elfdump -d ../tmp/libfoo.so | grep SONAME  SONAME 0x123 libfoo.so.1
When the link-editor processes a shared object that contains an soname, this is the name that is recorded as a dependency within the output file being generated.
If this new version of libfoo.so is used during the creation of the dynamic executable prog from the previous example, all three methods of creating the executable result in the same dependency recording.
$ cc -o prog main.o -L../tmp -lfoo $ elfdump -d prog | grep NEEDED  NEEDED 0x123 libfoo.so $ cc -o prog main.o ../tmp/libfoo.so $ elfdump -d prog | grep NEEDED  NEEDED 0x123 libfoo.so $ cc -o prog main.o /usr/tmp/libfoo.so $ elfdump -d prog | grep NEEDED  NEEDED 0x123 libfoo.so
In the previous examples, the –h option is used to specify a simple file name, that has no '/' in the name. This convention enables the runtime linker to use a set of rules to locate the actual file. See Locating Shared Object Dependencies for more details.
The mechanism of recording an soname within a shared object is essential if the shared object is ever processed from an archive library.
An archive can be built from one or more shared objects and then used to generate a dynamic object. Shared objects can be extracted from the archive to satisfy the requirements of the link-edit. Unlike the processing of relocatable objects, which are concatenated to the output file being created, any shared objects extracted from the archive are recorded as dependencies. See Archive Processing for more details on the criteria for archive extraction.
The name of an archive member is constructed by the link-editor and is a concatenation of the archive name and the object within the archive. For example.
$ cc -o libfoo.so.1 -G -K pic foo.c $ ar -r libfoo.a libfoo.so.1 $ cc -o main main.o libfoo.a $ elfdump -d main | grep NEEDED  NEEDED 0x123 libfoo.a(libfoo.so.1)
Because a file with this concatenated name is unlikely to exist at runtime, providing an soname within the shared object is the only means of generating a meaningful runtime file name for the dependency.
When shared objects are used to create a dynamic object, the link-editor performs several consistency checks. These checks ensure that any dependency names recorded in the output file are unique.
$ cc -o libfoo.so -G -K pic -h libsame.so.1 foo.c $ cc -o libbar.so -G -K pic -h libsame.so.1 bar.c $ cc -o prog main.o -L. -lfoo -lbar ld: fatal: recording name conflict: file './libfoo.so' and \ file './libbar.so' provide identical dependency names: libsame.so.1
A similar error condition occurs if the file name of a shared object that does not have a recorded soname matches the soname of another shared object used during the same link-edit.
If the runtime name of a shared object being generated matches one of its dependencies, the link-editor also reports a name conflict.
$ cc -o libbar.so -G -K pic -h libsame.so.1 bar.c -L. -lfoo ld: fatal: recording name conflict: file './libfoo.so' and \ -h option provide identical dependency names: libsame.so.1