Naming Conventions

Neither the link-editor nor the runtime linker interprets any file by virtue of its filename. All files are inspected to determine their ELF type (see "ELF Header"). From this information, the processing requirements of the file are deduced. 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 filenames as part of the command-line passed to the link-editor, it is more common that the -l option be used to take advantage of the link-editor's library search capabilities (see "Shared Object Processing").

For a shared object to be applicable to this link-editor processing, it should be designated with the prefix lib and the suffix .so. For example, /usr/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. If there is a /usr/lib/libc.so.1, for example, then the 64-bit counterpart would be found at /usr/lib/64/libc.so.1.

When used as part of the runtime environment, shared objects are read and processed by the runtime linker. Here it might be necessary to allow for change in the exported interface of the shared object over a series of software releases. This interface change can be anticipated and supported by providing the shared object as a versioned filename.

A versioned filename commonly takes the form of a .so suffix followed by a version number. For example, /usr/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(3DL). A suffix of .so is still recommended to indicate the actual file type, and a version number is strongly recommended to provide for the correct binding of the shared object across a series of software releases.

The shared object name used in a dlopen(3DL) is usually represented as a simple filename -- in other words there is no `/' in the name. This convention provides flexibility by allowing the runtime linker to use a set of rules to locate the actual file (see "Loading Additional Objects" for more details).

In Chapter 5, Versioning the concept of versioning a shared object's interface over a series of software releases is described in more detail. In addition, a mechanism for coordinating the naming conventions between shared objects used in both the compilation and runtime environments is presented. But first, a mechanism that allows a shared object to record its own runtime name is described.

Recording a Shared Object Name

The recording of a dependency in a dynamic executable or shared object will, by default, be the filename of the associated shared object as it is referenced by the link-editor. For example, the following dynamic executables, 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
$ dump -Lv prog | grep NEEDED
[1]     NEEDED   libfoo.so

$ cc -o prog main.o ../tmp/libfoo.so
$ dump -Lv prog | grep NEEDED
[1]     NEEDED   ../tmp/libfoo.so

$ cc -o prog main.o /usr/tmp/libfoo.so
$ dump -Lv prog | grep NEEDED
[1]     NEEDED   /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 filename 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. For example:

$ cc -o ../tmp/libfoo.so -G -K pic -h libfoo.so.1 foo.c

Here, the shared object's runtime name libfoo.so.1, is recorded within the file itself. This identification is known as an soname, and its recording can be displayed using dump(1) and referring to the entry that has the SONAME tag. For example:

$ dump -Lvp ../tmp/libfoo.so

../tmp/libfoo.so:
[INDEX] Tag      Value
[1]     SONAME   libfoo.so.1
.........

When the link-editor processes a shared object that contains an soname, it is this name that is recorded as a dependency within the output file being generated.

Therefore, 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
$ dump -Lv prog | grep NEEDED
[1]     NEEDED   libfoo.so.1

$ cc -o prog main.o ../tmp/libfoo.so
$ dump -Lv prog | grep NEEDED
[1]     NEEDED   libfoo.so.1

$ cc -o prog main.o /usr/tmp/libfoo.so
$ dump -Lv prog | grep NEEDED
[1]     NEEDED   libfoo.so.1

In the examples shown above, the -h option is used to specify a simple filename -- in other words there is no `/' in the name. This convention is recommended, as it provides flexibility by allowing the runtime linker to use a set of rules to locate the actual file (see "Locating Shared Object Dependencies" for more details).

Inclusion of Shared Objects in Archives

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 executable or shared object. Shared objects can be extracted from the archive to satisfy the requirements of the link-edit (see "Archive Processing" for more details on the criteria for archive extraction). However, unlike the processing of relocatable objects, which are concatenated to the output file being created, any shared objects extracted from the archive will be recorded as dependencies.

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
$ dump -Lv main | grep NEEDED
[1]     NEEDED   libfoo.a(libfoo.so.1)

As it is highly unlikely that a file with this concatenated name will exist at runtime, providing an soname within the shared object is the only means of generating a meaningful runtime filename for the dependency.

The runtime linker does not extract objects from archives. Therefore, in the above example it will be necessary for the required shared object dependencies to be extracted from the archive and made available to the runtime environment.

Recorded Name Conflicts

When shared objects are used to create a dynamic executable or another shared object, the link-editor performs several consistency checks to ensure that any dependency names that will be recorded in the output file are unique.

Conflicts in dependency names can occur if two shared objects used as input files to a link-edit both contain the same soname. For example:

$ 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: file ./libbar.so: recording name `libsame.so.1' \
           matches that provided by file ./libfoo.so
ld: fatal: File processing errors. No output written to prog

A similar error condition will occur if the filename 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 will also report a name conflict. For example:

$ cc -o libbar.so -G -K pic -h libsame.so.1 bar.c -L. -lfoo
ld: fatal: file ./libfoo.so: recording name `libsame.so.1'  \
           matches that supplied with -h option
ld: fatal: File processing errors. No output written to libbar.so