Generating an Auxiliary Filter

The creation of an auxiliary filter is essentially the same as creating a standard filter (see "Generating a Standard Filter" for more details). First define a filtee, libbar.so.1, on which this filter technology will be applied. This filtee might be built from several relocatable objects. One of these objects originates from the file bar.c, and supplies the symbol foo:

$ cat bar.c
char * foo()
{
        return("defined in bar.c");
}
$ cc -o libbar.so.1 -G -K pic .... bar.c ....

In the following example, an auxiliary filter, libfoo.so.1, is generated for the symbols foo and bar, and indicates the association to the filtee libbar.so.1. The environment variable LD_OPTIONS is used to circumvent the compiler driver from interpreting the -f option as one of its own.

$ cat foo.c
char * bar = "foo";

char * foo()
{
        return ("defined in foo.c");
}
$ LD_OPTIONS='-f libbar.so.1' \
cc -o libfoo.so.1 -G -K pic -h libfoo.so.1 -R. foo.c
$ ln -s libfoo.so.1 libfoo.so
$ dump -Lv libfoo.so.1 | egrep "SONAME|AUXILIARY"
[1]     SONAME    libfoo.so.1
[2]     AUXILIARY libbar.so.1

If the link-editor references the auxiliary filter libfoo.so.1 to create a dynamic executable or shared object, it will use the information from the filter's symbol table during symbol resolution. See "Symbol Resolution" for more details.

At runtime, any reference to the symbols of the filter will result in a search for the filtee libbar.so.1. If this filtee is found, the runtime linker will use this filtee to resolve any symbols defined by libfoo.so.1. If the filtee is not found, or a symbol from the filter is not found in the filtee, then the original value of the symbol within the filter is used.

For example, the following dynamic executable, prog, references the symbols foo and bar, which are resolved during link-edit from the filter libfoo.so.1.

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

main()
{
        (void) printf("foo() is %s: bar=%s\n", foo(), bar);
}
$ cc -o prog main.c -R. -L. -lfoo
$ prog
foo() is defined in bar.c: bar=foo

When the dynamic executable prog is executed, the function foo() is obtained from the filtee libbar.so.1, not from the filter libfoo.so.1. However, the data item bar is obtained from the filter libfoo.so.1, as this symbol has no alternative definition in the filtee libbar.so.1.

Auxiliary filters provide a mechanism for defining an alternative interface of an existing shared object. This mechanism is used in the Solaris operating environment to provide optimized functionality within platform specific shared objects. See "Instruction Set Specific Shared Objects" and "System Specific Shared Objects" for examples.