Generating a Standard Filter

To generate a standard filter, you first define a filtee, libbar.so.1, on which this filter technology is applied. This filtee might be built from several relocatable objects. In the following example, one of these objects originates from the file bar.c, and supplies the symbols foo and bar.

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

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

In the following example a standard 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 = 0;

char * foo(){}

$ 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|FILTER"
[1]     SONAME   libfoo.so.1
[2]     FILTER   libbar.so.1

If the link-editor references the standard 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 result in the additional loading of the filtee libbar.so.1. The runtime linker uses this filtee to resolve any symbols defined by libfoo.so.1.

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=bar

The execution of the dynamic executable prog results in the function foo(), and the data item bar, being obtained from the filtee libbar.so.1, not from the filter libfoo.so.1.

In this example, the filtee libbar.so.1 is uniquely associated to the filter libfoo.so.1 and is not available to satisfy symbol lookup from any other objects that might be loaded as a consequence of executing prog.

Standard filters provide a mechanism for defining a subset interface of an existing shared object, or an interface group spanning a number of existing shared objects. Several filters are used in the Solaris operating environment.

The /usr/lib/libsys.so.1 filter provides a subset of the standard C library /usr/lib/libc.so.1. This subset represents the ABI-conforming functions and data items that reside in the C library that must be imported by a conforming application.

The /usr/lib/libdl.so.1 filter defines the user interface to the runtime linker itself. This interface provides an abstraction between the symbols referenced in a compilation environment from libdl.so.1 and the actual implementation binding produced within the runtime environment from ld.so.1.

The /usr/lib/libxnet.so.1 filter uses multiple filtees. This library provides socket and XTI interfaces from /usr/lib/libsocket.so.1, /usr/lib/libnsl.so.1, and /usr/lib/libc.so.1.

Because the code in a standard filter is never referenced at runtime, there is no point in adding content to any functions defined within the filter. Filter code might require relocation, which would result in an unnecessary overhead when processing the filter at runtime. Functions are best defined as empty routines, or directly from a mapfile. See “Defining Additional Symbols”.

When generating data symbols within a filter, you should always initialize the data items to ensure that they result in references from dynamic executables.

Some of the more complex symbol resolutions carried out by the link-editor require knowledge of a symbol's attributes, including the symbol's size. See “Symbol Resolution” for more details. Therefore, you should generate the symbols in the filter so that their attributes match those of the symbols in the filtee. This ensures that the link-editing process analyzes the filter in a manner compatible with the symbol definitions used at runtime.

The link-editor uses the ELF class of the first input relocatable file it sees to govern the class of object it creates. Use the link-editor's -64 option to create a 64–bit filter solely from a mapfile.