Augmenting a Symbol Definition

The creation of an absolute data symbol within a shared object should be avoided. An external reference from a dynamic executable to a data item within a shared object typically requires the creation of a copy relocation. See Copy Relocations. To provide for this relocation, the data item should be associated with data storage. This association can be produced by defining the symbol within a relocatable object file. This association can also be produced by defining the symbol within a mapfile together with a size declaration and no value declaration. See SYMBOL_SCOPE / SYMBOL_VERSION Directives.

A data symbol can be filtered. See Shared Objects as Filters. To provide this filtering, an object file definition can be augmented with a mapfile definition. The following example creates a filter containing a function and data definition.

$ cat mapfile
$mapfile_version 2
SYMBOL_SCOPE {
        global:
                foo     { TYPE=FUNCTION;       FILTER=filtee.so.1 };
                bar     { TYPE=DATA; SIZE=0x4; FILTER=filtee.so.1 };
        local:
                *;
};
$ cc -o filter.so.1 -G -Kpic -h filter.so.1 -M mapfile -R.
$ elfdump -sN.dynsym filter.so.1 | egrep 'foo|bar'
       [1]  0x000105f8 0x00000004  OBJT GLOB  D    1 .data          bar
       [7]  0x00000000 0x00000000  FUNC GLOB  D    1 ABS            foo
$ elfdump -y filter.so.1 | egrep 'foo|bar'
       [1]  F        [0] filtee.so.1        bar
       [7]  F        [0] filtee.so.1        foo

At runtime, a reference from an external object to either of these symbols is resolved to the definition within the filtee.