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.