A quick example of how to:

• Export ELF version references in a library
• Select compatible version references when compiling/linking against a library

Simply run:

make run

to make the library and linking program, then run the result.

In the library, the version references can be configured through use of __asm__(".symver ...") and a .map file.

• __asm__(".symver <local-symbol>,<global-symbol>@") creates the Base version
• __asm__(".symver <local-symbol>,<global-symbol>@<version-symbol>") creates a normal version
• __asm__(".symver <local-symbol>,<global-symbol>@@<version-symbol>") creates the default version

The final option with @@ can only be used once.

The format of the .map file is:

<version-symbol> {
  global:
    <global-symbol>;
  local:
    <local-symbol>;
} <parent-version-symbol>;

When compiled as a shared object with the linker option --version-script, only global symbols are exported, and local symbols are hidden. In this way, we can achieve the following list of exported symbols:

$ objdump -T libexample.so | grep -i example
libexample.so:     file format elf64-x86-64
0000000000000849 g    DF .text	0000000000000013  EXAMPLE_2.0 example
0000000000000810 g    DF .text	0000000000000013 (Base)       example
0000000000000836 g    DF .text	0000000000000013 (EXAMPLE_1.2) example
0000000000000000 g    DO *ABS*	0000000000000000  EXAMPLE_1.1 EXAMPLE_1.1
0000000000000823 g    DF .text	0000000000000013 (EXAMPLE_1.1) example
0000000000000000 g    DO *ABS*	0000000000000000  EXAMPLE_1.2 EXAMPLE_1.2
0000000000000000 g    DO *ABS*	0000000000000000  EXAMPLE_2.0 EXAMPLE_2.0

The file example.c does not contain a function named example but rather a series of functions (example_10, example_11, example_12 and example_20) that are exported as the versioned example symbol with different versions.

Selection of non-default versions of a symbol in a linking program can be achieved also using __asm__(".symver ...) but with a different intent. Calling:

__asm__(".symver <symbol>,<symbol>@<version>")

will ensure that the linked symbol is the appropriate version from the library. Omitting this or a similar line will cause the linker to simply use the default version as specified by the library.

Based on the existence and content of the __asm__(".symver ...) line, we can have the default case, when omitted:

$ objdump -p main | grep -i example
  NEEDED               libexample.so
  required from libexample.so:
    0x0541e330 0x00 02 EXAMPLE_2.0

$ objdump -T main | grep -i example
0000000000000000      DF *UND*	0000000000000000  EXAMPLE_2.0 example

the base case, using __asm__(".symver example,example@"):

$ objdump -p main_base | grep -i example
  NEEDED               libexample.so

$ objdump -T main_base | grep -i example
0000000000000000      DF *UND*	0000000000000000              example

or the specified case, using __asm__(".symver example,example@EXAMPLE_1.1"):

$ objdump -p main_11 | grep -i example
  NEEDED               libexample.so
  required from libexample.so:
    0x0541e631 0x00 03 EXAMPLE_1.1

$ objdump -T main_11 | grep -i example
0000000000000000      DF *UND*	0000000000000000  EXAMPLE_1.1 example