Wednesday, August 25, 2010

Linker Sets

Reminder: When placing something in a dedicated section using __attribute__((section("foobar"))), the GNU toolchain will automatically add a symbol __start_foobar at the beginning and a symbol __stop_foobar at the end of the section.

However, you will need a reference to that symbol in order to prevent the linker from optimizing the symbol away. In other words, you need to declare something like extern void *__start_foobar; and use it.

When using the Microsoft toolchain, the symbols need to be added explicitly. To do that, one can make use of the fact that when the Microsoft linker encounters several sections with a "$_" in their name, it will merge the contents into one final output section. The name of the output section will be the common prefix of the declared sections. The beauty is that the contents are in the order of the section names.

Here's an example: Supposed you placed something into a section called "foobar$_something". You can then add a variable __start_foobar into a section "foobar$_a" and a variable __stop_foobar into a section "foobar$_z". The resulting binary will have one section "foobar" with the contents of variable __start_foobar placed at the beginning, followed the contents of everything in section "foobar$_something" and the contents of the variable __stop_foobar at the end.

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