What does libc_hidden_proto in the glibc standard libraries do?

I was trying to look up the prototype for the getpgrp() function in unistd.h but couldnt find it. The only line containing getpgrp in that file was the line libc_hidden_proto (tcgetpgrp) and I'm assuming this is a macro, but I dont know what it does.

I googled but couldn't find any explanations on the web for what libc_hidden_proto actually does. Any explanation about what the purpose of that macro is in glibc would be appreciated.

Solution

You've accidentally looked at the internal copy (include/unistd.h) instead of the public copy (posix/unistd.h). No, I don't understand how glibc's source is organized either.

From include/libc-symbols.h:

   The following macros are used for PLT bypassing within libc.so
   (and if needed other libraries similarly).
   First of all, you need to have the function prototyped somewhere,
   say in foo/foo.h:
   int foo (int __bar);
   If calls to foo within libc.so should always go to foo defined in libc.so,
   then in include/foo.h you add:
   libc_hidden_proto (foo)