What does libc_hidden_proto in the glibc standard libraries do?

Question

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.

Centesimo

What version of glibc are you using? I don't have libc_hidden_proto anywhere, but I do remember it from old times ... – Aerostatics

Thats strange. I'm using glibc 2.19 but Im also looking at the unistd.h header file in glibc 2.18, 2.20, 2.21, and 2.23 and I see libc_hidden_proto in all those versions. – Centesimo

Answer 1

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)

Aerostatics

Thanks. I've actually already read what it says in libc-symbols.h but I didnt understand what a PLT was and so I didnt mention libc-symbols in my question. But +1 for your answer anyways. Now I'm off to read about the PLT, GOT, ELF files, linkers, dynamic loading and the rest of the other million other things I dont know about :P – Centesimo

@jerry basically, it's like -fsemantic-interposition for just those symbols. And it only works within glibc due to all the other crazy stuff they do. – Aerostatics

Answer 2

Duplicate question libc_hidden_proto macro in gnu library C

Seems to be handeling the namespace.

Originally I thought it had something to do with strict aliasing si ce I found it on malloc and free but I have no other basis for that

Nibelungenlied

While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. - From Review – Stillas