What does gcc -D_REENTRANT really do?

Question

I am writing Java bindings for a C library, and therefore working with JNI. Oracle specifies, reasonably, that native libraries for use with Java should be compiled with multithread-aware compilers.

The JNI docs give the specific example that for gcc, this multithread-awareness requirement should be met by defining one of the macros _REENTRANT or _POSIX_C_SOURCE. That seems odd to me. _REENTRANT and _POSIX_C_SOURCE are feature-test macros. GCC and POSIX documentation describe their effects in terms of defining symbols and making declarations visible, just as I would expect for any feature-test macro.

If I do not need the additional symbols or functions, then do these macros in fact do anything useful for me? Does one or both cause gcc to generate different code than it otherwise would? Do they maybe cause my code's calls to standard library functions to be linked to different implementations? Or is Oracle just talking out of its nether regions?

Edit: Additionally, it occurs to me that reentrancy is a separate consideration from threading. Non-reentrancy can be an issue even for single-threaded programs, so Oracle's suggestion that defining _REENTRANT makes gcc multithread-aware now seems even more dubious.

Answer 1

The Oracle recommendation was written for Solaris, not for Linux.

On Solaris, if you compiled a .so without _REENTRANT and ended up loaded by a multi-threaded application then very bad things could happen (e.g. random data corruption of libc internals). This was because without the define you ended up with unlocked variants of some routines by default.

This was the case when I first read this documentation, which was maybe 15 years ago, the mention of the -mt flag for the sun studio compiler was added after I last read this document in any detail.

This is no longer the case - You always get the same routine now whether or not you compile with the _REENTRANT flag; it's now only a feature macro, and not a behaviour macro.