Do I need -D_REENTRANT with -pthreads?

Question

On Linux (kernel 2.6.5) our build system calls gcc with -D_REENTRANT.

Is this still required when using pthreads?

How is it related to gcc -pthread option? I understand that I should use -pthread with pthreads, do I still need -D_REENTRANT?

On a side note, is there any difference that you know off between the usage of REENTRANT between gcc 3.3.3 and gcc 4.x.x ?

When I use -pthread gcc option I can see that _REENTRANT gets defined. Will omitting -D_REENTRANT from command line make any difference, for example could some objects be compiled without multithreaded support and then linked into binary that uses pthreads and will cause problems?

I assume it should be ok just to use: g++ -pthread

> echo | g++          -E -dM -c - > singlethreaded > echo | g++ -pthread -E -dM -c - > multithreaded > diff singlethreaded multithreaded 39a40 > #define _REENTRANT 1 

We're compiling multiple static libraries and applications that link with the static libraries, both libraries and application use pthreads.

I believe it was required at some stage in the past but want to know if it is still required. Googling hasn't returned any recent information mentioning -D_REENTRANT with pthreads. Could you point me to links or references discussing the use in recent version of kernel/gcc/pthread?

Clarification: At the moment we're using -D_REENTRANT and -lpthread, I assume I can replace them with just g++ -pthread, looking at man gcc it sets the flags for both preprocessor and linker. Any thoughts?

Answer 1

For me the best answer was the comment from pts if only he bothered to submit it as answer:

You investigated properly and answered your own question. Use g++ -pthread, it is equivalent to g++ -lpthread -D_REENTRANT. Using g++ -D_REENTRANT would be different, it may not set all the linker flags. – pts May 18 at 0:30

Answer 2

From the gcc info pages:

`-pthread'      Adds support for multithreading with the "pthreads" library.  This      option sets flags for both the preprocessor and linker. 

So just the -pthread flag should be sufficient. I wouldn't recommend only passing it to some of your code, however.

As Chris suggested in the comments, using gcc -dumpspecs on Linux does indeed confirm that it sets preprocessor flags as well:

%{posix:-D_POSIX_SOURCE} %{pthread:-D_REENTRANT}