Significance of -pthread flag when compiling

In various multi threaded C and C++ projects I've seen the -pthread flag applied to both the compiling and linking stage while others don't use it at all and just pass -lpthread to the linking stage.

Is there any danger not compiling and linking with the -pthread flag - i.e. what does -pthread actually do ? I'm primarily interested in Linux platforms.

Solution 1:

Try:

gcc -dumpspecs | grep pthread

and look for anything that starts with %{pthread:.

On my computer, this causes files to be compiled with -D_REENTRANT, and linked with -lpthread. On other platforms, this could differ. Use -pthread for most portability.

Using _REENTRANT, on GNU libc, changes the way some libc headers work. As a specific example, it makes errno call a function returning a thread-local location.

Solution 2:

From man gcc:

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