Significance of -pthread flag when compiling

From man gcc:

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

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.