Significance of -pthread flag when compiling
Try:
gcc -dumpspecs | grep pthreadand 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.