Difference between -pthread and -lpthread while compiling

Question

What is the difference between gcc -pthread and gcc -lpthread which is used while compiling multithreaded programs?

Answer 1

There is an accepted answer, but, IMO, it doesn't provide enough context and insight. Hence this extra answer.

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-lpthread is a solution for a problem that no longer exists (since ~2005).

In the old days there were proprietary implementations of Pthreads API that weren't POSIX-compliant, like LinuxThreads. POSIX standard merely says that if one wants POSIX-compliant behaviour, then one must link with -lpthread, and linking that is required to link a POSIX-compliant implementation of Pthreads API, should there be many implementations of it.

There are no multiple implementations of Pthreads API in modern operating systems. And that is why -lpthread no longer serves any purpose.

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Compilers like gcc and clang (and, probably, all Linux-compatible compilers) require using -pthread command line option for both compiling and linking POSIX-compliant multi-threaded applications and that is what one must use.

At compile time, -pthread option manifests that Pthread API is requested (there can be multiple threading APIs, e.g. Solaris Threads) and defines platform-specific macros (_REENTRANT on Linux, _MT on Solaris).

At link time, -pthread links in required libraries (if any) that implement POSIX-compliant Pthreads API behaviour.

The above makes it clear why -lpthread is neither necessary nor sufficient.