Difference between -pthread and -lpthread while compiling

multithreading gcc thread-safety pthreads compiler-flags
Vishnuraj V picture Vishnuraj V · Apr 23, 2014 · Viewed 61.1k times · Source

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

Answer

Michael Burr picture Michael Burr · Apr 23, 2014

-pthread tells the compiler to link in the pthread library as well as configure the compilation for threads.

For example, the following shows the macros that get defined when the -pthread option gets used on the GCC package installed on my Ubuntu machine:

$ gcc -pthread -E -dM test.c > dm.pthread.txt
$ gcc          -E -dM test.c > dm.nopthread.txt
$ diff dm.pthread.txt dm.nopthread.txt 
152d151
< #define _REENTRANT 1
208d206
< #define __USE_REENTRANT 1

Using the -lpthread option only causes the pthread library to be linked - the pre-defined macros don't get defined.

Bottom line: you should use the -pthread option.

Note: the -pthread option is documented as a platform specific option in the GCC docs, so it might not always be available. However, it is available on platforms that the GCC docs don't explicitly list it for (such as i386 and x86-64) - you should use it when available.

Also note that other similar options have been used by GCC, such as -pthreads (listed as a synonym for -pthread on Solaris 2) and -mthread (for MinGW-specific thread support on i386 and x86-64 Windows). My understanding is that GCC is trying to move to using -pthread uniformly going forward.

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