This work:
gcc a.o b.o -o a -lpthread
This does not work:
gcc -lpthread a.o b.o -o a
and shows some errors like this:
undefined reference to `pthread_create'
So when I compile using Makefile, it generate
gcc -lpthread a.o b.o -o a
Which does not work.
Any ideas?
P.S: I am using gcc 4.6.3/Ubuntu 12.04
First of all, you want to use -pthread, not -lpthread, when compiling with pthread - -pthread may enable some compile-time options that are required for the program to work in multhithreaded environment.
The problem isn't really with your command line - it's a problem with GNU ld. GNU ld, upon encountering an -lXXX option, reads all the symbols defined by libXXX.so or libXXX.a, checks if any of those symbols have been mentioned in files that have been specified before the -lXXX option in the command line, and forgets about the rest of the symbols.
In other words: command line order of object files and libraries changes behavior of the linker.
This might've been reasonable back when computers had really limited memory, but nowadays it's just a silly artifact from long forgotten times, that should've been already fixed some time ago.
Of course, there might arise a problem of linking with several libraries defining the same symbol, but there certainly are saner ways to allow expected behavior for such cases.
Bottom line: for GNU ld (which is called by gcc), only the first command from your question is valid, because ld still lives in the previous computing era.
For extra sanity, use -pthread both when linking and compiling your pthread-related programs.