undefined reference to `pthread_mutex_trylock'

Question

I have the following test program.

#include <iostream>
#include <cstdlib>

using namespace std;    
pthread_mutex_t mymutex = PTHREAD_MUTEX_INITIALIZER;

int main(int argc, char *argv[])
{
  int iret;
  iret = pthread_mutex_trylock( & mymutex );
  cout << "Test2 !!! " << endl;
  pthread_mutex_unlock( & mymutex );
  return EXIT_SUCCESS;
}

If I compile it without adding the pthread library I get the error for unresolved error for pthread_mutex_trylock , but only for function pthread_mutex_trylock.

If I replace pthread_mutex_trylock with pthread_mutex_trylock the program is compiled and runnig well also without the -lpthread* option.

If I add the -lpthraed option to the compile commands all runs well this run well : $ g++ test2.c -o test2 -lpthread this warn unresolved : $ g++ test2.c -o test2

Example error output: $ g++ test2.c -o test2 /tmp/ccU1bBdU.o: In function main': test2.c:(.text+0x11): undefined reference topthread_mutex_trylock' collect2: ld returned 1 exit status

If I replace the instruction iret = pthread_mutex_trylock( & mymutex );

with iret = pthread_mutex_lock( & mymutex ); the program compiles and run without error also if didn't add the pthread libarry to the compile command I know that is right to have the unresolved error if I didn't use the -lpthread option , but why I have not the same unresolved error also for other pthread_ function ?

I'm using gcc 4.4.2 on fedora 12

$ g++ --version
g++ (GCC) 4.4.2 20091222 (Red Hat 4.4.2-20)
Copyright (C) 2009 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Do some have some suggestion about the meaning of this unreference only for pthread_mutex_trylock ?

thanks for the help, Enzo

Answer 1

You should perform both the compilation and linkage with the -pthread option, to be portable. On some systems, the compilation will have specific flags added (e.g., -D_REENTRANT) with -pthread specified.

If you're curious to see what -pthread will do to your compile and link flags, run gcc -dumpspecs.

Answer 2

If you use pthread functions you should link your object files with -lpthread and not worry about whether symbols are included in libc.

The rationale behind this is said to be such: some time ago the stubs in libc were used when application that used threads was run on a system without threading support. On such system, pthread_* functions became linked to libc stubs that returned errors showing that there's no threading functionality. While on "threaded" systems they were linked to pthread library and worked correctly.

Obviously, pthread_mutex_trylock function appeared after the policy changed to linking with -lpthread. So there's no stub for it.