function popen isn't thread-safe:

FILE *popen(const char *command, const char *modes)
{
...
        VFORK_LOCK;
        if ((pid = vfork()) == 0) {     /* Child of vfork... */
                // child-stuff
        }
-->     VFORK_UNLOCK; // Line 109

...

        if (pid > 0) {                          /* Parent of vfork... */
                // handling of popen_list
-->             VFORK_UNLOCK; // Line 121

                return fp;
        }
-->     // Line 125

As you see the mutex will be locked correct, but it will be unlocked always in line 109 of file popen.c. After that the function handles the popen_list and tries another unlock in line 121.

If 2 threads are calling popen at the same time, they can damage the popen_list.

I suggest to move the VFORK_UNLOCK-call from line 109 to line 125 to solve this.

Greetings, Andreas