Question

I’m writing a little application under Linux (embedded on ARM) that is running two threads. I do a “popen” in a function and this creates a deadlock for the second thread that enters the function. However, the first thread that entered the function first still runs correctly.

Here is some code sample:

pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;

    int sendCommand(
        const std::string& command, std::string& answer)
    {
      FILE* fd;                           // File descriptor to command output
      char answer_c[COMMAND_BUFFER_SIZE]; // The answer as char[]
      int answerLength = 0;               // The length of the answer

      pthread_mutex_lock( &mutex1 );

      // A probe
      cout << "VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV" << endl;

      fd = popen(command.c_str(), "r"); // <- Second thread entering is stuck here ...
      if(fd <= 0)
      {
        cout << "couldn't popoen !" << endl;
        return -1;
      }

      // A probe, never showed by second thread entering the function
      cout << "ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZzz" << endl;

      // ... Omitted code ...
      // Close the file descriptor
      pclose(fd);
      pthread_mutex_unlock( &mutex1 );

    }

I really have the felling that I’m missing something important. How could a deadlock happen with popen? Is the problem comming from the standard libc or Linux kernel ?

Any pointer is strongly appreciated !

Regards,

Answer 1

Since popen does a fork (which is not thread safe), then popen is not thread safe as well.

This question and answers might help you a bit.