主进程-> pthread-> fork + execvp

Question

I am seeing a strange issue.

Sometimes when i run my program long enough i see that there are two copies of my program running. The second is a child process of the first since i see that the parent PID of the second one is that of the first one.

I realized that i have a fork in my code and its only because of this that i can have two copies running -- i can otherwise never have two copies of my program running.

This happens very rarely but it does happen.

The architecture is as follows:

The main program gets an event and spawns a pthread. In that thread i do some processing and based on some result i do a fork immediately followed by an execvp.

I realize that its not best to call a fork from a pthread but in my design the main process gets many events and the only way to parallely work on all those events was to use pthreads. Each pthread does some processing and in certain cases it needs to call a different program (for which i use execvp). Since i had to call a different program i had to use fork

I am wondering if because i am eventually calling a fork from a thread context is it possible that multiple threads parallely call fork + execvp and this "somehow" results in two copies being created.

If this is indeed happening would it help if i protect the code that does fork+execvp with a mutex since that would result in only one thread calling the fork + execvp.

However, if i take a mutex before fork + excvp then i dont know when to release it.

Any help here would be appreciated.

thread code that does fork + execvp -- in case you guys can spot an issue there:

In main.c

    status = pthread_create(&worker_thread, tattr, 
                                 do_some_useful_work, some_pointer);

[clipped]

    void *do_some_useful_work (void * arg)
    {
          /* Do some processing and fill pArguments array */

          child_pid = fork();

          if (child_pid == 0)
          {
              char *temp_log_file;

              temp_log_file = (void *) malloc (strlen(FORK_LOG_FILE_LOCATION) +
                                        strlen("/logfile.") + 8);

               sprintf (temp_log_file, "%s/logfile.%d%c", FORK_LOG_FILE_LOCATION, getpid(),'\0');

               /* Open log file */
               int log = creat(temp_log_file, 0777);
               /* Redirect stdout to log file */
               close(1);
               dup(log);
               /* Redirect stderr to log file */
               close(2);
               dup(log);

               syslog(LOG_ERR, "Opening up log file %s\n", temp_log_file);

               free (temp_log_file);

               close (server_sockets_that_parent_is_listening_on);

               execvp ("jazzy_program", pArguments); 

         }

         pthread_exit (NULL);

         return NULL;
    }

I looked through this code and i see no reason why i would do a fork and not do an execvp -- so the only scenario that comes to my mind is that multiple threads get executed and they all call fork + execvp. This sometimes causes two copies of my main program to run.

Answer 1

In the case where execvp fails for any reason (perhaps too many processes, out of memory, etc.), you fail to handle the error; instead the forked copy of the thread keeps running. Calling pthread_exit (or any non-async-signal-safe) function in this process has undefined behavior, so it might not exit properly but hang or do something unexpected. You should always check for exec failure and immediately _exit(1) or similar when this happens. Also, while this probably isn't your problem, it's unsafe to call malloc after forking in a multithreaded process since it's non-async-signal-safe.