C, Some statements in fork() not executing

Question

I think I understand how fork() works, yet only some of my statements in the body execute. For example, in each conversion block, all the printf() will execute yet png_count, gif_count, and bmp_count never increment and remain 0 (as an be seen in the printf() output for child(x);

/* Create Processes */
    printf("\n+++++++++++CREATING PROCESSES+++++++++++\n");
    int i, pid;
    png_count = 0;
    gif_count = 0;
    bmp_count = 0;

for(i = 0; i < convert_count; i++) {
        pid = fork();
        if(pid < 0) {
            printf("Error: fork failed");
            exit(1);
        } 
        else if (pid == 0) {
            /* Child execution */ 

            /* PNG conversion */
            if ((getpid() % 2) == 0) {
                image_in_path = getInPath(input_dir, png_images[png_count]);
                image_out_path = getOutPath(output_dir, png_images[png_count]);
                printf("image_in_path: %s\n", image_in_path);
                printf("Child (%d): %d\n", png_count+1, getpid());
                png_count++;
                printf("Converting: %s\n", image_in_path);
                execlp("convert", "convert", image_in_path, "-resize", "200x200", image_out_path, NULL);
                exit(1);
            }
            /* GIF conversion */
            else if ((getpid() % 3) == 0) {
                image_in_path = getInPath(input_dir, gif_images[gif_count]);
                image_out_path = getOutPath(output_dir, gif_images[gif_count]);
                printf("image_in_path: %s\n", image_in_path);
                printf("Child (%d): %d\n", gif_count+1, getpid());
                gif_count++;
                printf("Converting: %s\n", image_in_path);
                execlp("convert", "convert", image_in_path, "-resize", "200x200", image_out_path, NULL);
                exit(1);
            }
            /* BMP conversion */
            else {
                image_in_path = getInPath(input_dir, bmp_images[bmp_count]);
                image_out_path = getOutPath(output_dir, bmp_images[bmp_count]);
                printf("image_in_path: %s\n", image_in_path);
                printf("Child (%d): %d\n", bmp_count+1, getpid());
                bmp_count++;
                printf("Converting: %s\n", image_in_path);
                execlp("convert", "convert", image_in_path, "-resize", "200x200", image_out_path, NULL);
                exit(1);
            }
        } 
        else {
            /* Parent Execution */
            wait(NULL);
        }
    }

output:

+++++++++++CREATING PROCESSES+++++++++++

image_in_path: idir/panda.bmp
Child (1): 5389
Converting: idir/panda.bmp
image_in_path: idir/panda.bmp
Child (1): 5393
Converting: idir/panda.bmp
image_in_path: idir/penguin.gif
Child (1): 5397
Converting: idir/penguin.gif
image_in_path: idir/bmp.png
Child (1): 5402
Converting: idir/bmp.png
image_in_path: idir/bmp.png
Child (1): 5408
Converting: idir/bmp.png
image_in_path: idir/panda.bmp
Child (1): 5413
Converting: idir/panda.bmp
image_in_path: idir/panda.bmp
Child (1): 5417
Converting: idir/panda.bmp
image_in_path: idir/penguin.gif
Child (1): 5421
Converting: idir/penguin.gif
image_in_path: idir/panda.bmp
Child (1): 5425
Converting: idir/panda.bmp
image_in_path: idir/panda.bmp
Child (1): 5429
Converting: idir/panda.bmp

Thanks in advance for the help! I've spent way too many hours trying to figure this out with no luck.

Answer 1

Forked child processes get a copy of their parent's memory (and share most open file descriptors with their parent and siblings).

They do not all share the same memory, so a variable change in a child will have no effect in the parent or a sibling.

To get the effect that I think you want, you need to make the changes in your parent before each fork() and then have the child perform its actions based on the variables as it sees them.

Answer 2

a programmer should always be very familiar with the system functions they call in their code.
the main thing to notice is a child gets a COPY of the parents data, not access to the actual data.

here is a copy of the 'man fork' output.

FORK(2) Linux Programmer's Manual FORK(2)

NAME fork - create a child process

SYNOPSIS #include

   pid_t fork(void);

DESCRIPTION fork() creates a new process by duplicating the calling process. The new process, referred to as the child, is an exact duplicate of the calling process, referred to as the parent, except for the following points:

   *  The child has its own unique process ID, and this PID does not match
      the ID of any existing process group (setpgid(2)).

   *  The  child's  parent  process ID is the same as the parent's process
      ID.

   *  The child does not inherit  its  parent's  memory  locks  (mlock(2),
      mlockall(2)).

   *  Process  resource  utilizations (getrusage(2)) and CPU time counters
      (times(2)) are reset to zero in the child.

   *  The child's set of pending  signals  is  initially  empty  (sigpend‐
      ing(2)).

   *  The  child  does  not  inherit semaphore adjustments from its parent
      (semop(2)).

   *  The child does not inherit record locks from its parent (fcntl(2)).

   *  The child does not inherit timers  from  its  parent  (setitimer(2),
      alarm(2), timer_create(2)).

   *  The  child  does not inherit outstanding asynchronous I/O operations
      from its parent (aio_read(3), aio_write(3)), nor does it inherit any
      asynchronous I/O contexts from its parent (seeio_setup(2)).

   The  process  attributes  in  the  preceding  list are all specified in
   POSIX.1-2001.  The parent and child also differ  with  respect  to  the
   following Linux-specific process attributes:

   *  The  child does not inherit directory change notifications (dnotify)
      from its parent (see the description of F_NOTIFY in fcntl(2)).

   *  The prctl(2) PR_SET_PDEATHSIG setting is reset  so  that  the  child
      does not receive a signal when its parent terminates.

   *  Memory mappings that have been marked with the madvise(2) MADV_DONT‐
      FORK flag are not inherited across a fork().

   *  The  termination  signal  of  the  child  is  always  SIGCHLD   (see
      clone(2)).

   Note the following further points:

   *  The  child  process  is  created with a single thread — the one that
      called fork().  The entire virtual address space of  the  parent  is
      replicated  in the child, including the states of mutexes, condition
      variables, and other pthreads objects; the use of  pthread_atfork(3)
      may be helpful for dealing with problems that this can cause.

   *  The  child inherits copies of the parent's set of open file descrip‐
      tors.  Each file descriptor in the child refers  to  the  same  open
      file  description (see open(2)) as the corresponding file descriptor
      in the parent.  This means that the two descriptors share open  file
      status  flags, current file offset, and signal-driven I/O attributes
      (see the description of F_SETOWN and F_SETSIG in fcntl(2)).

   *  The child inherits copies of the parent's set of open message  queue
      descriptors  (see  mq_overview(7)).   Each  descriptor  in the child
      refers to the same open message queue description as the correspond‐
      ing  descriptor  in the parent.  This means that the two descriptors
      share the same flags (mq_flags).

   *  The child inherits copies of the  parent's  set  of  open  directory
      streams  (see opendir(3)).  POSIX.1-2001 says that the corresponding
      directory streams in the parent and child may  share  the  directory
      stream positioning; on Linux/glibc they do not.

RETURN VALUE On success, the PID of the child process is returned in the parent, and 0 is returned in the child. On failure, -1 is returned in the parent, no child process is created, and errno is set appropriately.

ERRORS EAGAIN fork() cannot allocate sufficient memory to copy the parent's page tables and allocate a task structure for the child.

   EAGAIN It was not possible to create a new process because the caller's
          RLIMIT_NPROC  resource  limit  was  encountered.  To exceed this
          limit, the process must have either  the  CAP_SYS_ADMIN  or  the
          CAP_SYS_RESOURCE capability.

   ENOMEM fork()  failed  to  allocate  the  necessary  kernel  structures
          because memory is tight.

CONFORMING TO SVr4, 4.3BSD, POSIX.1-2001.

NOTES Under Linux, fork() is implemented using copy-on-write pages, so the only penalty that it incurs is the time and memory required to dupli‐ cate the parent's page tables, and to create a unique task structure for the child.

   Since  version  2.3.3,  rather than invoking the kernel's fork() system
   call, the glibc fork() wrapper that is provided as  part  of  the  NPTL
   threading  implementation  invokes clone(2) with flags that provide the
   same effect as the traditional system call.  The glibc wrapper  invokes
   any fork handlers that have been established using pthread_atfork(3).

EXAMPLE See pipe(2) and wait(2).

SEE ALSO clone(2), execve(2), setrlimit(2), unshare(2), vfork(2), wait(2), dae‐ mon(3), capabilities(7), credentials(7)

COLOPHON This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Linux 2009-04-27 FORK(2)