C、fork() 中的某些语句未执行

Question

我想我了解 fork() 的工作原理，但我在正文中只有一些语句执行。例如，在每个转换块中，所有 printf() 都将执行，但 png_count、gif_count 和 bmp_count 永远不会增加并保持为 0（如 child(x) 的 printf() 输出所示）；

/* 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);
        }
    }

输出：

+++++++++++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

提前感谢您的帮助！我花了太多时间试图解决这个问题，但没有运气。

Answer 1

分叉的子进程获得其父级内存的副本（并与其父级和兄弟级共享大多数打开的文件描述符）。

他们并非都共享相同的记忆，所以孩子的可变变化不会影响父母或兄弟姐妹。

要获得我认为您想要的效果，您需要在每个 fork() 之前在您的父级中进行更改，然后让子级根据看到的变量执行其操作。

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 程序员手册 FORK(2)

名称 fork - 创建一个子进程

概要 #包括

   pid_t fork(void);

描述 fork() 通过复制调用进程来创建一个新进程。这 新进程，称为子进程，是 调用进程，简称父进程，以下除外 分：

   *  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.

返回值 成功时，子进程的 PID 在父进程中返回，并且 在孩子中返回 0。失败时，在父节点中返回 -1， 没有创建子进程，并且正确设置了errno。

错误 EAGAIN fork() 无法分配足够的内存来复制父级的 页表并为子级分配任务结构。

   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.

符合 SVr4，4.3BSD，POSIX.1-2001。

注意事项 在 Linux 下，fork() 是使用写时复制页面实现的，因此 唯一的损失是复制所需的时间和内存 cate 父级的页表，并创建一个独特的任务结构 给孩子。

   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).

示例 参见 pipe(2) 和 wait(2)。

另请参阅 clone(2)、execve(2)、setrlimit(2)、unshare(2)、vfork(2)、wait(2)、dae- mon(3)、capabilities(7)、credentials(7)

COLOPHON 此页面是 Linux 手册页项目 3.23 版的一部分。一种 项目的描述，以及关于报告错误的信息，可以 可以在http://www.kernel.org/doc/man-pages/找到。

Linux 2009-04-27 FORK(2)