我想我理解fork（）是如何工作的，但只有我身体中的一些语句才能执行。例如，在每个转换块中，所有printf（）都将执行，但png_count，gif_count和bmp_count永远不会递增并保持为0（如在child（x）的printf（）输出中所见;
/* 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而没有运气。
答案 0 :(得分：3)
分叉子进程获得其父级内存的副本（并与父级和兄弟级共享大多数打开的文件描述符）。
他们不所有分享相同的内存，因此子项中的变量更改将不会影响父级或兄弟级。
要获得我认为您想要的效果，您需要在每个fork()之前在父级中进行更改，然后让子级根据它们看到的变量执行其操作。
答案 1 :(得分：1)
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）
NAME fork - 创建子进程
概要 #include
pid_t fork(void);
说明 fork（）通过复制调用进程来创建一个新进程。该 新的过程，被称为孩子，是一个完全相同的 调用进程，称为父进程，但以下情况除外 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将在父进程中返回，并且 孩子返回0。失败时，在父级中返回-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 页表并为子项分配任务结构。
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.
遵守 SVr4,4.3BSD，POSIX.1-2001。
NOTES 在Linux下，fork（）是使用copy-on-write页面实现的，所以 它所产生的唯一惩罚是重复所需的时间和记忆 输入父页面的表格，并创建一个独特的任务结构 对于孩子。
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).
实施例 见管道（2）并等待（2）。
另见 clone（2），execve（2），setrlimit（2），unshare（2），vfork（2），wait（2），dae- mon（3），capabilities（7），credentials（7）
后记 该页面是Linux man-pages项目3.23版的一部分。一个 项目的描述，以及有关报告错误的信息，可以 可以在http://www.kernel.org/doc/man-pages/找到。
Linux 2009-04-27 FORK（2）