How to call UNIX sort command on data in pipe

问题

I am creating a C program and with it I am setting up a pipe between separately forked process for interprocess communication.

The first process has written the data I need into the pipe. However, with the second process reading from the pipe, I am trying to exec the process to become the UNIX sort command. I want to somehow call sort on the data in the pipe.

How can I call sort on a pipe? On the commandline, I can sort by supplying the filename to sort as a commandline argument e.g. "sort -r MyFileToSort". I know that pipes are essentially considered files, but they are only described by their file descriptor, and as far as I know, sort won't know what to do with a fd.

Thanks for any help/feedback

回答1:

int p[2];
if (pipe(p) != 0) ...report error and do not continue...
pid_t pid = fork();
if (pid < 0) ...report error, close pipe descriptors, and do not continue...
if (pid == 0)
{
    /* Child - becomes sort */
    dup2(p[0], 0);
    close(p[0]);
    close(p[1]);
    int fd = open("output-file", O_CREAT | O_EXCL | O_WRONLY, 0644);
    if (fd < 0) ...report error and exit...
    dup2(fd, 1);
    close(fd);
    execlp("sort", "sort", (char *)0);
    ...report error and exit...
}
else
{
    /* Parent - writes data to sort */
    close(fd[0]);
    ...write data to fd[1]...
    close(fd[1]);
    int status;
    int corpse;
    while ((corpse = wait(&status)) > 0 && corpse != pid)
        ...consider reporting which child died...
    ...consider reporting sort status...
    ...continue with the rest of the program...
}

You can decide whether to report errors related to dup2() failing, or close() failing. There isn't much you can do in either case except report the problem and exit. Unless someone has subjected your program to cruel and unusual punishment by not supplying it with standard input, standard output and standard error (or something elsewhere in the program has closed any of the standard channels), then the pipe and file descriptors can't be the standard I/O descriptors, so the closes are safe. If you're not sure how sick your users are, you might protect the closes:

if (p[0] > FILENO_STDERR)
    close(p[0]);

That is normally unnecessarily paranoid (but it can be fun trying programs with missing standard I/O).

回答2:

You don't need to pass sort any arguments to specify input source or output sink at all in this case. Instead, before execing it, you should make attach your pipeline's file descriptors to its stdin (FD 0, if receiving data from a pipe) or stdout (FD 1, if writing data to a pipe), as appropriate.

See the dup2() call, which lets you set the destination to which you're copying a FD, for this purpose. As @JonathanLeffler points out, you'll want to be sure to close the original FDs (after duplicating them to the numbers you want) before your exec call.

Since you've clarified, in comments, that your goal is to write to a file, you would attach FD 1 to that destination file before calling exec, with FD 0 attached to the output side of the pipeline containing input.

来源：https://stackoverflow.com/questions/27809692/how-to-call-unix-sort-command-on-data-in-pipe

标签

unix

systems-programming