Non-blocking pipe using popen?

Question

I'd like to open a pipe using popen() and have non-blocking 'read' access to it.

How can I achieve this?

(The examples I found were all blocking/synchronous)

score 33 · Accepted Answer · answered Nov 15 '09 at 07:13

33

Setup like this:

FILE *f = popen("./output", "r");
int d = fileno(f);
fcntl(d, F_SETFL, O_NONBLOCK);

Now you can read:

ssize_t r = read(d, buf, count);
if (r == -1 && errno == EAGAIN)
    no data yet
else if (r > 0)
    received data
else
    pipe closed

When you're done, cleanup:

pclose(f);

answered Nov 15 '09 at 07:13

Matt Joiner

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the pipe, being a FILE pointer, is inherently buffered, is there any assurance that by using the file descriptor directly you're not going to miss something that was pulled into the file buffer, or can this be guaranteed as long as you don't call fget/fread/etc first? – stu Jun 03 '16 at 17:10

score 7 · Answer 2 · edited Jul 06 '18 at 06:36

7

popen() internally calls pipe(), fork(), dup2() (to point the child process's fds 0/1/2 to the pipes) and execve(). Have you considered using these instead? In that case, you can set the pipe you read to non-blocking using fcntl().

update: Here's an example, just for illustrative purposes:

int read_pipe_for_command(const char **argv)
{
   int p[2];

   /* Create the pipe. */
   if (pipe(p))
   {
      return -1;
   }

   /* Set non-blocking on the readable end. */
   if (fcntl(p[0], F_SETFL, O_NONBLOCK))
   {
      close(p[0]);
      close(p[1]);
      return -1;
   }

   /* Create child process. */
   switch (fork())
   {
      case -1:
          close(p[0]);
          close(p[1]);
          return -1;
      case 0:
          /* We're the child process, close read end of pipe */
          close(p[0]);
          /* Make stdout into writable end */
          dup2(p[1], 1);
          /* Run program */
          execvp(*argv, argv);
          /* If we got this far there was an error... */
          perror(*argv);
          exit(-1);
      default:
          /* We're the parent process, close write end of pipe */
          close(p[1]);
          return p[0];
   }
}

edited Jul 06 '18 at 06:36

Luca Ceresoli

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answered Nov 14 '09 at 22:29

asveikau

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Shouldn't that be: if (pipe(p) < 0) return -1; ? – Aktau Apr 14 '13 at 11:00
1

@Aktau I like my version better. The syscall will return 0 on success. The if statement tests for non zero. – asveikau Apr 15 '13 at 04:34
1

you're right, your version is also completely correct, I was thinking of other syscalls! – Aktau Apr 15 '13 at 12:15
Note the parent and child cases are swapped: kill() returns 0 to the child, not the parent. – Luca Ceresoli Jul 04 '18 at 15:23
Thanks Luca, you are right. I approved the edit. (Just 9 years after my answer!) – asveikau Jul 06 '18 at 06:37

score 2 · Answer 3 · answered Nov 14 '09 at 22:27

2

Never tried it but I don't see why you couldn't grab the file descriptors with fileno(), use fcntl() to set to non-blocking, and use read()/write(). Worth a try.

answered Nov 14 '09 at 22:27

Duck

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Non-blocking pipe using popen?

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