UNIX/Linux IPC : Reading from a pipe. How to know length of data at runtime?

Question

I have a child process which generates some output of variable length and then sends it to the parent using a half duplex pipe. In the parent, how do I use the read() function? Since the data can be of different length each time, how can I at run time know the size of the data to do any malloc() for a buffer? Can the fstat() function be used on a pipe file descriptor?

I know that the read() function will read a specified number of bytes but will return 0 if the end of file (not the EOF character) is reached before the bytes requested have been read.

I am specifically running Ubuntu GNU/Linux with a 2.6.27-9 Kernel.

All examples in Richard Stevens' Advanced Programming in the UNIX Environment have specified either the length of data while writing into the pipe or have relied on fgets() stdio.h function. Since I am concerned with speed, I want to stay away from using stdio.h as much as possible.

Will this be necessarily faster with shared memory?

Thanks, -Dhruv

Answer 1

Since it seems that you intend to make a single read of all the data from the pipe, I think the following will serve you better than delimiter+encoding or miniheader techniques suggested in other answers:

From the pipe (7) manpage:

If all file descriptors referring to the write end of a pipe have been closed, then an attempt to read(2) from the pipe will see end-of-file (read(2) will return 0).

The following example was taken from the pipe (2) manpage and reversed so that the child does the writing, the parent the reading (just to be sure). I also added a variable size buffer. The child will sleep for 5 seconds. The delay will ensure that the exit() of the child can have nothing to do with pipeio (the parent will print a complete line before the child exits).

#include <sys/wait.h>
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

char *
slurpfd(int fd)
{
    const int bytes_at_a_time = 2;
    char *read_buffer = NULL;
    int buffer_size = 0;
    int buffer_offset = 0;
    int chars_io;
    while (1) {
      if (buffer_offset + bytes_at_a_time > buffer_size) {
        buffer_size = bytes_at_a_time + buffer_size * 2;
        read_buffer = realloc(read_buffer, buffer_size);
        if (!read_buffer) {
          perror("memory");
          exit(EXIT_FAILURE);
        }
      }

      chars_io = read(fd,
                  read_buffer + buffer_offset,
                  bytes_at_a_time);
      if (chars_io <= 0) break;
      buffer_offset += chars_io;
    }

    if (chars_io < 0) {
      perror("read");
      exit(EXIT_FAILURE);
    }

    return read_buffer; /* caller gets to free it */
}

int
main(int argc, char *argv[])
{
  int pipefd[2];
  pid_t cpid;

  assert(argc == 2);

  if (pipe(pipefd) == -1) {
    perror("pipe");
    exit(EXIT_FAILURE);
  }

  cpid = fork();
  if (cpid == -1) {
    perror("fork");
    exit(EXIT_FAILURE);
  }

  if (cpid == 0) {     /* Child writes argv[1] to pipe */
    close(pipefd[0]);  /* Close unused read end */

    write(pipefd[1], argv[1], strlen(argv[1]) + 1);

    close(pipefd[1]);  /* Reader will see EOF */
    /* sleep before exit to make sure that there
       will be a delay after the parent prints it's
       output */
    sleep(5);
    exit(EXIT_SUCCESS);
  } else {             /* Parent reads from pipe */
    close(pipefd[1]);  /* Close unused write end */

    puts(slurpfd(pipefd[0]));

    close(pipefd[0]);
    wait(NULL);        /* Wait for child */
    _exit(EXIT_SUCCESS);
  }
}

---

From your comment I see now that you may want to read the data as it becomes available, to update the UI or whatever, to reflect your system's status. To do that open the pipe in non-blocking (O_NONBLOCK) mode. Read repeatedly whatever is available until -1 is returnd and errno == EAGAIN and do your parsing. Repeat unil read returns 0, which indicates that the child has closed the pipe.

To use an in-memory buffer for File* functions you can use fmemopen() in the GNU C library.