可以在c编程中将一个fifo重定向到stdout吗？

Question

I want to redirect a fifo to stdout and I read the doc http://man7.org/linux/man-pages/man2/tee.2.html

It says tee(int fd_in, int fd_out,...)

but when I throw a fifo fd to the 1st arguments, it says invalid error.

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>
int main() {
    int num = 0, fd;
    char fifo[] = "/tmp/tmpfifo";

    fd = open(fifo, O_RDONLY, 0644);
    if (fd == -1) {
        perror("open");
        exit(EXIT_FAILURE);
    }
    num = tee(fd, STDOUT_FILENO, INT_MAX, SPLICE_F_NONBLOCK);
    if (num < 0) {
        perror("tee");
        exit(EXIT_FAILURE);
    }
    fprintf(stderr,"%d\n", num);
    return 0;
}

The console shows: tee:invalid arguments. The 1st argu should be stdin?

Answer 1

Make sure your stdout is a pipe.

rm -f /tmp/tmpfifo
mkfifo /tmp/tmpfifo
echo hello world > /tmp/tmpfifo & 
./a.out | cat #ensure that the program's stdout is a pipe

(where a.out is your program) works for me.

Answer 2

From tee() 's man pages:

tee() duplicates up to len bytes of data from the pipe referred to by the file descriptor fd_in to the pipe referred to by the file descriptor fd_out.

So, both file descriptors must refer to pipes.

In your call to tee() :

tee(fd, STDOUT_FILENO, INT_MAX, SPLICE_F_NONBLOCK);

fd is a fifo, which is in turn a pipe, but STDOUT_FILENO may not refer to a pipe.

STDIN_FILENO and STDOUT_FILENO are not necessarily pipes.

---

If you want STDOUT_FILENO to refer to a pipe, you can run your program at the shell's command line in the following way:

yourProgram | cat