Ton van den Heuvel Ton van den Heuvel - 28 days ago 5
C Question

tee always returns EINVAL

I am trying to figure out how to correctly use

tee
. In my application,
tee
always returns
EINVAL
for some reason. I was getting desperate, and tried to run the example application listed in the man page of
tee
(for example: https://linux.die.net/man/2/tee), only to find out that even that example code always fails with:
tee: Invalid argument
, for example when using it as follows:
cat tee.c | ./tee tee.log
. Any idea why this might happen?

The example code from die.net:

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>

int
main(int argc, char *argv[])
{
int fd;
int len, slen;
if (argc != 2) {
fprintf(stderr, "Usage: %s <file>\n", argv[0]);
exit(EXIT_FAILURE);
}
fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
do {
/*
* tee stdin to stdout.
*/
len = tee(STDIN_FILENO, STDOUT_FILENO,
INT_MAX, SPLICE_F_NONBLOCK);
if (len < 0) {
if (errno == EAGAIN)
continue;
perror("tee");
exit(EXIT_FAILURE);
} else
if (len == 0)
break;
/*
* Consume stdin by splicing it to a file.
*/
while (len > 0) {
slen = splice(STDIN_FILENO, NULL, fd, NULL,
len, SPLICE_F_MOVE);
if (slen < 0) {
perror("splice");
break;
}
len -= slen;
}
} while (1);
close(fd);
exit(EXIT_SUCCESS);
}

Answer

tee requires a pipe for both file descriptors, fd_in and fd_out.

Your invocation does not supply a pipe for the second file descriptor, but a file descriptor referring to a TTY. Note also that the example in the manpage specifically uses a trailing | cat:

The example below implements a basic tee(1) program using the tee() system call.
Here is an example of its use:

    $ date |./a.out out.log | cat
    Tue Oct 28 10:06:00 CET 2014
    $ cat out.log
    Tue Oct 28 10:06:00 CET 2014

Not using a pipe file descriptor for the second (or first, for that matter) argument would qualify for EINVAL:

EINVAL fd_in  or  fd_out  does not refer to a pipe; or fd_in and fd_out refer to
       the same pipe.
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