Hello Jens,
While editing and extending your draft man pages for
tee(), splice(), vmsplice() I've been testing out
the splice()/tee() calls using a modified version of
the program you provided in the tee.2 manual page.
The most notable differences between my program and yours
are:
* I print some debugging info to stderr.
* I don't pass SPLICE_F_NONBLOCK to tee().
I'm running this on kernel 2.6.17, using the following
command line:
$ ls *.c | ktee r | wc
On different runs I see:
a) No output from ls through the pipeline:
tee returned 0
0 0 0
b) Very many instances of EAGAIN followed by expected results:
...
EAGAIN
EAGAIN
EAGAIN
EAGAIN
EAGAIN
EAGAIN
tee returned 19
splice returned 19
tee returned 0
2 2 19
In some of these cases the elapsed time to run the command-line
is 1 or 2 seconds in this case (instead of the more typical
0.05 seconds).
c) Occasionally the command line just hangs, producing no output.
In this case I can't kill it with ^C or ^\. This is a
hard-to-reproduce behaviour on my (x86) system, but I have
seen it several times by now.
Assuming I'm not messing up with my test method, some
observations:
Result a) seems to be occurring because tee() returns 0 if its
in_fd is not yet "ready" to deliver data. Shouldn't tee()
be blocking in this case? And should not 0 only
be returned for EOF? on the input file descriptor?
If I uncomment the usleep() line in the program, this behavior
does not occur--the program always just produces the expected
output:
tee returned 19
splice returned 19
tee returned 0
2 2 19
For behaviour b) -- why does tee() give EAGAIN when
I haven't specified SPLICE_F_NONBLOCK? (This is a
philosophical question; I can see that there are code paths
that lead to EAGAIN without SPLICE_F_NONBLOCK, but that
seems confusing behaviour for userland.)
Behaviour c) hints of a bug in tee().
Your thoughts?
Cheers,
Michael
====
/* ktee.c */
#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <assert.h>
#include <errno.h>
#include <limits.h>
#if defined(__i386__)
#define __NR_splice 313
#define __NR_tee 315
#else
#error unsupported arch
#endif
#define SPLICE_F_MOVE (0x01) /* move pages instead of copying */
#define SPLICE_F_NONBLOCK (0x02) /* don't block on the pipe splicing (but */
/* we may still block on the fd we splice */
/* from/to, of course */
#define SPLICE_F_MORE (0x04) /* expect more data */
#define SPLICE_F_GIFT (0x08) /* pages passed in are a gift */
static inline int splice(int fdin, loff_t *off_in, int fdout,
loff_t *off_out, size_t len, unsigned int flags)
{
return syscall(__NR_splice, fdin, off_in, fdout, off_out, len, flags);
}
static inline int tee(int fdin, int fdout, size_t len, unsigned int flags)
{
return syscall(__NR_tee, fdin, fdout, len, flags);
}
int
main(int argc, char *argv[])
{
int fd;
int len, slen;
assert(argc == 2);
fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0644);
if (fd == -1) {
perror("open");
exit(EXIT_FAILURE);
}
//usleep(100000);
do {
/*
* tee stdin to stdout.
*/
len = tee(STDIN_FILENO, STDOUT_FILENO,
INT_MAX, 0);
if (len < 0) {
if (errno == EAGAIN) {
fprintf(stderr, "EAGAIN\n");
continue;
}
perror("tee");
exit(EXIT_FAILURE);
}
fprintf(stderr, "tee returned %ld\n", (long) len);
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;
}
fprintf(stderr, "splice returned %ld\n", (long) slen);
len -= slen;
}
} while (1);
close(fd);
exit(EXIT_SUCCESS);
}
--
Michael Kerrisk
maintainer of Linux man pages Sections 2, 3, 4, 5, and 7
Want to help with man page maintenance?
Grab the latest tarball at
ftp://ftp.win.tue.nl/pub/linux-local/manpages/,
read the HOWTOHELP file and grep the source
files for 'FIXME'.
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