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Subjectsplice() on two pipes
Hi,

when I read about the splice() system call, I thought it was obvious
that it could copy data between two pipes. I was surprised that this
assumption is wrong, it's not possible on 2.6.29, I get EINVAL. Can
anybody please explain this limitation?

Background: I want to forward data between two subprocesses, which are
connected to me with a pipe().

I have attached a small test program which prints a table of supported
splice operations. Here's the output on 2.6.29.1:

in\out pipe sock reg chr
pipe no yes yes yes
sock no no no no
reg yes no no no
chr no no no no

Max
/*
* Copyright (C) 2009 Max Kellermann <max@duempel.org>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* - Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
*
* - Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the
* distribution.
*/
/*
* This tiny program prints a matrix: which file descriptor
* combinations are supported by splice()?
*/
#define _GNU_SOURCE
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/socket.h>

static struct {
const char *const name;
int in, out;
} fds[] = {
{ .name = "pipe", },
{ .name = "sock", },
{ .name = "reg", },
{ .name = "chr", },
};
enum {
NUM_FDS = sizeof(fds) / sizeof(fds[0]),
};
int main(int argc, char **argv)
{
int f[2], ret;
unsigned x, y;
char template1[] = "/tmp/test_splice.XXXXXX";
char template2[] = "/tmp/test_splice.XXXXXX";
(void)argc;
(void)argv;

/* open two file descriptors of each kind */
fds[0].in = pipe(f) >= 0 ? f[0] : -1;
fds[0].out = pipe(f) >= 0 ? f[1] : -1;
fds[1].in = socketpair(AF_UNIX, SOCK_STREAM, 0, f) >= 0 ? f[0] : -1;
fds[1].out = socketpair(AF_UNIX, SOCK_STREAM, 0, f) >= 0 ? f[0] : -1;
fds[2].in = mkstemp(template1);
fds[2].out = mkstemp(template2);
fds[3].in = open("/dev/zero", O_RDONLY);
fds[3].out = open("/dev/null", O_WRONLY);

/* print table header */
printf("in\\out");
for (x = 0; x < NUM_FDS; ++x)
printf("\t%s", fds[x].name);
putchar('\n');

for (y = 0; y < NUM_FDS; ++y) {
fputs(fds[y].name, stdout);
for (x = 0; x < NUM_FDS; ++x) {
putchar('\t');
if (fds[x].out < 0 || fds[y].in < 0) {
fputs("n/a", stdout);
continue;
}
ret = splice(fds[y].in, NULL, fds[x].out, NULL, 1,
SPLICE_F_NONBLOCK);
if (ret >= 0 || errno == EAGAIN || errno == EWOULDBLOCK)
/* EAGAIN or EWOULDBLOCK means that the kernel has
accepted this combination, but can't move pages
right now */
fputs("yes", stdout);
else if (errno == EINVAL)
/* the kernel doesn't support this combination */
fputs("no", stdout);
else if (errno == ENOSYS)
/* splice() isn't supported at all */
fputs("ENOSYS", stdout);
else
/* an unexpected error code */
fputs("err", stdout);
}
putchar('\n');
}
unlink(template1);
unlink(template2);
}
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