proc-net-dev-lseek.c

 1/*
 2 * Copyright (c) 2025 Alexey Dobriyan <adobriyan@gmail.com>
 3 *
 4 * Permission to use, copy, modify, and distribute this software for any
 5 * purpose with or without fee is hereby granted, provided that the above
 6 * copyright notice and this permission notice appear in all copies.
 7 *
 8 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 9 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15 */
16#undef _GNU_SOURCE
17#define _GNU_SOURCE
18#undef NDEBUG
19#include <assert.h>
20#include <errno.h>
21#include <fcntl.h>
22#include <string.h>
23#include <unistd.h>
24#include <sched.h>
25/*
26 * Test that lseek("/proc/net/dev/", 0, SEEK_SET)
27 * a) works,
28 * b) does what you think it does.
29 */
30int main(void)
31{
32	/* /proc/net/dev output is deterministic in fresh netns only. */
33	if (unshare(CLONE_NEWNET) == -1) {
34		if (errno == ENOSYS || errno == EPERM) {
35			return 4;
36		}
37		return 1;
38	}
39
40	const int fd = open("/proc/net/dev", O_RDONLY);
41	assert(fd >= 0);
42
43	char buf1[4096];
44	const ssize_t rv1 = read(fd, buf1, sizeof(buf1));
45	/*
46	 * Not "<=", this file can't be empty:
47	 * there is header, "lo" interface with some zeroes.
48	 */
49	assert(0 < rv1);
50	assert(rv1 <= sizeof(buf1));
51
52	/* Believe it or not, this line broke one day. */
53	assert(lseek(fd, 0, SEEK_SET) == 0);
54
55	char buf2[4096];
56	const ssize_t rv2 = read(fd, buf2, sizeof(buf2));
57	/* Not "<=", see above. */
58	assert(0 < rv2);
59	assert(rv2 <= sizeof(buf2));
60
61	/* Test that lseek rewinds to the beginning of the file. */
62	assert(rv1 == rv2);
63	assert(memcmp(buf1, buf2, rv1) == 0);
64
65	/* Contents of the file is not validated: this test is about lseek(). */
66
67	return 0;
68}