Monitor for changes in mount points via /proc/mounts

mount-mon.c

#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

#define MOUNT_PROC "/proc/mounts"

int main(int argc, char **argv)
{
    int mfd = open(MOUNT_PROC, O_RDONLY, 0);

    if (mfd < 0) {
        fprintf(stderr, "ERROR: Unable to open %s.", MOUNT_PROC);
        exit(EXIT_FAILURE);
    }

    fd_set rfds;
    struct timeval tv;
    int rv;

    FD_ZERO(&rfds);
    FD_SET(mfd, &rfds);
    tv.tv_sec = 5;
    tv.tv_usec = 0;


    int changes = 0;
    while ((rv = select(mfd+1, &rfds, NULL, NULL, &tv)) >= 0) {
        if (FD_ISSET(mfd, &rfds)) {
            fprintf(stdout, "Mount points changed. %d.\n", changes++);
        }

        FD_ZERO(&rfds);
        FD_SET(mfd, &rfds);
        tv.tv_sec = 5;
        tv.tv_usec = 0;

        if (changes > 10) {
            exit(EXIT_FAILURE);
        }
    }

    return 0;
}

strace output on the failing machine:

strace ./mount-mon-simple >/dev/null
execve("./mount-mon-simple", ["./mount-mon-simple"], [/* 43 vars */]) = 0
brk(0) = 0x9fc5000
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb775e000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=78022, ...}) = 0
mmap2(NULL, 78022, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb774a000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
open("/lib/tls/i686/cmov/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0000m\1\0004\0\0\0"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1405508, ...}) = 0
mmap2(NULL, 1415592, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x9c4000
mprotect(0xb17000, 4096, PROT_NONE) = 0
mmap2(0xb18000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x153) = 0xb18000
mmap2(0xb1b000, 10664, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xb1b000
close(3) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7749000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb77496c0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xb18000, 8192, PROT_READ) = 0
mprotect(0x8049000, 4096, PROT_READ) = 0
mprotect(0xeef000, 4096, PROT_READ) = 0
munmap(0xb774a000, 78022) = 0
open("/proc/mounts", O_RDONLY) = 3
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999994})
fstat64(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
ioctl(1, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbfb723d4) = -1 ENOTTY (Inappropriate ioctl for device)
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb775d000
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999994})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
select(4, [3], NULL, NULL, {5, 0}) = 1 (in [3], left {4, 999995})
write(1, "Mount points changed. 0.\nMount p"..., 276) = 276
exit_group(1) = ?