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POMonitoring mount point changes via /proc/mounts
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  1. POMonitoring mount point changes via /proc/mounts
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    copied!<p>According proc manual, one can monitor for mount point changes in linux system by opening "/proc/mounts", and adding the file descriptor to read <code>fd_set</code> in <code>select()</code> call.</p> <p>Following piece of code works on Ubuntu 9.04, and not in Ubuntu 10.04 (with 2.6.32 linux kernel):</p> <pre><code>int mfd = open("/proc/mounts", O_RDONLY, 0); fd_set rfds; struct timeval tv; int rv; FD_ZERO(&amp;rfds); FD_SET(mfd, &amp;rfds); tv.tv_sec = 5; tv.tv_usec = 0; int changes = 0; while ((rv = select(mfd+1, &amp;rfds, NULL, NULL, &amp;tv)) &gt;= 0) { if (FD_ISSET(mfd, &amp;rfds)) { fprintf(stdout, "Mount points changed. %d.\n", changes++); } FD_ZERO(&amp;rfds); FD_SET(mfd, &amp;rfds); tv.tv_sec = 5; tv.tv_usec = 0; if (changes &gt; 10) { exit(EXIT_FAILURE); } } </code></pre> <p><a href="https://gist.github.com/837605" rel="noreferrer">Compilable snippet.</a></p> <p>The file descriptor is always readable in one machine, and hence it keeps popping up in the select call. Even there are no changes in mounts.</p> <p>Am I missing something here?</p> <p>Thanks in advance for any help!</p> <p><a href="http://linux.die.net/man/5/proc" rel="noreferrer">man 5 proc:</a></p> <p>/proc/[pid]/mounts (since Linux 2.4.19)</p> <p>This is a list of all the file systems currently mounted in the process's mount namespace. The format of this file is documented in fstab(5). Since kernel version 2.6.15, this file is pollable: after opening the file for reading, a change in this file (i.e., a file system mount or unmount) causes select(2) to mark the file descriptor as readable, and poll(2) and epoll_wait(2) mark the file as having an error condition.</p>
 

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