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    <p>In addition of the sequence given in <a href="https://stackoverflow.com/a/17304744/841108">David Schwartz answer</a> you could perhaps use advisory locks with e.g. <a href="http://man7.org/linux/man-pages/man2/flock.2.html" rel="nofollow noreferrer">flock(2)</a> syscall (or maybe <a href="http://man7.org/linux/man-pages/man3/lockf.3.html" rel="nofollow noreferrer">lockf(3)</a> i.e. <a href="http://man7.org/linux/man-pages/man2/fcntl.2.html" rel="nofollow noreferrer">fcntl(2)</a> with <code>F_SETLK</code> ....)</p> <p>That would mean to add, just after</p> <pre><code> FILE * fil = fopen("/etc/routing_table.tmp", "wb"); </code></pre> <p>the lines</p> <pre><code> if (!fil) { perror("/etc/routing_table.tmp"); exit(EXIT_FAILURE); }; if (flock(fileno(fil), LOCK_EX)) { perror("flock LOCK_EX"); exit(EXIT_FAILURE); }; </code></pre> <p>and at the end, you would</p> <pre><code> if (fflush(fil)) /* flush the file before unlocking it!!*/ { perror("fflush"); exit(EXIT_FAILURE); }; if (flock(fileno(fil), LOCK_UN)) { perror("flock LOCK_UN"); exit(EXIT_FAILURE); }; if (fclose (fil)) { perror("fclose"); exit(EXIT_FAILURE); };; if (rename("/etc/routing_table.tmp", "/etc/routing_table")) { perror("rename"); exit(EXIT_FAILURE); }; </code></pre> <p>Using such advisory locking would ensure that even if two processes of your program are running, only one would write the file.</p> <p>But it is overkill probably.</p> <p>BTW, you seems to write binary data in <code>/etc/</code>. I believe it is against the habits or the conventions (see <a href="http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/Linux-Filesystem-Hierarchy.html" rel="nofollow noreferrer">Linux Filesystem Hierarchy</a>, or <a href="http://www.linuxfoundation.org/collaborate/workgroups/lsb" rel="nofollow noreferrer">Linux Standard Base</a>). I expect files under <code>/etc</code> to be textual. Perhaps you want your file under <code>/var/lib</code> ?</p> <p>See also <a href="http://advancedlinuxprogramming.com/" rel="nofollow noreferrer">Advanced Linux Programming</a> book online.</p>
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