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copied!<p>Note that you've provided a mapping for the file that is exactly the size of the file. If you create the file in your call to <code>open(2)</code>, it will have a length of <code>0</code>, and I wouldn't be surprised if the kernel doesn't bother setting up any kind of memory mapping from a <code>0</code> length mapping. (Maybe it does? I've never tried...)</p> <p>I would suggest using <code>ftruncate(2)</code> to extend the length of your file <em>before</em> performing the mapping. (Note that extending files using <code>ftruncate(2)</code> isn't very portable; not all platforms provide extending functionality and not all filesystem drivers support the extending functionality. See your system's manpage for details.)</p> <p>You must use the <code>MAP_SHARED</code> mapping for your file modifications to be saved to disk.</p> <p>Your use of <code>perror(3)</code> isn't quite correct; <code>perror(3)</code> will <em>not</em> terminate your program, so it will continue executing with incorrect assumptions:</p> <pre><code>if((ret=fstat(fd,&st)) < 0) perror("Error in fstat"); </code></pre> <p>Should read:</p> <pre><code>if((ret=fstat(fd,&st)) < 0) { perror("Error in fstat"); exit(1); } </code></pre> <p>(Or <code>exit(EXIT_FAILURE)</code> if you want to be more portable -- I find that a little harder on the eyes but I live in Linux-land.)</p> <p><code>strcat(3)</code> expects to find an ASCII NUL character (byte value <code>0x00</code>, C representation <code>'\0'</code>) -- the usual C end-of-string marker -- at the end of the <code>dest</code> string. Your file will not contain an ASCII NUL if you create it in this program -- its length is zero, after all -- and I don't know the consequences of trying to read a zero-byte file via <code>mmap(2)</code>. If the file already exists and has data in it, it <em>probably</em> doesn't have an ASCII NUL encoded in the file. <code>strcat(3)</code> is almost certainly the wrong tool to write into your file. (No one wants ASCII NULs in their files anyway.) Try <code>memcpy(3)</code> instead.</p>

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