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    <p>This works for me. Note that I had to open the destination O_RDWR. I suspect the kernel attempts to map whole pages from the file into memory (reading it) because you're updating it a byte or word at a time, and that might not change the whole page.</p> <p>A couple of other points:</p> <ol> <li><p>You don't need to close and unmap stuff on error if you're just going to exit.</p></li> <li><p>Use memcpy and don't write your own byte-copying loop. Memcpy will be a lot better optimised in general. (Though it's not always the absolute best.)</p></li> <li><p>You might want to read the source code to FreeBSD's "cp" utility. Take a look here and search for the use of mmap. <a href="http://svnweb.freebsd.org/base/stable/9/bin/cp/utils.c?revision=225736&amp;view=markup" rel="nofollow">http://svnweb.freebsd.org/base/stable/9/bin/cp/utils.c?revision=225736&amp;view=markup</a></p></li> </ol> <hr> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;fcntl.h&gt; #include &lt;unistd.h&gt; #include &lt;sys/mman.h&gt; #include &lt;string.h&gt; #include &lt;sys/stat.h&gt; int main(int argc, char *argv[]) { int s, d; struct stat st; void *sp, *dp; s = open(argv[1], O_RDONLY); if (s == -1) { perror("open source"); exit(1); } d = open(argv[2], O_RDWR | O_CREAT | O_TRUNC, 0644); if (d == -1) { perror("open destintation"); exit(1); } if (fstat(s, &amp;st)) { perror("stat source"); exit(1); } if (ftruncate(d, st.st_size)) { perror("truncate destination"); exit(1); } sp = mmap(NULL, st.st_size, PROT_READ, MAP_SHARED, s, 0); if (sp == MAP_FAILED) { perror("map source"); exit(1); } dp = mmap(NULL, st.st_size, PROT_WRITE | PROT_READ, MAP_SHARED, d, 0); if (dp == MAP_FAILED) { perror("map destintation"); exit(1); } memcpy(dp, sp, st.st_size); return 0; } </code></pre>
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    1. POWhy does mmap() fail with permission denied for the destination file of a file copy program?
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    1. USrptb1
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    1. COThank you for the answer! I was always forced to unload stuff, free memory and all even before the program was to terminate but I always thought that the OS should de-allocate and close all resources a program was using upon completion. I'll certainly look at that link you posted to learn more about the matter.
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      1. USJames Russell
    2. CO@JamesRussell: Performing those cleanups is unnecessary from a correctness point of view, but it does help sanitize the code when using a memory debugger.
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      1. USjxh
    3. COYou can be sure that a Unix-like system will clean up most resources when your process exits (or is killed). The exceptions are mostly things with names in the filesystem (files, devices, named sockets, etc.)
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