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    <p>Because you're using <code>shm_unlink()</code> immediately before <code>shm_open()</code>, your two processes never open the same shared memory object - each one is creating a new, distinct object (even though they have the same name).</p> <hr> <p>It's not clear what you mean by "doesn't work". The following minimal example, based on your code, works fine for me. What does it do for you?</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;sys/mman.h&gt; #include &lt;sys/types.h&gt; #include &lt;fcntl.h&gt; #include &lt;unistd.h&gt; #include &lt;semaphore.h&gt; typedef struct container { sem_t resource, mutex; int counter; } container; int main() { container *memory; int fd = shm_open("MYSHM", O_RDWR|O_CREAT|O_EXCL, 0); if(fd == -1) { perror("shm_open"); return 1; } ftruncate(fd, sizeof(container)); memory = mmap(NULL, sizeof(container), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); memory-&gt;counter = 5; printf("%d\n", memory-&gt;counter); return 0; } </code></pre> <hr> <p>On reviewing your code, it's failing because you are trying to access the file descriptor in a child process after <code>exec()</code>, and <code>shm_open</code> sets the <code>FD_CLOEXEC</code> flag by default so the file descriptor is no longer open in the child. So you just need to unset that flag in the main process (eg. right after checking for the error after <code>shm_open</code>):</p> <pre><code>fdflags = fcntl(fd, F_GETFD); fdflags &amp;= ~FD_CLOEXEC; fcntl(fd, F_SETFD, fdflags); </code></pre>
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    1. COthat's not it; memory->counter = 5; does not work even if use it only in main app.
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    2. CO@blid..pl: The `ftruncate()` call should also be before the `mmap()` call.
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    3. CO@caf: ftruncate() has apparently nothing to do with my problem.
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