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POHow does Linux Kernel assigns memory pointers when a process uses shm_open()?
 

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  1. POHow does Linux Kernel assigns memory pointers when a process uses shm_open()?
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    <p>I'm on Linux 2.6 and I have a weird problem. I have 3 concurrent processes (forked from the same process) which need to obtain 3 DIFFERENT shared memory segments, one for each process. Each of the process executes this code (please note that 'message' type is user-defined)</p> <pre><code> message *m; int fd = shm_open("message", O_CREAT|O_RDWR, S_IRUSR|S_IWUSR); ftruncate(fd, sizeof(message)); m = mmap(NULL, sizeof(message), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0); char messagename[16]; snprintf(messagename, sizeof(messagename), "%p", m); char path[32] = "/dev/shm/"; strcat(path, messagename); rename("/dev/shm/message", path); </code></pre> <p>Let me explain a bit: I want every process to allocate a shared memory zone which contains a message. To make sure another process (the message receiver) can access the same shm, I then rename my shm file from "message" to a string named after the message pointer (this because the process which receives the message already knows the pointer).</p> <p>When executing the program, though, I tried to print (for debugging purpose) the pointers that every process received when mmapping the fd obtained with shm_open, and I noticed that all of them got the SAME pointer. How is it possible? I thought that maybe other processes did the shm_open() after the first one did and before it renamed the segment, so I also tried to make these lines of code an atomic operation by using a process shared mutex, but the problem persists.</p> <p>I would really appreciate any kind of help or suggestion.</p>
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