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# 1  
Old 10-24-2001
Shared memory

Dear Reader,

Is is necessary to attach / dettach the shared memory segments for write operations , if more than one program is accessing same shared memory segments..
I have used semaphore mutex and still I'm getting segmentation fault when I write to the segment when other program is already attached to it... Is something else wrong..

Thanks,
# 2  
Old 10-24-2001
Only one process (at a time) may have an open file descriptor for a shared memory segment. The interprocess communication that is used to insure proper file access is the semaphore.

When a semaphore is used correctly (and it can be tricky!) the problem with shared memory segmentation error will be solved (more than likely, unless you have a pointer or other C programming error).

Here is a basic algorithm:
[list=1][*]Create a shared memory segment.[*]Create a semaphore.[*]Process A acquires the semaphore used for the shared memory segment.[*]Process A opens the shared memory.[*]Process A does cool things with shared memory.[*]Process A closed the shared memory segment.[*]Process A releases the semaphore.[*]Process B has been waiting for the semaphore to be released.[*]Now that semaphore has been released......[*]Process B acquires the semaphore used for the shared memory segment.[*]Process B opens the shared memory.[*]Process B does cool things with shared memory.[*]Process B closed the shared memory segment.[*]Process B releases the semaphore.[/list=1]

Yadda, yadda, yadda....... does this help?

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