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Full Discussion: shmat() with fixed address
Top Forums Programming shmat() with fixed address Post 302477560 by DoxieLvr on Sunday 5th of December 2010 08:49:23 PM
Old 12-05-2010
The problem is that a valid address running one process may not be valid/available in another. In fact, an address that may be valid on one system may not be valid on another system. That's why it is recommended to use an automatic address assignment.

If all of the communicating processes are the same and fork from one program, you can use shmat() before forking and each process will automatically have the memory segment at the same address.

Otherwise, the best course of action is to not use absolute pointers but instead use some for of relative pointer: for example, an integer offset from the base of the segment. Unfortunately, this requires a bit of code to dereference these "pointers".
 

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explain_shmat_or_die(3) 				     Library Functions Manual					   explain_shmat_or_die(3)

NAME
explain_shmat_or_die - shared memory attach and report errors SYNOPSIS
#include <libexplain/shmat.h> void *explain_shmat_or_die(int shmid, const void *shmaddr, int shmflg); void *explain_shmat_on_error(int shmid, const void *shmaddr, int shmflg); DESCRIPTION
The explain_shmat_or_die function is used to call the shmat(2) system call. On failure an explanation will be printed to stderr, obtained from the explain_shmat(3) function, and then the process terminates by calling exit(EXIT_FAILURE). The explain_shmat_on_error function is used to call the shmat(2) system call. On failure an explanation will be printed to stderr, obtained from the explain_shmat(3) function, but still returns to the caller. shmid The shmid, exactly as to be passed to the shmat(2) system call. shmaddr The shmaddr, exactly as to be passed to the shmat(2) system call. shmflg The shmflg, exactly as to be passed to the shmat(2) system call. RETURN VALUE
The explain_shmat_or_die function only returns on success, see shmat(2) for more information. On failure, prints an explanation and exits, it does not return. The explain_shmat_on_error function always returns the value return by the wrapped shmat(2) system call. EXAMPLE
The explain_shmat_or_die function is intended to be used in a fashion similar to the following example: void *result = explain_shmat_or_die(shmid, shmaddr, shmflg); SEE ALSO
shmat(2) shared memory attach explain_shmat(3) explain shmat(2) errors exit(2) terminate the calling process COPYRIGHT
libexplain version 0.52 Copyright (C) 2011 Peter Miller explain_shmat_or_die(3)
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