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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shmop(2) System Calls Manual shmop(2)
Name
shmop, shmat, shmdt - shared memory operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
char *shmat (shmid, shmaddr, shmflg)
int shmid;
char *shmaddr;
int shmflg;
int shmdt (shmaddr)
char *shmaddr;
Description
The system call attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of
the calling process. The segment is attached at the address specified by one of the following criteria:
If shmaddr is equal to zero, the segment is attached at the first available address as selected by the system.
If shmaddr is not equal to zero and (shmflg & SHM_RND ) is true, the segment is attached at the address given by (shmaddr- (shmaddr modulus
SHMLBA )).
If shmaddr is not equal to zero and (shmflg & SHM_RND ) is false, the segment is attached at the address given by shmaddr.
The segment is attached for reading if (shmflg & SHM_RDONLY ) is true. Otherwise, it is attached for reading and writing.
The system call detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr.
Return Values
Upon successful completion, the return values are as follows:
o The system call returns the data segment start address of the attached shared memory segment.
o The system call returns a value of zero (0).
Otherwise, a value of -1 is returned, and errno is set to indicate the error.
Diagnostics
The system call fails and not attach the shared memory segment, if any of the following is true:
[EINVAL] The shmid is not a valid shared memory identifier.
[EACCES] Operation permission is denied to the calling process. For further information, see
[ENOMEM] The available data space is not large enough to accommodate the shared memory segment.
[EINVAL] The shmaddr is not equal to zero, and the value of (shmaddr- (shmaddr modulus SHMLBA )) is an illegal address.
[EINVAL] The shmaddr is not equal to zero, (shmflg & SHM_RND ) is false, and the value of shmaddr is an illegal address.
[EMFILE] The number of shared memory segments attached to the calling process would exceed the system imposed limit.
The fails and does not detach the shared memory segment if:
[EINVAL] The shmaddr is not the data segment start address of a shared memory segment.
See Also
execve(2), exit(2), fork(2), shmctl(2), shmget(2)
shmop(2)