shmop(2)

shmop: shmat, shmdt -- shared memory operations

Synopsis

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/shm.h>
   
   void shmat(int shmid, const void shmaddr, int shmflg);
   

   int shmdt(const void shmaddr);

Description

shmat 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 (void *) 0, the segment is attached at the first available address as selected by the system.
If shmaddr is not equal to (void *) 0 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 (void *) 0 and (shmflg&SHM_RND) is false, the segment is attached at the address given by shmaddr.

shmdt detaches from the calling process's data segment the shared memory segment located at the address specified by shmaddr.

The segment is attached for reading if (shmflg&SHM_RDONLY) is true {READ}, otherwise it is attached for reading and writing {READ/WRITE}.

Return values

On success:

shmat returns the data segment start address of the attached shared memory segment.
shmdt returns 0.

On failure, shmat and shmdt return -1 and set errno to identify the error.

Errors

In the following conditions, shmat fails and sets errno to:

EINVAL: shmid is not a valid shared memory identifier.
EACCES: Operation permission is denied to the calling process [see intro(2)].
ENOMEM: The available data space is not large enough to accommodate the shared memory segment.
EINVAL: shmaddr is not equal to zero, and the value of (shmaddr - (shmaddr modulus SHMLBA)) is an illegal address.
EINVAL: 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.

In the following conditions, shmdt fails and sets errno to:

EINVAL: shmaddr is not the data segment start address of a shared memory segment.

References

exec(2), exit(2), fork(2), intro(2), shmctl(2), shmget(2)

Notices

The user must explicitly remove shared memory segments after the last reference to them has been removed.