shmop(S)

shmop: shmat, shmdt -- shared memory operations

Syntax

cc . . . -lc

#include  <sys/shm.h>

void *shmat(int shmid, const void *shmaddr, int shmflg);

int shmdt(const void *shmaddr);

Description

shmat- attaches shared memory segment

shmdt- detaches shared memory segment

The shmat 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.

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}.

shmat fails and does not attach the shared memory segment if one or more of the following is true:

[EACCES]: Operation permission is denied to the calling process (see Intro(S)).
[EINVAL]: shmid is not a valid shared memory identifier.
[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.
[EINVAL]: shmdt fails and does not detach the shared memory segment if shmaddr is not the data segment start address of a shared memory segment.
[EMFILE]: The number of shared memory segments attached to the calling process would exceed the system-imposed limit.
[ENOMEM]: The available data space is not large enough to accommodate the shared memory segment.

Diagnostics

Upon successful completion, the return value is as follows:

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

Otherwise, a value of -1 is returned, and errno is set to indicate the error.

Notes

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

Standards conformance

shmat and shmdt are conformant with: