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 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. 

The segment is attached for reading if (shmflg & SHM_RDONLY ) is "true", otherwise it is attached for reading and writing. 

The shmdt system call detaches from the calling process’s data segment the shared memory segment located at the address specified by shmaddr. 

RETURN VALUE

Upon successful completion, the return values are as follows:

The shmat system call returns the data segment start address of the attached shared memory segment.

The shmdt system call returns a value of 0.

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

DIAGNOSTICS

The shmat system call will fail and not attach the shared memory segment if one or more of the following are true:

[EINVAL] The shmid is not a valid shared memory identifier. 

[EACCES] Operation permission is denied to the calling process.  For further information, see errno(.).

[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 shmdt will fail and 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)

System Calls