shmget(2)
Name
shmget − get shared memory segment
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget (key, size, shmflg)
key_t key;
int size, shmflg;
Description
The shmget system call returns the shared memory identifier associated with key. A shared memory identifier and associated data structure and shared memory segment of size size bytes are created for key, if one of the following is true:
The key is equal to IPC_PRIVATE. For further information, see intro(.).
The key does not already have a shared memory identifier associated with it, and (shmflg & IPC_CREAT ) is true.
Upon creation, the data structure associated with the new shared memory identifier is initialized as follows:
The shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and shm_perm.gid are set equal to the effective user ID and effective group ID of the calling process.
The low-order nine bits of shm_perm.mode are set equal to the low-order nine bits of shmflg. The shm_segsz is set equal to the value of size.
The shm_lpid, shm_nattch, shm_atime, and shm_dtime are set equal to zero (0). The shm_ctime is set equal to the current time.
Return Values
Upon successful completion, a non-negative integer, namely, a shared memory identifier is returned. Otherwise, a value of −1 is returned and errno is set to indicated the error.
Diagnostics
The shmget system call fails if any of the following is true:
[EINVAL] The size is less than the system-imposed minimum or greater than the system-imposed maximum.
[EACCES] A shared memory identifier exists for key, but operations permission, as specified by the low-order nine bits of shmflg, would not be granted. For further information, see errno(.).
[EINVAL] A shared memory identifier exists for key, but the size of the segment associated with it is less than size and size is not equal to zero.
[ENOENT] A shared memory identifier does not exist for key, and (shmflg & IPC_CREAT ) is false.
[ENOSPC] A shared memory identifier is to be created, but the system-imposed limit on the maximum number of allowed shared memory identifiers would be exceeded.
[ENOMEM] A shared memory identifier and the associated shared memory segment are to be created, but the amount of available physical memory is not sufficient to fill the request.
[EEXIST] A shared memory identifier exists for key, but ((shmflg & IPC_CREAT ) and (shmflg & IPC_EXCL )) is true.