Manual browser: shmget(2)
|SHMGET(2)||System Calls Manual||SHMGET(2)|
NAMEshmget — get shared memory segment
LIBRARYStandard C Library (libc, -lc)
shmget(key_t key, size_t size, int shmflg);
DESCRIPTIONshmget() returns the shared memory identifier associated with the key key.
A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated with it, and the IPC_CREAT bit is set in shmflg. If both the IPC_CREAT bit and the IPC_EXCL bit are set in shmflg, and key has a shared memory segment identifier associated with it already, the operation will fail.
If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as follows:
- shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process.
- shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process.
- shm_perm.mode is set to the lower 9 bits of shmflg.
- shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
- shm_ctime is set to the current time.
- shm_segsz is set to the value of size.
RETURN VALUESUpon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.
- A shared memory segment is already associated with key and the caller has no permission to access it.
- Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.
No shared memory segment is to be created, and a shared memory segment exists for key, but the size of the segment associated with it is less than size, which is non-zero.
A shared memory segment is to be created, and size is less than the system imposed minimum, or greater than the system imposed maximum (refer to the
kern.ipcvalues in sysctl(7)).
- IPC_CREAT is not set in shmflg and no shared memory segment associated with key was found.
- There is not enough memory left to create a shared memory segment of the requested size.
- A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers has been reached.
STANDARDSThe shmget system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).
HISTORYShared memory segments appeared in the first release of AT&T System V UNIX.
|October 27, 2008||NetBSD 7.0|