shmget - get shared memory segment
Standard C Library (libc, -lc)
shmget(key_t key, size_t size, int shmflg);
shmget() returns the shared memory identifier associated with the key
key. The shmflg should be a combination of the following flags:
IPC_CREAT create if segment does not exist
IPC_EXCL fail if segment already exists
IPC_NOWAIT return error instead of waiting
and nine bits of access bits, based on:
IPC_R read permission
IPC_W write permission
IPC_M changing control info permission
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 a new shared memory segment is created, the data structure associated
with it (the shmid_ds structure, see shmctl(2)) is initialized as follows:
+o shm_perm.cuid and shm_perm.uid are set to the effective uid of the
+o shm_perm.gid and shm_perm.cgid are set to the effective gid of the
+o shm_perm.mode is set to the lower 9 bits of shmflg.
+o shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.
+o shm_ctime is set to the current time.
+o shm_segsz is set to the value of size.
Upon 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.
[EACCES] A shared memory segment is already associated with key
and the caller has no permission to access it.
[EEXIST] Both IPC_CREAT and IPC_EXCL are set in shmflg, and a
shared memory segment is already associated with key.
[ENOSPC] A new shared memory identifier could not be created
because the system limit for the number of shared memory
identifiers has been reached.
[ENOENT] IPC_CREAT is not set in shmflg and no shared memory
segment associated with key was found.
[ENOMEM] There is not enough memory left to create a shared
memory segment of the requested size.
mmap(2), shmat(2), shmctl(2), ftok(3)
The shmget system call conforms to X/Open System Interfaces and Headers
Issue 5 (``XSH5'').
Shared memory segments appeared in the first release of AT&T System V
BSD August 25, 1999 BSD
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