HP-UX Reference (11i v1 05/09) - 2 System Calls (vol 5)

s
shmget(2) shmget(2)
NAME
shmget - get shared memory segment
SYNOPSIS
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTION
shmget() returns the shared memory identifier associated with key.
A shared memory identifier and associated data structure and shared memory segment of size size bytes
(see glossary (9)) are created for key if one of the following is true:
• key is equal to
IPC_PRIVATE . This call creates a new identifier, subject to available resources.
The identifier will never be returned by another call to
shmget() until it has been released by a
call to
shmctl(). The identifier should be used among the calling process and its descendents;
however, it is not a requirement. The resource can be accessed by any process having the proper
permissions.
• key does not already have a shared memory identifier associated with it, and (shmflg &
IPC_CREAT) is ‘‘true’’. If IPC_CREAT is set in shmflg the shared memory segment created can
only be shared by processes of the same executable type. That is, an application compiled as a 32-
bit process will be able to share the same memory segment with other 32-bit processes, and an
application compiled as a 64-bit process will be able to share the same memory segment with other
64-bit processes. If a 64-bit bit process wants to create a shared memory segment which can also be
shared with 32-bit processes, the 64-bit process must specify
IPC_SHARE32 in addition to
IPC_CREAT in shmflg. The 32-bit process does not need to specify IPC_SHARE32 .
On HP Scalable Computing Architecture (SCA) systems with multiple locality domains, specifying
IPC_MEM_LOCAL in shmflag targets the shared memory to the locality on which the allocating thread is
executing at the time of the call. This overrides the default allocation policy of "first touch", in which
memory is allocated from the locality domain on which the thread is executing at the time of the fault.
When
IPC_MEM_LOCAL is specified:
• Should there not be enough memory available in the specified locality domain to satisfy the
request, memory is allocated from the domain on which the thread is currently executing.
• On systems with a single locality domain, the call has no effect.
Upon creation, the data structure associated with the new shared memory identifier is initialized as follows:
•
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, respectively, of the calling process.
•
shm_perm.cuid, the low-order 9 bits of
shm_perm.mode are set equal to the low-order 9 bits
of shmflg.
shm_segsz is set equal to the value of size.
• shm_lpid, shm_nattch , shm_atime, and shm_dtime are set equal to 0.
• shm_ctime is set equal to the current time.
EXAMPLES
The following call to shmget() returns a unique shmid for the newly created shared memory segment of
4096 bytes:
int myshmid;
myshmid = shmget (IPC_PRIVATE, 4096, 0600);
RETURN VALUE
Upon successful completion, a non-negative integer, namely a shared memory identifier is returned. Other-
wise, a value of −1 is returned and errno is set to indicate the error.
ERRORS
shmget() fails if any of the following conditions are encountered:
[EINVAL] size is less than the system-imposed minimum or greater than the system-imposed
maximum.
HP-UX 11i Version 1: September 2005 − 1 − Hewlett-Packard Company Section 2−−349