shmget

Returns a shared memory identifier.

Syntax

   #include <sys/types.h>

   #include <sys/ipc.h>

   #include <sys/shm.h>



   int shmget (key_t key, int size, int shmflg);

Parameters

key

Passes either a user-defined key value to rendezvous with the shared memory area, or IPC_PRIVATE. If IPC_PRIVATE is specified, a new shared memory area is created, but other processes cannot rendezvous by key. Refer to the description of ftok() for details about obtaining user-defined key values

size

Passes the size, in bytes, of the shared memory area. The maximum shared memory area size is 256 megabytes.

shmflg

Valid flags for this function are:

IPC_CREAT: If a shared memory area is not already associated with key, a new shared memory area identifier is allocated and a shared memory area and data structure are associated with it. If a shared memory area is already associated with key, shmctl() returns the shared memory identifier associated with key.
IPC_EXCL: If specified with IPC_CREAT, shmget() returns an error if a shared memory identifier is already associated with key.
MODE: The lower nine bits of shmflg contain the access permission bits (similar to the nine bit mask found in file entries). They define access permissions for the owner, the group, and other users on the system.
SHM_NO_PID: Allocate a shared memory area without PID protection. (Refer to "Implementation Considerations" for more information about using this flag.)
SHM_PRIV_ACCESS: Allocate a shared memory area accessible only to a calling process that has MPE/iX user privileged mode (PM). (Refer to "Implementation Considerations" for more information about using this flag.)

Return Values

>=0: Success. A shared memory area identifier is returned.
-1: An error occurred, and errno is set to indicate the error condition.

Description

The shmget() function returns a shared memory identifier associated with the value passed in key. A new shared memory identifier is allocated and an associated data structure and shared memory area of size bytes are associated with it if:

The value passed in key is equal to IPC_PRIVATE.
The value passed in key does not already have a shared memory identifier associated with it, and shmflg specifies IPC_CREAT.

The data structure associated with the new shared memory identifier is initialized to the following values:

shm_perm.cuid: Effective user ID of the calling process (creator user ID)
shm_perm.uid: Effective user ID of the calling process (owner user ID)
shm_perm.cgid: Effective group ID of the calling process (creator group ID)
shm_perm.gid: Effective group ID of the calling process (owner group ID)
shm_perm.mode: Low-order 9 bits are set equal to the low-order 9 bits of shmflg
shm_segsz: Value passed in size
msg_qnum: Zero
shm_lpid: Zero
shm_nattch: Zero
shm_atime: Zero
shm_dtime: Zero
shm_ctime: Current time

Implementation Considerations

The maximum shared memory area size is 256 megabytes.

On MPE/iX, two flags, SHM_NO_PID and SHM_PRIV_ACCESS are available to the shmget() function that are not defined by SVID. (Refer to the description of shmflg above.)




	NOTE: The `SHM_NO_PID` and `SHM_PRIV_ACCESS` flags are available only on 900 Series HP 3000 computer systems. These two flags are not recommended for portable applications. Specifying these flags on a different computer system may produce unpredictable results. In addition, setting `SHM_NO_PID` increases the risk of data corruption, since the shared memory area will not be protected by normal MPE/iX data memory protection traps.

An MPE/iX system manager can use the MPE/iX SVIPC utility to interactively configure:

The minimum and maximum size of the shared memory area
The total number of shared memory areas allowed system wide.

Refer to the section "Managing SVID IPC Services" for more information.

Errors

If an error occurs, errno is set to one of the following values:

`EACCES`	CAUSE	A shared memory identifier exists for `key` but the calling process does not have permission (as specified by the low-order 9 bits of `shmflg`).
∅	ACTION	Ensure that the calling process has appropriate permissions to obtain access to the existing shared memory identifier.
`EEXIST`	CAUSE	A shared memory identifier exists for `key` and `shmflg` specifies both `IPC_CREATE` and `IPC_EXCL`.
∅	ACTION	To access the existing shared memory identifier, remove the `IPC_EXCL` option. Otherwise, a unique `key` value must be specified for a new shared memory identifier to be created.
`EINVAL`	CAUSE	`size` is either less than the system-defined minimum or greater than the system-defined maximum, or a shared memory identifier exists for `key` and the size of the shared memory area associated with it is less than `size` and `size` is not equal to zero.
∅	ACTION	Check to see that `size` is within the system-defined valid range, and that if a shared memory identifier already exists for `key`, that `size` is within that shared memory area's valid range.
`ENOENT`	CAUSE	A shared memory identifier does not exist for `key` and `shmflg` does not specify `IPC_CREATE`.
∅	ACTION	To create a shared memory area for `key` when one does not already exist, make sure `IPC_CREAT` is specified.
`ENOMEM`	CAUSE	The available data space is not large enough to create a shared memory identifier and associated shared memory area.
∅	ACTION	None. The operation can be retried if another shared memory identifier is removed from the system.
`ENOSPC`	CAUSE	The number of shared memory identifiers would exceed the system-defined limit.
∅	ACTION	None. The operation can be retried if another shared memory identifier is removed from the system.
`ESYSERR`	CAUSE	An operating system error occurred that does not map directly to any of the above errors.
∅	ACTION	Examine the MPE/iX process error stack for the type of system error.

shmget

Technical documentation

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