shmctl Subroutine

Purpose

Controls shared memory operations.

Library

Standard C Library (libc.a)

Syntax

#include <sys/shm.h>

int shmctl (SharedMemoryID, Command, Buffer)
int  SharedMemoryID, Command;
struct shmid_ds * Buffer;

Description

The shmctl subroutine performs a variety of shared-memory control operations as specified by the Command parameter.

The following limits apply to shared memory:

Parameters

Item Description
SharedMemoryID Specifies an identifier returned by the shmget subroutine.
Buffer Indicates a pointer to the shmid_ds structure. The shmid_ds structure is defined in the sys/shm.h file.
Command The following commands are available:
IPC_STAT
Obtains status information about the shared memory segment identified by the SharedMemoryID parameter. This information is stored in the area pointed to by the Buffer parameter. The calling process must have read permission to run this command. The shm_pagesize and shm_lba fields of the shmid_ds data structure pointed to by the Buffer parameter are not updated by this command.
IPC_ SET
Sets the user and group IDs of the owner as well as the access permissions for the shared memory segment identified by the SharedMemoryID parameter. This command sets the following fields:
shm_perm.uid  /* owning user ID       */
shm_perm.gid  /* owning group ID      */
shm_perm.mode /* permission bits only */

You must have an effective user ID equal to root or to the value of the shm_perm.cuid or shm_perm.uid field in the shmid_ds data structure identified by the SharedMemoryID parameter.

IPC_RMID
Removes the shared memory identifier specified by the SharedMemoryID parameter from the system and erases the shared memory segment and data structure associated with it. This command is only executed by a process that has an effective user ID equal either to that of superuser or to the value of the shm_perm.uid or shm_perm.cuid field in the data structure identified by the SharedMemoryID parameter.
SHM_SIZE
Sets the size of the shared memory segment to the value specified by the shm_segsz field of the structure specified by the Buffer parameter. This value can be larger or smaller than the current size. The limit is the maximum shared-memory segment size. This command is only executed by a process that has an effective user ID equal either to that of a process with the appropriate privileges or to the value of the shm_perm.uid or shm_perm.cuid field in the data structure identified by the SharedMemoryID parameter. This command is not supported for regions created with the environment variable EXTSHM=ON. This results in a return value of -1 with errno set to EINVAL. Attempting to use the SHM_SIZE on a shared memory region larger than 256MB or attempting to increase the size of a shared memory region larger than 256MB results in a return value of -1 with errno set to EINVAL.
 
SHM_BSR
Backs the shared memory region identified by the SharedMemoryID parameter with barrier synchronization register (BSR) memory. BSR shared memory can be used for efficiently implementing barrier synchronization constructs that are commonly used in highly parallel workloads. The Buffer parameter must be set to NULL when using this command. This command can only be used by a process that has an effective user ID equal to that of superuser or to the value of the shm_perm.uid or shm_perm.cuid fields in the shmid_ds data structure identified by the SharedMemoryIDparameter. A non-root user must have the CAP_BYPASS_RAC_VMM capability in order to allocate BSR memory and PV_KER_RAC privilege if using RBAC. If insufficient BSR memory is available to satisfy the request, shmctl() will fail with errno set to ENOMEM. In order to use BSR memory for a shared memory region, this command must be used on the shared memory region immediately after it has been created and before any process has attached to the shared memory region. This command cannot be used with shared memory regions that have been created with the SHM_PIN flag or shared memory regions that have been locked with the SHM_LOCK shmctl() command. This command also cannot be used on shared memory regions whose page size has been changed with the SHM_PAGESIZE shmctl() command, as well as shared memory regions created with the EXTSHM=ON environment variable.
 
SHM_PAGESIZE
Sets the page size backing the shared memory segment identified by the SharedMemoryID parameter. This command will set the page size backing the specified shared memory segment to the value of the shm_pagesize field of the shmid_ds structure specified by the Buffer parameter. The shm_pagesize field is interpreted as a page size in bytes. This command can only be used by a process that has an effective user ID with permissions set equal either to that of superuser or to the value of the shm_perm.uid or shm_perm.cuid field in the shmid_ds data structure identified by the SharedMemoryID parameter. In order to change the page size backing a shared memory segment, this command must be used on the shared memory segment immediately after it has been created and before any process has attached to the shared memory segment. Also, this command must be used before pinning the pages in a shared memory segment. Thus, this command cannot be used with shared memory segments that have been created with the SHM_PIN flag or shared memory segments that have been pinned with the SHM_LOCK shmctl() command. This command cannot be used with shared memory regions created with the EXTSHM=ON environment variable.
Note: A system's supported page sizes can be queried by specifying the VM_GETPSIZES command to the vmgetinfo() system call.
Command continued The following commands are available:
SHM_LOCK
Pins all of the pages in the shared memory segment identified by the SharedMemoryID parameter. Pinning the pages in a shared memory segment will ensure that page faults do not occur for memory references to the shared memory region. This command can only be used by a process that has an effective user ID equal to that of superuser or to the value of the shm_perm.uid or shm_perm.cuid field in the shmid_ds data structure identified by the SharedMemoryID parameter. A non-superuser user must also have the CAP_BYPASS_RAC_VMM capability in order to use this command. This command cannot be used with shared memory regions created with the EXTSHM=ON environment variable or shared memory regions created with the SHM_PIN flag. The Buffer parameter must be set to NULL when using this command.
 
SHM_UNLOCK
Unpins all of the pages in the shared memory segment identified by the SharedMemoryID parameter. This command can only be used by a process that has an effective user ID equal either to that of superuser or to the value of the shm_perm.uid or shm_perm.cuid field in the shmid_ds data structure identified by the SharedMemoryID parameter. This command will fail if called on shared memory segments created with the SHM_PIN flag. Also, this command can only be used when the specified shared memory segment is not attached by any process, and there is no outstanding I/O to the shared memory segment. The Buffer parameter must be set to NULL when using this command.
SHM_GETLBA
Obtains the minimum alignment of the address at which the shared memory segment identified by the SharedMemoryID parameter can be attached by the shmat() subroutine. This command will store the minimum alignment in the shm_lba field of the shmid_ds struct pointed to by the Buffer parameter. The alignment is reported in bytes. The calling process must have read permission to a shared memory region in order to use this command.

Return Values

When completed successfully, the shmctl subroutine returns a value of 0. Otherwise, it returns a value of -1 and the errno global variable is set to indicate the error.

Error Codes

The shmctl subroutine is unsuccessful if one or more of the following are true:

Item Description
EACCES The Command parameter is equal to the IPC_STAT or SHM_GETLBA value and read permission is denied to the calling process.
EFAULT The Buffer parameter points to a location outside the allocated address space of the process.
EINVAL The SharedMemoryID parameter is not a valid shared memory identifier.
EINVAL The Command parameter is not a valid command.
EINVAL The Command parameter is equal to the SHM_SIZE value and the value of the shm_segsz field of the structure specified by the Buffer parameter is not valid.
EINVAL The Command parameter is equal to the SHM_SIZE, SHM_PAGESIZE, SHM_LOCK or SHM_BSR value and the shared memory region was created with the environment variable EXTSHM=ON.
EINVAL The Command parameter is equal to the SHM_PAGESIZE value and the value of the shm_pagesize field of the structure specified by the Buffer parameter is not supported.
EINVAL The Command parameter is equal to SHM_UNLOCK, and the specified shared memory segment was not previously locked by a SHM_LOCK operation.
EINVAL The Command parameter is equal to SHM_LOCK SHM_UNLOCK, or SHM_BSR and the Buffer parameter is not NULL.
EINVAL The Command parameter is equal to SHM_BSR, and the shared memory region’s page size has previously been changed via the SHM_PAGESIZE command.
ENOMEM The Command parameter is SHM_BSR, and there is insufficient BSR memory available to back the entire shared memory segment.
ENOMEM The Command parameter is equal to the SHM_SIZE value, and the attempt to change the segment size is unsuccessful because the system does not have enough memory.
ENOMEM The Command parameter is SHM_LOCK, and locking the pages in the specified shared memory segment would exceed the limit on the amount of memory the calling process may lock.
ENOMEM The Command parameter is SHM_PAGESIZE, and there are insufficient pages of the specified page size to back the entire shared memory segment.
EOVERFLOW The Command parameter is IPC_STAT and the size of the shared memory region is greater than or equal to 4G bytes. This only happens with 32-bit programs.
EPERM The Command parameter is IPC_RMID, SHM_SIZE, SHM_PAGESIZE, SHM_LOCK, or SHM_UNLOCK, and the effective user ID of the calling process is not equal to the value of the shm_perm.uid or shm_perm.cuid field in the data structure identified by the SharedMemoryID parameter. The effective user ID of the calling process is not the root user ID.
EPERM The Command parameter is SHM_PAGESIZE, and the calling process does not have the appropriate privilege to allocate pages of the specified page size.
EPERM The Command parameter is SHM_LOCK SHM_UNLOCK, or SHM_BSR and the calling process does not have the appropriate privilege to perform the requested operation.
EBUSY The Command parameter is SHM_LOCK or SHM_UNLOCK, and the specified shared memory segment is currently being used for I/O or is attached by one or more processes.
EBUSY The Command parameter is SHM_PAGESIZE or SHM_BSR and the specified shared memory segment has already been attached by one or more processes or has been pinned via SHM_PIN or SHM_LOCK.

Examples

The following example allocates a 32MB shared memory region, changes the page size for the shared memory region to 64K, and then pins all of the pages in the shared memory region:

int    id;
size_t shm_size;
struct shmid_ds shm_buf = { 0 };
psize_t psize_64k;

psize_64k = 64 * 1024;

/* Create a 32MB shared memory region */
shm_size = 32*1024*1024;

/* Allocate the shared memory region */
if ((id = shmget(IPC_PRIVATE, shm_size, IPC_CREAT)) < 0)
{
	perror("shmget() failed");
	return -1;
}

/* Use 64K pages for the shared memory region */
shm_buf.shm_pagesize = psize_64k;
if (shmctl(id, SHM_PAGESIZE, &shm_buf))
{
	perror("shmctl(SHM_PAGESIZE) failed");
}

/* Pin all of the pages in the shared memory region */
if (shmctl(id, SHM_LOCK, NULL))
{
	perror("shmctl(SHM_LOCK) failed");
}

The following example allocates a 16MB shared memory region and determines the minimum alignment of the address at which an application can shmat() the shared memory region:

int    id;
size_t shm_size;
struct shmid_ds shm_buf = { 0 };

/* Create a 16MB shared memory region */
shm_size = 16*1024*1024;

/* Allocate the shared memory region */
if ((id = shmget(IPC_PRIVATE, shm_size, IPC_CREAT)) < 0)
{
	perror("shmget() failed");
	return -1;
}

/* Determine the address alignment requirements */
if (shmctl(id, SHM_GETLBA, &shm_buf))
{
	perror("shmctl(SHM_GETLBA) failed");
}
else
{
	printf("shmlba = %08llx\n", shm_buf.shm_lba);
}