shmctl(S)

Synopsis

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/shm.h>
   

   int shmctl(int shmid, int cmd, ... /* struct shmid_ds *buf */);

Description

IPC_STAT

Place the current value of each member of the data structure associated with shmid into the structure pointed to by buf. The contents of this structure are defined in intro(S). {READ}

IPC_SET

Set the value of the following members of the data structure associated with shmid to the corresponding value found in the structure pointed to by buf:

   shm_perm.uid
   shm_perm.gid
   shm_perm.mode / only access permission bits /

This command can be executed only by a process that has an effective user ID equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid, or by a process that has the P_OWNER privilege.

IPC_RMID

Remove the shared memory identifier specified by shmid from the system and destroy the shared memory segment and data structure associated with it. This command can be executed only by a process that has an effective user ID equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid, or by a process that has the P_OWNER privilege.

SHM_LOCK

Lock the shared memory segment specified by shmid in memory. This command can be executed only by a process that has the P_SYSOPS privilege.

SHM_UNLOCK

Unlock the shared memory segment specified by shmid. This command can be executed only by a process that has the P_SYSOPS privilege.

SHM_SETPLACE

Set placement information for part or all of the shared memory segment. The calling process must have the P_OWNER privilege to execute this command.

Details are provided in the shmid_ds data structure, where the relevant fields are:

off_t shm_off

The offset, in bytes, from the start of the shared memory segment of the portion to which the new policy should apply. It should be a multiple of the page size.

size_t shm_nbytes

The length of the portion of shared memory segment to which the new policy should apply. The system will round up shm_nbytes to a multiple of the page size.

If shm_off and shm_nbytes are both set to zero, the policy applies to the entire shared memory segment.

int shm_placepolicy

Controls the placement of the physical memory used by the shared memory segment. It overrides any previous placement policy set either when the shared memory segment was created (see shmget(S)) or by a previous call to shmctl.

The following are the possible values for int shm_placepolicy:

SHM_BALANCED

Indicates that the specified portion of shared memory will be accessed read/write, or readonly, but that replication at each CPU-group in a partitioned system is not desired. It is further expected that the portion of shared memory will be accessed from many CPU-groups, so it is appropriate to allocate physical memory so as to spread it evenly across these CPU-groups.

The granularity of the balanced allocations is given by shm_granularity. This will be rounded up if necessary to an integral multiple of the page size.

If shm_granularity is 0, the system default granularity should apply

The balanced allocation will be for future memory allocations only.

SHM_BALANCED|SHM_MIGRATE

The same as SHM_BALANCED, but existing allocations should be modified to match the new placement policy.

SHM_CPUGROUP

Specifies that the physical memory for the required portion of the shared memory segment should be allocated from the particular CPU-Group specified in shm_cpugroup.

A value of NOCGID for shm_cpugroup specifies that the CPU-Group where the caller is currently executing should be used for the allocations.

The CPU-Group allocation will be for future memory allocations only.

SHM_CPUGROUP|SHM_MIGRATE

The same as SHM_CPUGROUP, but existing allocations should be modified to match the new placement policy.

SHM_FIRSTUSAGE

Specifies that the pages of physical memory for the shared memory segment are to be allocated on the CPU-Group where the thread that makes the initial access to the shared memory happens to be executing at the time.

SHM_FIRSTUSAGE|SHM_MIGRATE

The same as SHM_FIRSTUSAGE, but existing allocations should be modified to match the new placement policy. The physical memory pages should be allocated anew wherever they are accessed after the shmctl call.

SHM_PLC_DEFAULT

Specifies that system default policies apply for new allocations.

SHM_PLC_DEFAULT|SHM_MIGRATE

The same as SHM_PLC_DEFAULT, but existing allocations should be modified to match the new placement policy.

cgid_t shm_cg

Used if shm_placepolicy indicates that allocation from a particular CPU-Group is required. It specifies the required CPU-Group.

unsigned int shm_granularity

Used if shm_placepolicy indicates that blanced allocation across all CPU-Groups is required. It specifies the required allocation granularity.

Return values

On success, shmctl returns 0. On failure, shmctl returns -1 and sets errno to identify the error.

Errors

In the following conditions, shmctl fails and sets errno to:

EACCES

cmd is equal to IPC_STAT and {READ} operation permission is denied to the calling process (see intro(S)).

EINVAL

shmid is not a valid shared memory identifier.

EINVAL

cmd is not a valid command.

EINVAL

cmd is IPC_SET and shm_perm.uid or shm_perm.gid is not valid.

EINVAL

cmd is SHM_SETPLACE and SHM_SETPLACE and one or more of the values in shmid_ds is incorrect.

EOVERFLOW

cmd is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by buf.

EPERM

cmd is equal to IPC_RMID or IPC_SET and the effective user is not equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid and the process does not have the P_OWNER privilege.

EPERM

cmd is equal to SHM_LOCK or SHM_UNLOCK and the calling process does not have the P_SYSOPS privilege.

EPERM

cmd is equal to SHM_SETPLACE and the effective user is not equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid and the process does not have the P_OWNER privilege.

EAGAIN

cmd is SHM_SETPLACE and the call failed due to resource constraints within the kernel.

ENOTSUP

cmd is SHM_SETPLACE and the system does not support the requested placement style.

EFBIG

cmd is SHM_SETPLACE and the system cannot honor the command on the specified shared memory segment because SHM_FINEGRAIN was not specified when the segment was created. In other words, the system has been allowed to optimize access to the shared memory in such a way that fine-grained placement control via SHM_SETPLACE is not applicable.

EFAULT

buf points to an illegal address.

ENOMEM

cmd is equal to SHM_LOCK and there is not enough memory.

The SHM_SETPLACE command can be executed only by a process that has an effective user ID equal to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid or by a process that has the P_OWNER privilege.

References

Notices