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SHMCTL(2) NetBSD System Calls Manual SHMCTL(2)
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shmctl -- shared memory control operations
Standard C Library (libc, -lc)
shmctl(int shmid, int cmd, struct shmid_ds *buf);
The shmctl() system call performs control operations on the shared memory
segment specified by shmid.
Each shared memory segment has a shmid_ds structure associated with it
which contains the following members:
struct ipc_perm shm_perm; /* operation permissions */
size_t shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* pid of last shm op */
pid_t shm_cpid; /* pid of creator */
shmatt_t shm_nattch; /* # of current attaches */
time_t shm_atime; /* last shmat() time */
time_t shm_dtime; /* last shmdt() time */
time_t shm_ctime; /* last change by shmctl() */
The ipc_perm structure used inside the shmid_ds structure is defined in
<sys/ipc.h> and contains the following members:
uid_t cuid; /* creator user id */
gid_t cgid; /* creator group id */
uid_t uid; /* user id */
gid_t gid; /* group id */
mode_t mode; /* permission (lower 9 bits) */
The operation to be performed by shmctl() is specified in cmd and is one
IPC_STAT Gather information about the shared memory segment and place
it in the structure pointed to by buf.
IPC_SET Set the value of the shm_perm.uid, shm_perm.gid and
shm_perm.mode fields in the structure associated with shmid.
The values are taken from the corresponding fields in the
structure pointed to by buf. This operation can only be exe-
cuted by the super-user, or a process that has an effective
user id equal to either shm_perm.cuid or shm_perm.uid in the
data structure associated with the shared memory segment.
IPC_RMID Remove the shared memory segment specified by shmid and
destroy the data associated with it. Only the super-user or a
process with an effective uid equal to the shm_perm.cuid or
shm_perm.uid values in the data structure associated with the
segment can do this.
SHM_LOCK Lock the shared memory segment specified by shmid in memory.
This operation can only be executed by the super-user.
Unlock the shared memory segment specified by shmid. This
operation can only be executed by the super-user.
The read and write permissions on a shared memory identifier are deter-
mined by the shm_perm.mode field in the same way as is done with files
(see chmod(2)), but the effective uid can match either the shm_perm.cuid
field or the shm_perm.uid field, and the effective gid can match either
shm_perm.cgid or shm_perm.gid.
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable errno is set to indicate the error.
shmctl() will fail if:
[EACCES] The command is IPC_STAT and the caller has no read
permission for this shared memory segment.
[EFAULT] buf specifies an invalid address.
[EINVAL] shmid is not a valid shared memory segment identifier.
cmd is not a valid command.
[ENOMEM] The cmd is equal to SHM_LOCK and there is not enough
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the caller is
not the super-user, nor does the effective uid match
either the shm_perm.uid or shm_perm.cuid fields of the
data structure associated with the shared memory seg-
An attempt was made to increase the value of
shm_qbytes through IPC_SET but the caller is not the
The cmd is equal to SHM_LOCK or SHM_UNLOCK and the
caller is not the super-user.
ipcrm(1), ipcs(1), shmat(2), shmget(2)
The shmctl 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
NetBSD 6.0 November 25, 2006 NetBSD 6.0