SHMCTL(2) BSD System Calls Manual SHMCTL(2)NAME
shmctl — shared memory control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
The shmctl() system call performs some control operations on the shared
memory area specified by shmid. Each shared memory segment has a data
structure associated with it, parts of which may be altered by shmctl()
and parts of which determine the actions of shmctl(). This structure is
defined as follows in ⟨sys/shm.h⟩:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permissions */
int shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* pid of last shm op */
pid_t shm_cpid; /* pid of creator */
short 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() */
void *shm_internal; /* sysv stupidity */
};
The ipc_perm structure used inside the shmid_ds structure is defined in
⟨sys/ipc.h⟩ and looks like this:
struct ipc_perm {
ushort cuid; /* creator user id */
ushort cgid; /* creator group id */
ushort uid; /* user id */
ushort gid; /* group id */
ushort mode; /* r/w permission (see chmod(2)) */
ushort seq; /* sequence # (to generate unique msg/sem/shm id) */
key_t key; /* user specified msg/sem/shm key */
};
The operation to be performed by shmctl() is specified in cmd and is one
of:
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
queue can do this.
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.
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is
returned and the global variable errno is set to indicate the error.
ERRORSshmctl() will fail if:
[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‐
ment. An attempt is made to increase the value of
shm_qbytes through IPC_SET but the caller is not the
super-user.
[EACCES] The command is IPC_STAT and the caller has no read
permission for this shared memory segment.
[EINVAL] shmid is not a valid shared memory segment identifier.
cmd is not a valid command.
[EFAULT] buf specifies an invalid address.
SEE ALSOshmat(2), shmdt(2), shmget(2)BSD August 17, 1995 BSD
