NAME
shmctl
—
shared memory control
operations
SYNOPSIS
#include
<sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.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
<sys/ipc.h>
and looks like
this:
struct ipc_perm { 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; /* r/w permission (see chmod(2)) */ u_short 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 executed by the superuser, 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
- Mark the shared memory segment specified by shmid for removal when it is no longer in use by any process. When it is removed, all data associated with it will be destroyed too. Only the superuser 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 determined 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.
ERRORS
shmctl
() will fail if:
- [
EPERM
] - cmd is equal to
IPC_SET
orIPC_RMID
and the caller is not the superuser, 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 segment.An attempt is made to increase the value of shm_qbytes through
IPC_SET
but the caller is not the superuser. - [
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 ALSO
STANDARDS
Segments which are marked for removal (but not yet removed since they are still in use) can be attached to by new callers using shmat(2). This is permitted as an extension beyond the standards.