SHMCTL(2) BSD System Calls Manual SHMCTL(2)NAMEshmctl — shared memory control
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <machine/param.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
int
shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
Performs the action specified by cmd on the shared memory segment identi‐
fied by shmid:
IPC_STAT Fetch the segment's struct shmid_ds, storing it in the mem‐
ory pointed to by buf.
IPC_SET Changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode
members of the segment's struct shmid_ds to match those of
the struct pointed to by buf. The calling process's effec‐
tive uid must match either shm_perm.uid or shm_perm.cuid, or
it must have superuser privileges.
IPC_RMID Removes the segment from the system. The removal will not
take effect until all processes having attached the segment
have exited; however, once the IPC_RMID operation has taken
place, no further processes will be allowed to attach the
segment. For the operation to succeed, the calling
process's effective uid must match shm_perm.uid or
shm_perm.cuid, or the process must have superuser privi‐
leges.
The shmid_ds structure is defined as follows:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permission structure */
size_t shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* process ID of last shared memory op */
pid_t shm_cpid; /* process ID of creator */
int shm_nattch; /* number of current attaches */
time_t shm_atime; /* time of last shmat() */
time_t shm_dtime; /* time of last shmdt() */
time_t shm_ctime; /* time of last change by shmctl() */
};
RETURN VALUES
The shmctl() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The shmctl() system call will fail if:
[EINVAL] Invalid operation, or no shared memory segment was
found corresponding to shmid.
[EPERM] The calling process's effective uid does not match the
uid of the shared memory segment's owner or creator.
[EACCES] Permission denied due to mismatch between operation
and mode of shared memory segment.
SEE ALSOshmat(2), shmdt(2), shmget(2), ftok(3)BSD July 17, 1995 BSD
