SHMCTL(2) MidnightBSD System Calls Manual SHMCTL(2)

NAME

shmctl — 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 identified by shmid:

IPC_STAT

Fetch the segment’s struct shmid_ds, storing it in the memory 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 effective 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 privileges.

The shmid_ds structure is defined as follows:

struct shmid_ds {
struct ipc_perm shm_perm; /* operation permission structure */
int 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 */
short 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() */
void *shm_internal; /* sysv stupidity */
};

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 ALSO

shmat(2), shmdt(2), shmget(2), ftok(3)

MidnightBSD 0.3 July 17, 1995 MidnightBSD 0.3