SHMCTL(2) — SYSTEM CALLS

NAME

shmctl − shared memory control operations

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>

int shmctl (shmid, cmd, buf)
int shmid, cmd;
struct shmid_ds ∗buf;

DESCRIPTION

shmctl() provides a variety of shared memory control operations as specified by cmd. The following cmds are available:

IPC_STAT Place the current value of each member of the data structure associated with shmid into the structure pointed to by buf. The contents of this structure are defined in intro(2). {READ}

IPC_SET Set the value of the following members of the data structure associated with shmid to the corresponding value found in the structure pointed to by buf:

shm_perm.uid
shm_perm.gid
shm_perm.mode /∗ only low 9 bits ∗/

This cmd can only be executed by a process that has an effective user ID equal to that of super-user, or to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid.

IPC_RMID
Remove the shared memory identifier specified by shmid from the system. If no processes are currently mapped to the corresponding shared memory segment, then the segment is removed and the associated resources are reclaimed. Otherwise, the segment will persist, although shmget(2) will not be able to locate it, until it is no longer mapped by any process. This cmd can only be executed by a process that has an effective user ID equal to that of super-user, or to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid.

RETURN VALUE

Upon successful completion, a value of 0 is returned. Otherwise, a value of −1 is returned and errno is set to indicate the error.

ERRORS

shmctl() will fail if one or more of the following are true:

EINVAL shmid is not a valid shared memory identifier.

EINVAL cmd is not a valid command.

EACCES cmd is equal to IPC_STAT and {READ} operation permission is denied to the calling process (see intro(2)).

EPERM cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of super-user, or to the value of shm_perm.cuid or shm_perm.uid in the data structure associated with shmid.

EFAULT buf points to an illegal address.

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