shmctl (2)

Leading comments

Copyright (c) 1995 David Hovemeyer <daveho@infocom.com>

All rights reserved.

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(The comments found at the beginning of the groff file "man2/shmctl.2freebsd".)

NAME

shmctl - shared memory control

LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In sys/ipc.h In sys/shm.h Ft int Fn shmctl int shmid int cmd struct shmid_ds *buf

DESCRIPTION

Performs the action specified by Fa cmd on the shared memory segment identified by Fa shmid :

IPC_STAT

Fetch the segment's Fa struct shmid_ds , storing it in the memory pointed to by Fa buf .

IPC_SET

Changes the Fa shm_perm.uid , Fa shm_perm.gid , and Fa shm_perm.mode members of the segment's Fa struct shmid_ds to match those of the struct pointed to by Fa buf . The calling process's effective uid must match either Fa shm_perm.uid or Fa 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 Fa shm_perm.uid or Fa shm_perm.cuid , or the process must have superuser privileges.

The Vt 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

Rv -std shmctl

ERRORS

The Fn shmctl system call will fail if:

Bq Er EINVAL

Invalid operation, or no shared memory segment was found corresponding to Fa shmid .

Bq Er EPERM

The calling process's effective uid does not match the uid of the shared memory segment's owner or creator.

Bq Er EACCES

Permission denied due to mismatch between operation and mode of shared memory segment.

SEE ALSO

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