SHMCTL(2)                 FreeBSD System Calls Manual                SHMCTL(2)

NAME
     shmctl - shared memory control

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #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.  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.  If the kern.ipc.shm_allow_removed sysctl(3)
                  variable is set to 0, once the IPC_RMID operation has taken
                  place, no further processes will be allowed to attach the
                  segment.

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

FreeBSD 13.1-RELEASE-p6        October 10, 2015        FreeBSD 13.1-RELEASE-p6