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)

Debian                                          October 10, 2015                                       SHMCTL(2)