SHMCTL(2)                   BSD System Calls Manual                  SHMCTL(2)

NAME
     shmctl -- shared memory control operations

SYNOPSIS
     ##include <>
     ##include <>
     ##include <>

     int
     shmctl(int shmid, int cmd, struct shmidds *buf);

DESCRIPTION
     The shmctl() system call performs some control operations on the shared
     memory area specified by shmid.  Each shared memory segment has a data
     structure associated with it, parts of which may be altered by shmctl()
     and parts of which determine the actions of shmctl().  This structure is
     defined as follows in :

     struct shmidds {
         struct ipcperm shmperm;     /* operation permissions */
         int             shmsegsz;    /* size of segment in bytes */
         pidt           shmlpid;     /* pid of last shm op */
         pidt           shmcpid;     /* pid of creator */
         short           shmnattch;   /* # of current attaches */
         timet          shmatime;    /* last shmat() time*/
         timet          shmdtime;    /* last shmdt() time */
         timet          shmctime;    /* last change by shmctl() */
         void           *shminternal; /* sysv stupidity */
     };
     The ipcperm structure used inside the shmidds structure is defined in
      and looks like this:

     struct ipcperm {
         ushort cuid; /* creator user id */
         ushort cgid; /* creator group id */
         ushort uid;  /* user id */
         ushort gid;  /* group id */
         ushort mode; /* r/w permission (see chmod(2)) */
         ushort seq;  /* sequence # (to generate unique msg/sem/shm id) */
         keyt key;   /* user specified msg/sem/shm key */
     };
     The operation to be performed by shmctl() is specified in cmd and is one
     of:

     IPCSTAT   Gather information about the shared memory segment and place
                it in the structure pointed to by buf.

     IPCSET    Set the value of the shmperm.uid, shmperm.gid and
                shmperm.mode fields in the structure associated with shmid.
                The values are taken from the corresponding fields in the
                structure pointed to by buf.  This operation can only be exe-
                cuted by the super-user, or a process that has an effective
                user id equal to either shmperm.cuid or shmperm.uid in the
                data structure associated with the shared memory segment.

     IPCRMID   Remove the shared memory segment specified by shmid and
                destroy the data associated with it. Only the super-user or a
                process with an effective uid equal to the shmperm.cuid or
                shmperm.uid values in the data structure associated with the
                queue can do this.
     The read and write permissions on a shared memory identifier are deter-
     mined by the shmperm.mode field in the same way as is done with files
     (see chmod(2) ), but the effective uid can match either the shmperm.cuid
     field or the shmperm.uid field, and the effective gid can match either
     shmperm.cgid or shmperm.gid.

RETURN VALUES
     Upon successful completion, a value of 0 is returned. Otherwise, -1 is
     returned and the global variable errno is set to indicate the error.

ERORS
     shmctl() will fail if:

     [EPERM]            cmd is equal to IPCSET or IPCRMID and the caller is
                        not the super-user, nor does the effective uid match
                        either the shmperm.uid or shmperm.cuid fields of the
                        data structure associated with the shared memory seg-
                        ment.  An attempt is made to increase the value of
                        shmqbytes through IPCSET but the caller is not the
                        super-user.

     [EACES]           The command is IPCSTAT and the caller has no read
                        permission for this shared memory segment.

     [EINVAL]           shmid is not a valid shared memory segment identifier.
                        cmd is not a valid command.

     [EFAULT]           buf specifies an invalid address.

SEE ALSO
     shmat(2), shmdt(2), shmget(2)

BSD                             August 17, 1995                            BSD