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

     shmctl -- shared memory control operations

     #include <&lt;sys/types.h>&gt;
     #include <&lt;sys/ipc.h>&gt;
     #include <&lt;sys/msg.h>&gt;

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

     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 <sys/shm.h>:

     struct shmid_ds {
         struct ipc_perm shm_perm;     /* operation permissions */
         int             shm_segsz;    /* size of segment in bytes */
         pid_t           shm_lpid;     /* pid of last shm op */
         pid_t           shm_cpid;     /* pid of creator */
         short           shm_nattch;   /* # of current attaches */
         time_t          shm_atime;    /* last shmat() time*/
         time_t          shm_dtime;    /* last shmdt() time */
         time_t          shm_ctime;    /* last change by shmctl() */
         void           *shm_internal; /* sysv stupidity */
     The ipc_perm structure used inside the shmid_ds structure is defined in
     <sys/ipc.h> and looks like this:

     struct ipc_perm {
         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) */
         key_t key;   /* user specified msg/sem/shm key */
     The operation to be performed by shmctl() is specified in cmd and is one

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

     IPC_SET    Set the value of the shm_perm.uid, shm_perm.gid and
                shm_perm.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 shm_perm.cuid or shm_perm.uid in the
                data structure associated with the shared memory segment.

     IPC_RMID   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 shm_perm.cuid or
                shm_perm.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 shm_perm.mode field in the same way as is done with files
     (see chmod(2) ), but the effective uid can match either the shm_perm.cuid
     field or the shm_perm.uid field, and the effective gid can match either
     shm_perm.cgid or shm_perm.gid.

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

     shmctl() will fail if:

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

     [EACCES]           The command is IPC_STAT 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.

     shmat(2), shmdt(2), shmget(2)

BSD                             August 17, 1995                            BSD