MirBSD manpage: shmctl(2)

SHMCTL(2)                  BSD Programmer's Manual                   SHMCTL(2)


     shmctl - shared memory control operations


     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

     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 ac-
     tions 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 {
             uid_t           cuid;   /* creator user id */
             gid_t           cgid;   /* creator group id */
             uid_t           uid;    /* user id */
             gid_t           gid;    /* group id */
             mode_t          mode;   /* r/w permission (see chmod(2)) */
             u_short         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 superuser, 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 des-
                troy the data associated with it. Only the superuser or a pro-
                cess 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 superuser, 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 segment.

                   An attempt is made to increase the value of shm_qbytes
                   through IPC_SET but the caller is not the superuser.

     [EACCES]      The command is IPC_STAT and the caller has no read permis-
                   sion 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), shmget(2)

MirBSD #10-current             August 17, 1995                               1

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