shmctl(2) - NetBSD Manual Pages

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


NAME

     shmctl - shared memory control operations


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <sys/shm.h>

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


DESCRIPTION

     The shmctl() system call performs control operations on the shared memory
     segment specified by shmid.

     Each shared memory segment has a shmid_ds structure associated with it
     which contains the following members:

         struct ipc_perm shm_perm;   /* operation permissions */
         size_t          shm_segsz;  /* size of segment in bytes */
         pid_t           shm_lpid;   /* pid of last shm op */
         pid_t           shm_cpid;   /* pid of creator */
         shmatt_t        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() */

     The ipc_perm structure used inside the shmid_ds structure is defined in
     <sys/ipc.h> and contains the following members:

         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; /* permission (lower 9 bits) */

     The operation to be performed by shmctl() is specified in cmd and is one
     of:

     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 de-
                stroy 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.


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.


ERRORS

     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 segment.

                   An attempt was made to increase the value of shm_qbytes
                   through IPC_SET but the caller is not the super-user.

     [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.


SEE ALSO

     shmat(2), shmget(2)


STANDARDS

     The shmctl system call conforms to X/Open System Interfaces and Headers
     Issue 5 (``XSH5'').


HISTORY

     Shared memory segments appeared in the first release of AT&T System V
     UNIX.

NetBSD 1.5.1                    August 25, 1999                              2