shmctl(2) - NetBSD Manual Pages

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


NAME

     shmctl - shared memory control operations


SYNOPSIS

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

     int
     shmctl(int shmid, int cmd, struct shmid_ds *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 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 {
         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
     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)

NetBSD                          August 17, 1995                              2