MSGCTL(S) UNIX System V MSGCTL(S)
Name
msgctl - message control operations
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgctl (msqid, cmd, buf)
int msqid, cmd;
msqid_ds *buf;
Description
The msgctl system call provides a variety of message control
operations as specified by cmd. The following cmds are
available:
IPC_STAT Place the current value of each member of the
data structure associated with msqid into the
structure pointed to by buf. The contents of
this structure are defined in intro(S).
{READ}
IPC_SET Set the value of the following members of the
data structure associated with msqid to the
corresponding value found in the structure
pointed to by buf:
msg_perm.uid
msg_perm.gid
msg_perm.mode /* only low 9 bits */
msg_qbytes
This cmd can only be executed by a process
that has an effective user ID equal to either
that of super-user, or to the value of
msg_perm.cuid or msg_perm.uid in the data
structure associated with msqid. Only
super-user can raise the value of msg_qbytes.
IPC_RMID Remove the message queue identifier specified
by msqid from the system and destroy the
message queue and data structure associated
with it. This cmd can only be executed by a
process that has an effective user ID equal
to either that of super-user, or to the value
of msg_perm.cuid or msg_perm.uid in the data
structure associated with msqid.
The msgctl system call will fail if one or more of the
following is true:
[EINVAL] The msqid argument is not a valid message
queue identifier.
[EINVAL] The cmd argument is not a valid command.
[EACCES] The cmd argument is equal to IPC_STAT and
{READ} operation permission is denied to the
calling process (see intro(S)).
[EPERM] The cmd argument is equal to IPC_RMID or
IPC_SET. The effective user ID of the
calling process is not equal to that of
super-user, or to the value of msg_perm.cuid
or msg_perm.uid in the data structure
associated with msqid.
[EPERM] The cmd argument is equal to IPC_SET, an
attempt is being made to increase to the
value of msg_qbytes, and the effective user
ID of the calling process is not equal to
that of super-user.
[EFAULT] The buf argument points to an illegal
address.
See Also
intro(S), msgget(S), msgop(S)
Diagnostics
Upon successful completion, a value of 0 is returned.
Otherwise, a value of -1 is returned, and errno is set to
indicate the error.
Standards Conformance
msgctl is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 6/20/89)