Standards
| Standards / Extensions |
C or C++ |
Dependencies |
|---|
XPG4
XPG4.2
Single UNIX Specification, Version 3
|
both |
|
Format
#define _XOPEN_SOURCE
#include <sys/msg.h>
int msgctl(int msgid, int cmd, struct msqid_ds *buf);
General description
The msgctl()
function provides message control operations as specified by
cmd.
The following values for
cmd, and the message
control operations they specify, are (These symbolic constants are
defined by the <sys/ipc.h> header):
- IPC_STAT
- Place the current
value of each member of the msqid_ds data structure
associated with msgid into the structure
pointed to by buf. The contents of this
structure are defined in <sys/msg.h>. This command requires
read permission.
- IPC_SET
- Set the value
of the following members of the msqid_ds data structure
associated with msgid to the corresponding
value found in the structure pointed to by buf:
- msg_perm.uid
- msg_perm.gid
- msg_perm.mode
- msg_qbytes
IPC_SET can only be executed by a process with the appropriate
privileges or that has an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in
the msqid_ds data structure associated with msgid.
Only a process with appropriate privileges can raise the value of msg_gbytes.
- IPC_RMID
- Remove the message
queue identifier specified by msgid from
the system and destroy the message queue and msqid_ds data
structure associated with it. IPC_RMID can only be executed
by a process with appropriate privileges or one that has an effective
user ID equal to the value of msg_perm.cuid or msg_perm.uid in
the msqid_ds data structure associated with msgid.
Returned value
If successful, msgctl()
returns 0.
If unsuccessful, msgctl() returns -1 and sets errno
to one of the following values:
- Error Code
- Description
- EACCES
- The argument cmd is IPC_STAT and
the calling process does not have read permission.
- EINVAL
- The value of msgid is not a valid message
queue identifier, or the value of cmd is
not a valid command.
- EPERM
- The argument cmd is IPC_RMID or IPC_SET and
the effective user ID of the calling process is not equal to that
of a process with appropriate privileges and it is not equal to the
value of msg_perm.cuid or msg_perm.uid in
the data structure associated with msgid.
Or the argument cmd is IPC_SET,
an attempt is being made to increase the value of msg_qbytes,
and the effective user ID of the calling process does not have appropriate
privileges.