msgctl(2)
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;
struct msqid_ds ∗buf;
DESCRIPTION
The msgctl system call provides a variety of message control operations as specified by cmd. The specified cmd can be:
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(2).
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 in:
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 either of superuser or of msg_perm.uid in the data structure associated with msqid. Only the superuser 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 as well as its associated data structure. This cmd can only be executed by a process that has an effective user ID equal to either that of superuser or to the value of msg_perm.uid in the data structure associated with msqid.
RETURN VALUE
If unsuccessful, returns a −1, and the global variable errno indicates the error code.
DIAGNOSTICS
The msgctl call will fail if:
[EINVAL] The specified msqid is not a valid message queue identifier.
[EINVAL] The specified cmd is not a valid command.
[EACCES] The specified cmd is equal to IPC_STAT, and operation permission is denied to the calling process. For further information, see intro(2).
[EPERM] The specified cmd is either IPC_RMID or IPC_SET. The effective user ID of the calling process is not that of superuser, and it is not equal to the value of msg_perm.uid in the data structure associated with msqid.
[EPERM] The specified cmd 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 superuser.
[EFAULT] The buf points to an illegal address.