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 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(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:
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 superuser or to the value of 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 command 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 msquid.
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.
DIAGNOSTICS
The msgctl system call will fail if one or more of the following are true:
[EINVAL] The msqid is not a valid message queue identifier.
[EINVAL] The cmd is not a valid command.
[EACCES] The cmd is equal to IPC_STAT and read operation permission is denied to the calling process. For further information, see intro(2).
[EPERM] The cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of superuser and it is not equal to the value of msg_perm.uid in the data structure associated with msqid.
[EPERM] The 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] Buf points to an illegal address.