NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON
MSGCTL(2) Linux Programmer's Manual MSGCTL(2)
msgctl - message control operations
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
msgctl() performs the control operation specified by cmd on the message queue
with identifier msqid.
The msqid_ds data structure is defined in <sys/msg.h> as follows:
struct msqid_ds {
struct ipc_perm msg_perm; /* Ownership and permissions */
time_t msg_stime; /* Time of last msgsnd(2) */
time_t msg_rtime; /* Time of last msgrcv(2) */
time_t msg_ctime; /* Time of last change */
unsigned long __msg_cbytes; /* Current number of bytes in
queue (non-standard) */
msgqnum_t msg_qnum; /* Current number of messages
in queue */
msglen_t msg_qbytes; /* Maximum number of bytes
allowed in queue */
pid_t msg_lspid; /* PID of last msgsnd(2) */
pid_t msg_lrpid; /* PID of last msgrcv(2) */
};
The ipc_perm structure is defined in <sys/ipc.h> as follows (the highlighted
fields are settable using IPC_SET):
struct ipc_perm {
key_t __key; /* Key supplied to msgget(2) */
uid_t uid; /* Effective UID of owner */
gid_t gid; /* Effective GID of owner */
uid_t cuid; /* Effective UID of creator */
gid_t cgid; /* Effective GID of creator */
unsigned short mode; /* Permissions */
unsigned short __seq; /* Sequence number */
};
Valid values for cmd are:
IPC_STAT
Copy information from the kernel data structure associated with msqid
into the msqid_ds structure pointed to by buf. The caller must have
read permission on the message queue.
IPC_SET
Write the values of some members of the msqid_ds structure pointed to
by buf to the kernel data structure associated with this message queue,
updating also its msg_ctime member. The following members of the
structure are updated: msg_qbytes, msg_perm.uid, msg_perm.gid, and (the
least significant 9 bits of) msg_perm.mode. The effective UID of the
calling process must match the owner (msg_perm.uid) or creator
(msg_perm.cuid) of the message queue, or the caller must be privileged.
Appropriate privilege (Linux: the CAP_IPC_RESOURCE capability) is
required to raise the msg_qbytes value beyond the system parameter
MSGMNB.
IPC_RMID
Immediately remove the message queue, awakening all waiting reader and
writer processes (with an error return and errno set to EIDRM). The
calling process must have appropriate privileges or its effective user
ID must be either that of the creator or owner of the message queue.
IPC_INFO (Linux-specific)
Returns information about system-wide message queue limits and
parameters in the structure pointed to by buf. This structure is of
type msginfo (thus, a cast is required), defined in <sys/msg.h> if the
_GNU_SOURCE feature test macro is defined:
struct msginfo {
int msgpool; /* Size in kibibytes of buffer pool
used to hold message data;
unused within kernel */
int msgmap; /* Maximum number of entries in message
map; unused within kernel */
int msgmax; /* Maximum number of bytes that can be
written in a single message */
int msgmnb; /* Maximum number of bytes that can be
written to queue; used to initialize
msg_qbytes during queue creation
(msgget(2)) */
int msgmni; /* Maximum number of message queues */
int msgssz; /* Message segment size;
unused within kernel */
int msgtql; /* Maximum number of messages on all queues
in system; unused within kernel */
unsigned short int msgseg;
/* Maximum number of segments;
unused within kernel */
};
The msgmni, msgmax, and msgmnb settings can be changed via /proc files
of the same name; see proc(5) for details.
MSG_INFO (Linux-specific)
Returns a msginfo structure containing the same information as for
IPC_INFO, except that the following fields are returned with
information about system resources consumed by message queues: the
msgpool field returns the number of message queues that currently exist
on the system; the msgmap field returns the total number of messages in
all queues on the system; and the msgtql field returns the total number
of bytes in all messages in all queues on the system.
MSG_STAT (Linux-specific)
Returns a msqid_ds structure as for IPC_STAT. However, the msqid
argument is not a queue identifier, but instead an index into the
kernel's internal array that maintains information about all message
queues on the system.
On success, IPC_STAT, IPC_SET, and IPC_RMID return 0. A successful IPC_INFO
or MSG_INFO operation returns the index of the highest used entry in the
kernel's internal array recording information about all message queues. (This
information can be used with repeated MSG_STAT operations to obtain
information about all queues on the system.) A successful MSG_STAT operation
returns the identifier of the queue whose index was given in msqid.
On error, -1 is returned with errno indicating the error.
On failure, errno is set to one of the following:
EACCES The argument cmd is equal to IPC_STAT or MSG_STAT, but the calling
process does not have read permission on the message queue msqid, and
does not have the CAP_IPC_OWNER capability.
EFAULT The argument cmd has the value IPC_SET or IPC_STAT, but the address
pointed to by buf isn't accessible.
EIDRM The message queue was removed.
EINVAL Invalid value for cmd or msqid. Or: for a MSG_STAT operation, the
index value specified in msqid referred to an array slot that is
currently unused.
EPERM The argument cmd has the value IPC_SET or IPC_RMID, but the effective
user ID of the calling process is not the creator (as found in
msg_perm.cuid) or the owner (as found in msg_perm.uid) of the message
queue, and the process is not privileged (Linux: it does not have the
CAP_SYS_ADMIN capability).
SVr4, POSIX.1-2001.
The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the ipcs(8) program
to provide information on allocated resources. In the future these may
modified or moved to a /proc file system interface.
Various fields in the struct msqid_ds were typed as short under Linux 2.2 and
have become long under Linux 2.4. To take advantage of this, a recompilation
under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and
new calls by an IPC_64 flag in cmd.)
msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7), svipc(7)
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-08-06 MSGCTL(2)