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MSGCTL(2) Linux Programmer's Manual MSGCTL(2)
NAME
msgctl - message control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
DESCRIPTION
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 (nonstandard) */
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 associ-
ated 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 effec-
tive 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.
RETURN VALUE
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.
ERRORS
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).
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
The IPC_INFO, MSG_STAT and MSG_INFO operations are used by the ipcs(1) 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.)
SEE ALSO
msgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7), svipc(7)
COLOPHON
This page is part of release 3.25 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)
