MSGCTL(2) Linux Programmer's Manual MSGCTL(2)NAMEmsgctl - 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);
DESCRIPTIONmsgctl() 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() */
time_t msg_rtime; /* Time of last msgrcv() */
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() */
pid_t msg_lrpid; /* PID of last msgrcv() */
};
The ipc_perm structure is defined in <sys/ipc.h> as follows (the high‐
lighted fields are settable using IPC_SET):
struct ipc_perm {
key_t key; /* Key supplied to msgget() */
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. Appro‐
priate privilege (Linux: the CAP_IPC_RESOURCE capability) is
required to raise the msg_qbytes value beyond the system parame‐
ter 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 bytes of buffer pool used
to hold message data; unused */
int msgmap; /* Max. # of entries in message
map; unused */
int msgmax; /* Max. # of bytes that can be
written in a single message */
int msgmnb; /* Max. # of bytes that can be written to
queue; used to initialize msg_qbytes
during queue creation (msgget()) */
int msgmni; /* Max. # of message queues */
int msgssz; /* Message segment size; unused */
int msgtql; /* Max. # of messages on all queues
in system; unused */
unsigned short int msgseg;
/* Max. # of segments; unused */
};
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 cur‐
rently 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 opera‐
tion, 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 capa‐
bility).
NOTES
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 shorts under Linux 2.2 and
have become longs under Linux 2.4. To take advantage of this, a recom‐
pilation under glibc-2.1.91 or later should suffice. (The kernel dis‐
tinguishes old and new calls by an IPC_64 flag in cmd.)
CONFORMING TO
SVr4, POSIX.1-2001.
SEE ALSOmsgget(2), msgrcv(2), msgsnd(2), capabilities(7), mq_overview(7),
svipc(7)Linux 2.6.9 2004-11-10 MSGCTL(2)
