NAME¶
msgctl - System V 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
System V 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 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_SYS_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. The third argument to msgctl() is ignored in this
case.
- IPC_INFO (Linux-specific)
- Return 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)
- Return 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)
- Return 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 in the
user namespace that governs its IPC namespace.
- 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 caller is not
privileged (Linux: does not have the CAP_SYS_ADMIN
capability).
- EPERM
- An attempt (IPC_SET) was made to increase msg_qbytes beyond
the system parameter MSGMNB, but the caller is not privileged
(Linux: does not have the CAP_SYS_RESOURCE capability).
POSIX.1-2001, POSIX.1-2008, SVr4.
NOTES¶
The inclusion of
<sys/types.h> and
<sys/ipc.h> isn't
required on Linux or by any version of POSIX. However, some old
implementations required the inclusion of these header files, and the SVID
also documented their inclusion. Applications intended to be portable to such
old systems may need to include these header files.
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 filesystem 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 4.10 of the Linux
man-pages project. A
description of the project, information about reporting bugs, and the latest
version of this page, can be found at
https://www.kernel.org/doc/man-pages/.