msgctl() - Unix, Linux System Call
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NAME
msgctl - message control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
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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() */
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() */
};
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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() */
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 */
};
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Valid values for
cmd are:
Tag | Description |
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.
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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.
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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 */
};
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The
msgmni, msgmax, and
msgmnb settings can be changed via
/proc files of the same name; see
proc(5)
for details.
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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.
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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 kernels internal array that maintains information about
all message queues on the system.
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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
kernels 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:
Tag | Description |
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.
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EFAULT |
The argument
cmd has the value
IPC_SET or
IPC_STAT, but the address pointed to by
buf isnt accessible.
|
EIDRM |
The message queue was removed.
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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.
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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).
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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 recompilation under glibc-2.1.91 or later should suffice.
(The kernel distinguishes old and new calls by an IPC_64 flag in
cmd.) CONFORMING TO
SVr4, POSIX.1-2001.
SEE ALSO
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