MSGOP(2) Linux Programmer's Manual MSGOP(2)
NAME
msgop - message operations
SYNOPSIS
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/msg.h>
int msgsnd ( int msqid, struct msgbuf *msgp, int msgsz,
int msgflg )
int msgrcv ( int msqid, struct msgbuf *msgp, int msgsz,
long msgtyp, int msgflg )
DESCRIPTION
To send or receive a message, the calling process allo
cates a structure that looks like the following
struct msgbuf {
long mtype; /* message type, must be > 0 */
char mtext[1]; /* message data */
};
but with an array mtext of size msgsz, a non-negative
integer value. The structure member mtype must have a
strictly positive integer value that can be used by the
receiving process for message selection (see the section
about msgrcv).
The calling process must have write access permissions to
send and read access permissions to receive a message on
the queue.
The msgsnd system call enqueue a copy of the message
pointed to by the msgp argument on the message queue whose
identifier is specified by the value of the msqid argu
ment.
The argument msgflg specifies the system call behaviour if
enqueuing the new message will require more than
msg_qbytes in the queue. Asserting IPC_NOWAIT the message
will not be sent and the system call fails returning with
errno set to EAGAIN. Otherwise the process is suspended
until the condition for the suspension no longer exists
(in which case the message is sent and the system call
succeeds), or the queue is removed (in which case the sys
tem call fails with errno set to EIDRM), or the process
receives a signal that has to be caught (in which case the
system call fails with errno set to EINTR).
Upon successful completion the message queue data struc
ture is updated as follows:
msg_lspid is set to the process-ID of the calling
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process.
msg_qnum is incremented by 1.
msg_stime is set to the current time.
The system call msgrcv reads a message from the message
queue specified by msqid into the msgbuf pointed to by the
msgp argument removing from the queue, on success, the
read message.
The argument msgsz specifies the maximum size in bytes for
the member mtext of the structure pointed to by the msgp
argument. If the message text has length greater than
msgsz, then if the msgflg argument asserts MSG_NOERROR,
the message text will be truncated (and the truncated part
will be lost), otherwise the message isn't removed from
the queue and the system call fails returning with errno
set to E2BIG.
The argument msgtyp specifies the type of message
requested as follows:
If msgtyp is 0, then the message on the queue's
front is read.
If msgtyp is greater than 0, then the first message
on the queue of type msgtyp is read if MSG_EXCEPT
isn't asserted by the msgflg argument, otherwise
the first message on the queue of type not equal to
msgtyp will be read.
If msgtyp is less than 0, then the first message on
the queue with the lowest type less than or equal
to the absolute value of msgtyp will be read.
The msgflg argument asserts none, one or more (or-ing
them) among the following flags:
IPC_NOWAIT For immediate return if no message of
the requested type is on the queue. The system
call fails with errno set to ENOMSG.
MSG_EXCEPT Used with msgtyp greater than 0 to read
the first message on the queue with message type
that differs from msgtyp.
MSG_NOERROR To truncate the message text if longer
than msgsz bytes.
If no message of the requested type is available and
IPC_NOWAIT isn't asserted in msgflg, the calling process
is blocked until one of the following conditions occurs:
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A message of the desired type is placed on the
queue.
The message queue is removed from the system. In
such a case the system call fails with errno set to
EIDRM.
The calling process receives a signal that has to
be caught. In such a case the system call fails
with errno set to EINTR.
Upon successful completion the message queue data struc
ture is updated as follows:
msg_lrpid is set to the process-ID of the calling
process.
msg_qnum is decremented by 1.
msg_rtime is set to the current time.
RETURN VALUE
On a failure both functions return -1 with errno indicat
ing the error, otherwise msgsnd returns 0 and msgrvc
returns the number of bytes actually copied into the mtext
array.
ERRORS
When msgsnd fails, at return errno will be set to one
among the following values:
EAGAIN The message can't be sent due to the msg_qbytes
limit for the queue and IPC_NOWAIT was asserted
in mgsflg.
EACCES The calling process has no write access permis
sions on the message queue.
EFAULT The address pointed to by msgp isn't accessi
ble.
EIDRM The message queue was removed.
EINTR Sleeping on a full message queue condition, the
process received a signal that had to be
caught.
EINVAL Invalid msqid value, or nonpositive mtype
value, or invalid msgsz value (less than 0 or
greater than the system value MSGMAX).
ENOMEM The system has not enough memory to make a copy
of the supplied msgbuf.
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When msgrcv fails, at return errno will be set to one
among the following values:
E2BIG The message text length is greater than msgsz
and MSG_NOERROR isn't asserted in msgflg.
EACCES The calling process has no read access permis
sions on the message queue.
EFAULT The address pointed to by msgp isn't accessi
ble.
EIDRM While the process was sleeping to receive a
message, the message queue was removed.
EINTR While the process was sleeping to receive a
message, the process received a signal that had
to be caught.
EINVAL Illegal msgqid value, or msgsz less than 0.
ENOMSG IPC_NOWAIT was asserted in msgflg and no mes
sage of the requested type existed on the mes
sage queue.
NOTES
The followings are system limits affecting a msgsnd system
call:
MSGMAX Maximum size for a message text: the implemen
tation set this value to 4080 bytes.
MSGMNB Default maximum size in bytes of a message
queue: policy dependent. The super-user can
increase the size of a message queue beyond
MSGMNB by a msgctl system call.
The implementation has no intrinsic limits for the system
wide maximum number of message headers (MSGTQL) and for
the system wide maximum size in bytes of the message pool
(MSGPOOL).
CONFORMING TO
SVr4, SVID.
SEE ALSO
ipc(5), msgctl(2), msgget(2), msgrcv(2), msgsnd(2).
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