msgop(2) SYSTEM CALLS msgop(2)
NAME
msgop: msgsnd, msgrcv - message operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd(int msqid, const void *msgp,
sizet msgsz, int msgflg);
int msgrcv(int msqid, void *msgp,
sizet msgsz, long msgtyp, int msgflg);
DESCRIPTION
msgsnd sends a message to the queue associated with the mes-
sage queue identifier specified by msqid. msgp points to a
user defined buffer that must contain first a field of type
long integer that will specify the type of the message, and
then a data portion that will hold the text of the message.
The following is an example of members that might be in a
user defined buffer.
long mtype; /* message type */
char mtext[]; /* message text */
mtype is a positive integer that can be used by the receiv-
ing process for message selection. mtext is any text of
length msgsz bytes. msgsz can range from 0 to a system
imposed maximum.
msgflg specifies the action to be taken if one or more of
the following are true:
The number of bytes already on the queue is equal to
msgqbytes [see intro(2)].
The total number of messages on all queues system-wide
is equal to the system-imposed limit.
These actions are as follows:
If (msgflg&IPCNOWAIT) is true, the message is not sent
and the calling process returns immediately.
If (msgflg&IPCNOWAIT) is false, the calling process
suspends execution until one of the following occurs:
The condition responsible for the suspension no
longer exists, in which case the message is
sent.
msqid is removed from the system [see
msgctl(2)]. When this occurs, errno is set to
1
msgop(2) SYSTEM CALLS msgop(2)
EIDRM, and a value of -1 is returned.
The calling process receives a signal that is
to be caught. In this case the message is not
sent and the calling process resumes execution
in the manner prescribed in signal(2).
msgsnd fails and sends no message if one or more of the fol-
lowing are true:
EINVAL msqid is not a valid message queue identif-
ier.
EACCES Operation permission is denied to the calling
process [see intro(2)].
EINVAL mtype is less than 1.
EAGAIN The message cannot be sent for one of the
reasons cited above and (msgflg&IPCNOWAIT)
is true.
EINVAL msgsz is less than zero or greater than the
system-imposed limit.
EFAULT msgp points to an illegal address.
Upon successful completion, the following actions are taken
with respect to the data structure associated with msqid
[see intro (2)].
msgqnum is incremented by 1.
msglspid is set to the process ID of the calling pro-
cess.
msgstime is set to the current time.
msgrcv reads a message from the queue associated with the
message queue identifier specified by msqid and places it in
the user defined structure pointed to by msgp. The struc-
ture must contain a message type field followed by the area
for the message text (see the structure mymsg above). mtype
is the received message's type as specified by the sending
process. mtext is the text of the message. msgsz specifies
the size in bytes of mtext. The received message is trun-
cated to msgsz bytes if it is larger than msgsz and
(msgflg&MSGNOERROR) is true. The truncated part of the
message is lost and no indication of the truncation is given
to the calling process.
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msgop(2) SYSTEM CALLS msgop(2)
msgtyp specifies the type of message requested as follows:
If msgtyp is 0, the first message on the queue is
received.
If msgtyp is greater than 0, the first message of type
msgtyp is received.
If msgtyp is less than 0, the first message of the
lowest type that is less than or equal to the absolute
value of msgtyp is received.
msgflg specifies the action to be taken if a message of the
desired type is not on the queue. These are as follows:
If (msgflg&IPCNOWAIT) is true, the calling process
returns immediately with a return value of -1 and sets
errno to ENOMSG.
If (msgflg&IPCNOWAIT) is false, the calling process
suspends execution until one of the following occurs:
A message of the desired type is placed on the
queue.
msqid is removed from the system. When this
occurs, errno is set to EIDRM, and a value of
-1 is returned.
The calling process receives a signal that is
to be caught. In this case a message is not
received and the calling process resumes execu-
tion in the manner prescribed in signal(2).
msgrcv fails and receives no message if one or more of the
following are true:
EINVAL msqid is not a valid message queue identif-
ier.
EACCES Operation permission is denied to the calling
process.
EINVAL msgsz is less than 0.
E2BIG The length of mtext is greater than msgsz and
(msgflg&MSGNOERROR) is false.
ENOMSG The queue does not contain a message of the
desired type and (msgtyp&IPCNOWAIT) is true.
EFAULT msgp points to an illegal address.
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msgop(2) SYSTEM CALLS msgop(2)
Upon successful completion, the following actions are taken
with respect to the data structure associated with msqid
[see intro (2)].
msgqnum is decremented by 1.
msglrpid is set to the process ID of the calling pro-
cess.
msgrtime is set to the current time.
SEE ALSO
intro(2), msgctl(2), msgget(2), signal(2).
DIAGNOSTICS
If msgsnd or msgrcv return due to the receipt of a signal, a
value of -1 is returned to the calling process and errno is
set to EINTR. If they return due to removal of msqid from
the system, a value of -1 is returned and errno is set to
EIDRM.
Upon successful completion, the return value is as follows:
msgsnd returns a value of 0.
msgrcv returns the number of bytes actually placed into
mtext.
Otherwise, a value of -1 is returned and errno is set to
indicate the error.
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