msgop(2) msgop(2)
NAME
msgop: msgsnd, msgrcv - send/receive message
SYNOPSIS
#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 specified by the identifier
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 members can 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 receiving process
for message selection. mtext is any text of length msgsz bytes. msgsz
can range from 0 to a system imposed maximum.
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msgop(2) msgop(2)
msgflg specifies the action to be taken if one or more of the follow-
ing apply:
- The number of bytes already on the queue is equal to msgqbytes.
- 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 exe-
cution 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 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 following
apply:
EINVAL msqid is not a valid message queue identifier.
EACCES Operation permission is denied to the calling process.
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.
- msgqnum is incremented by 1.
- msglspid is set to the process ID of the calling process.
- msgstime is set to the current time.
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msgop(2) msgop(2)
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 structure must contain a message
type field followed by the area for the message text (see the struc-
ture mymsg above). mtype is the received message's type as specified
by the sending process. mtext is the text of the message. msgsz speci-
fies the size in bytes of mtext. The received message is truncated 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.
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 immedi-
ately with a result of -1 and sets errno to ENOMSG.
⊕ If (msgflg&IPCNOWAIT) is false, the calling process suspends exe-
cution 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 execution in the manner prescribed in signal(2).
msgrcv() fails and receives no message if one or more of the following
apply:
EINVAL msqid is not a valid message queue identifier.
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
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msgop(2) msgop(2)
(msgtyp&IPCNOWAIT) is true.
EFAULT msgp points to an illegal address.
Upon successful completion, the following actions are taken with
respect to the data structure associated with msqid.
- msgqnum is decremented by 1.
- msglrpid is set to the process ID of the calling process.
- msgrtime is set to the current time.
RESULT
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 result 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.
SEE ALSO
msgctl(2), msgget(2), signal(2).
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