msgop(2) 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,
size_t msgsz, int msgflg);
int msgrcv(int msqid, void *msgp,
size_t msgsz, long msgtyp, int msgflg);
DESCRIPTION
msgsnd sends a message to the queue associated with the
message 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 receiving
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
msg_qbytes [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&IPC_NOWAIT) is true, the message is not sent
and the caller returns immediately.
If (msgflg&IPC_NOWAIT) is false, the caller suspends
execution until one of the following occurs:
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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 caller receives a signal that is to be caught.
In this case the message is not sent and the
caller resumes execution in the manner prescribed
in signal(5).
On success, the following actions are taken with respect to
the data structure associated with msqid [see intro(2)].
msg_qnum is incremented by 1.
msg_lspid ID of the caller.
msg_stime 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 structure
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 truncated to msgsz
bytes if it is larger than msgsz and (msgflg&MSG_NOERROR) 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.
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msgop(2) msgop(2)
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&IPC_NOWAIT) is true, the caller returns
immediately with a return value of -1 and sets errno to
ENOMSG.
If (msgflg&IPC_NOWAIT) is false, the caller 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 caller receives a signal that is to be caught.
In this case a message is not received and the
caller resumes execution in the manner prescribed
in signal(2).
On success, the following actions are taken with respect to
the data structure associated with msqid [see intro (2)].
msg_qnum is decremented by 1.
msg_lrpid is set to the process ID of the caller.
msg_rtime is set to the current time.
Return Values
On success:
msgsnd returns 0.
msgrcv returns the number of bytes actually placed into
mtext.
On failure, msgsnd and msgrcv return -1 and set errno to
identify the error.
Errors
In the following conditions, msgsnd and msgrcv fail and set
errno to:
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EINTR msgsnd or msgrcv returned due to the receipt of a
signal.
EIDRM msgsnd or msgrcv returned due to removal of msqid from
the system.
In the following conditions, msgsnd fails and sets errno to:
EINVAL msqid is not a valid message queue identifier.
EACCES Write permission is denied to the caller. [see
intro(2)].
EINVAL mtype is less than 1.
EAGAIN The message cannot be sent for one of the
reasons cited above and (msgflg&IPC_NOWAIT) is
true.
EINVAL msgsz is less than zero or greater than the
system-imposed limit.
EFAULT msgp points to an illegal address.
In the following conditions, msgrcv fails and sets errno to:
EINVAL msqid is not a valid message queue identifier.
EACCES Read permission is denied to the caller.
EINVAL msgsz is less than 0.
E2BIG The length of mtext is greater than msgsz and
(msgflg&MSG_NOERROR) is false.
ENOMSG The queue does not contain a message of the
desired type and (msgtyp&IPC_NOWAIT) is true.
EFAULT msgp points to an illegal address.
REFERENCES
intro(2), msgctl(2), msgget(2), signal(2)
NOTICES
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msgop(2) msgop(2)
Considerations for Threads Programming
While one thread is blocked, siblings might still be
executing.
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