msgop(2) msgop(2)
msgop: msgsnd, msgrcv - message operations
#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);
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<b>&IPC_NOWAIT) is true, the message is not sent and the
calling process returns immediately.
If (msgflg<b>&IPC_NOWAIT) 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 EIDRM, and a value of -1 is
returned.
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msgop(2) msgop(2)
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 are
true:
EINVAL msqid is not a valid message queue identifier.
EACCES Write 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<b>&IPC_NOWAIT) 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)].
msg_qnum is incremented by 1.
msg_lspid is set to the process ID of the calling process.
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<b>&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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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<b>&IPC_NOWAIT) is true, the calling process returns
immediately with a return value of -1 and sets errno to ENOMSG.
If (msgflg<b>&IPC_NOWAIT) 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 execution 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 identifier.
EACCES Read permission is denied to the calling process.
EINVAL msgsz is less than 0.
E2BIG The length of mtext is greater than msgsz and
(msgflg<b>&MSG_NOERROR) is false.
ENOMSG The queue does not contain a message of the desired type
and (msgtyp<b>&IPC_NOWAIT) 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 [see intro (2)].
msg_qnum is decremented by 1.
msg_lrpid is set to the process ID of the calling process.
msg_rtime is set to the current time.
intro(2), msgctl(2), msgget(2), signal(2).
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
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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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