MSGOP(2) SysV MSGOP(2)
NAME
msgop: msgsnd, msgrcv - message operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd (msqid, msgp, msgsz, msgflg)
int msqid;
struct msgbuf *msgp;
int msgsz, msgflg;
int msgrcv (msqid, msgp, msgsz, msgtyp, msgflg)
int msqid;
struct msgbuf *msgp;
int msgsz;
long msgtyp;
int msgflg;
DESCRIPTION
msgsnd is used to send a message to the queue associated with the message
queue identifier specified by msqid. {WRITE} msgp points to a structure
containing the message. This structure is composed of the following
members:
long mtype; /* message type */
char mtext[]; /* message text */
mtype is a positive integer that can be used by the receiving process for
message selection (see msgrcv below). 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 will not be sent and
the calling process will return immediately.
⊕ If (msgflg & IPC_NOWAIT) is "false", the calling process will suspend
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 equal 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).
Following a successful call to msgsnd, the following actions are taken
with respect to the data structure associated with msqid (see intro (2)).
1. msg_qnum is incremented by 1.
2. msg_lspid is set equal to the process ID of the calling process.
3. msg_stime is set equal 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 structure pointed to
by msgp. {READ} This structure is composed of the following members:
long mtype; /* message type */
char mtext[]; /* message text */
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 equal to 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 & IPC_NOWAIT) is "true", the calling process will return
immediately with a return value of -1 and errno set to ENOMSG.
If (msgflg & IPC_NOWAIT) is "false", the calling process will suspend
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
equal 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).
Following a successful call to msgrcv, the following actions are taken
with respect to the data structure associated with msqid (see intro (2)).
1. msg_qnum is decremented by 1.
2. msg_lrpid is set equal to the process ID of the calling process.
3. msg_rtime is set equal to the current time.
ERRORS
msgsnd will fail and no message will be sent if one or more of the
following are true:
[EINVAL] msqid is not a valid message queue identifier.
[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 & IPC_NOWAIT) is "true".
[EINVAL] msgsz is less than 0 or greater than the system-imposed limit.
[EFAULT] msgp points to an illegal address.
msgrcv will fail and no message will be received if one or more of the
following are true:
[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] 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.
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 a value equal to the number of bytes actually placed
into mtext.
Otherwise, a value of -1 is returned and errno is set to indicate the
error.