MSGOP(S) UNIX System V MSGOP(S)
Name
msgop: msgsnd, msgrcv - message operations
Syntax
#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
The msgsnd system call 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 */
The mtype integer is positive and can be used by the
receiving process for message selection (see msgrcv below).
The array mtext is any text of length msgsz bytes. The
msgsz argument can range from 0 to a system-imposed maximum.
msgflg specifies the action to be taken if one or more of
the following is true:
The number of bytes already on the queue is equal to
msg_qbytes (see intro(S)).
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.
The msqid argument is removed from the system
(see msgctl(S)). 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(S).
msgsnd will fail and no message will be sent if one or more
of the following is true:
[EINVAL] msqid is not a valid message queue
identifier.
[EACCES] Operation permission is denied to the calling
process (see intro(S)).
[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.
Upon successful completion, the following actions are taken
with respect to the data structure associated with msqid
(see intro(S)).
msg_qnum is incremented by 1.
msg_lspid is set equal to the process ID of the calling
process.
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(S).
msgrcv will fail and no message will be received if one or
more of the following is 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.
Upon successful completion, the following actions are taken
with respect to the data structure associated with msqid
(see intro(S)).
msg_qnum is decremented by 1.
msg_lrpid is set equal to the process ID of the calling
process.
msg_rtime is set equal to the current time.
See Also
intro(S), msgctl(S), msgget(S), signal(S)
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.
Standards Conformance
msgrcv and msgsnd are conformant with:
AT&T SVID Issue 2, Select Code 307-127;
and The X/Open Portability Guide II of January 1987.
(printed 6/20/89)