msgop.2 (2010 09)

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msgop(2) msgop(2)

NAME

msgsnd, msgrcv - message operations

SYNOPSIS

#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

The msgsnd() system call 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 that speciﬁes the type of

the message, followed by a data portion that will hold the data bytes of the message. The structure below

is an example of what this user-deﬁned buffer might look like:

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 is true:

• The number of bytes already on the queue is equal to

msg_qbytes (see message queue

identiﬁer in glossary (9)).

• 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 calling process returns

immediately.

If (msgﬂg

& IPC_NOWAIT) is false, the calling process suspends execution until one of the follow-

ing 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 to be caught. In this case, the message is not sent and the

calling process resumes execution in the manner prescribed in signal (5).

Upon successful completion, the following actions are taken with respect to the data structure associated

with msqid:

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.

The

msgrcv() system call 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. This structure is composed of the

following members:

HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1

Summary of content (4 pages)

PAGE 1
msgop(2) msgop(2) NAME msgsnd, msgrcv - message operations SYNOPSIS #include 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 The msgsnd() system call sends a message to the queue associated with the message queue identifier specified by msqid .
PAGE 2
msgop(2) msgop(2) long char mtype; mtext[]; /* message type */ /* 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.
PAGE 3
msgop(2) msgop(2) [EAGAIN] The message cannot be sent for one of the reasons cited above and (msgflg IPC_NOWAIT) is true. [EFAULT] msgp points to an illegal address. The reliable detection of this error is implementation dependent. [EIDRM] The message queue identifier msqid has been removed from the system. [EINTR] msgsnd() was interrupted by a signal. [EINVAL] msqid is not a valid message queue identifier. [EINVAL] mtype is less than 1.
PAGE 4
(Notes) A (Notes) mA 4 Hewlett-Packard Company −1− HP-UX 11i Version 3: September 2010