msgrcv(3)

NAME

msgrcv - receive a message from a message queue

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int
msgflg);

DESCRIPTION

The msgrcv() function receives a message from the message

queue specified
in msqid, and places it into the structure pointed to by

msgp. This
structure should consist of the following members:

long mtype; /* message type */
char mtext[1]; /* body of message */

mtype is an integer greater than 0 that can be used for se

lecting messages, mtext is an array of bytes, with a size up to that of

the system
limit (MSGMAX).

The value of msgtyp has one of the following meanings:

+o The msgtyp argument is greater than 0. The first mes

sage of type

msgtyp will be received.

+o The msgtyp argument is equal to 0. The first message on

the queue

will be received.

+o The msgtyp argument is less than 0. The first message

of the lowest

message type that is less than or equal to the absolute

value of
msgtyp will be received.

The msgsz argument specifies the maximum length of the re

quested message.
If the received message has a length greater than msgsz it

will be
silently truncated if the MSG_NOERROR flag is set in msgflg,

otherwise an
error will be returned.

If no matching message is present on the message queue spec

ified by
msqid, the behavior of msgrcv() depends on whether the

IPC_NOWAIT flag is
set in msgflg or not. If IPC_NOWAIT is set, msgrcv() will

immediately
return a value of -1, and set errno to ENOMSG. If

IPC_NOWAIT is not set,
the calling process will be blocked until:

+o A message of the requested type becomes available on the

message

queue.

+o The message queue is removed, in which case -1 will be

returned, and

errno set to EINVAL.

+o A signal is received and caught. -1 is returned, and

errno set to

EINTR.

If a message is successfully received, the data structure

associated with
msqid is updated as follows:

+o msg_cbytes is decremented by the size of the message.

+o msg_lrpid is set to the pid of the caller.

+o msg_lrtime is set to the current time.

+o msg_qnum is decremented by 1.

RETURN VALUES

Upon successful completion, msgrcv() returns the number of

bytes received
into the mtext field of the structure pointed to by msgp.

Otherwise, -1
is returned, and errno set to indicate the error.

ERRORS

The msgrcv() function will fail if:

[EINVAL] The msqid argument is not a valid message

queue iden

tifier.

The message queue was removed while

msgrcv() was waiting for a message of the requested type

to become
available on it.

The msgsz argument is less than 0.

[E2BIG] A matching message was received, but its

size was

greater than msgsz and the MSG_NOERROR

flag was not
set in msgflg.

[EACCES] The calling process does not have read

access to the

message queue.

[EFAULT] The msgp argument points to an invalid

address.

[EINTR] The system call was interrupted by the

delivery of a

signal.

[ENOMSG] There is no message of the requested type

available on

the message queue, and IPC_NOWAIT is set

in msgflg.

SEE ALSO

msgctl(3), msgget(3), msgsnd(3)

HISTORY

Message queues appeared in the first release of AT&T System

V UNIX.

BSD June 15, 2005

BSD