NAME
msgrcv —
receive a message from a message
queue
LIBRARY
library “libc”
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
Themsgrcv()
system call 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 selecting 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:
- The msgtyp argument is greater than 0. The first message of type msgtyp will be received.
- The msgtyp argument is equal to 0. The first message on the queue will be received.
- 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 requested 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 specified 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 EAGAIN.
If IPC_NOWAIT is not set, the calling process will
be blocked until:
- A message of the requested type becomes available on the message queue.
- The message queue is removed, in which case -1 will be returned, and
errno set to
EINVAL. - 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:
- msg_cbytes is decremented by the size of the message.
- msg_lrpid is set to the pid of the caller.
- msg_lrtime is set to the current time.
- 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.
ENVIRONMENT
The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the sysvipcd(8) daemon has to be running.
If the USR_SYSVIPC variable is set in a
process' environment, the process and its children will use the userspace
implementation.
ERRORS
The msgrcv() system call will fail if:
- [
EINVAL] - The msqid argument is not a valid message queue
identifier.
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_NOERRORflag 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.
- [
EAGAIN] - There is no message of the requested type available on the message queue,
and
IPC_NOWAITis set in msgflg.
SEE ALSO
HISTORY
Message queues appeared in the first release of AT&T System V UNIX.
AUTHORS
The DragonFly specific userspace implementation (see ENVIRONMENT) was written by Larisa Grigore.
BUGS
NetBSD, DragonFly,
and FreeBSD do not define the
EIDRM error value, which should be used in the case
of a removed message queue, nor the ENOMSG value,
which should be used when no suitable message is available and
IPC_NOWAIT is set.