msgrcv(2)msgrcv(2)NAMEmsgrcv - Receive a message from a message queue
SYNOPSIS
#include <sys/msg.h>
ssize_t msgrcv(
int msqid,
void *msgp,
size_t msgsz,
long msgtyp,
int msgflg );
Application developers may want to specify #include statements for
<sys/types.h> and <sys/ipc.h> before the one for <sys/msg.h> if pro‐
grams are being developed for multiple platforms. The additional
#include statements are not required on Tru64 UNIX systems or by ISO or
XSH standards, but may be required on other vendors' systems that con‐
form to these standards.
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
msgrcv(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Specifies the ID of the message queue from which to receive the mes‐
sage. Specifies a pointer to the msgbuf structure that is to receive
the message. See the NOTES section for more information. Specifies
the maximum number of bytes allowed for the received data. The msgsz
parameter is only used to specify the size of the msgbuf.mtext field.
It cannot include the msgbuf.mtype field or else the msgbuf buffer is
overwritten. Specifies the message type to read from the queue. Spec‐
ifies the action to be taken by the kernel if there are no msgtyp mes‐
sages on the queue.
DESCRIPTION
The msgrcv() function receives a message from the queue associated with
the msqid parameter. The function returns the number of bytes in the
received message.
The msgp parameter points to a user-defined msgbuf structure. This
structure receives the message read from the queue.
The msgsz parameter specifies the maximum size allowed for the received
data. If the message is longer than msgsz, the following occurs,
depending upon the setting of the MSG_NOERROR flag: If MSG_NOERROR is
not set, the kernel returns an [E2BIG] error to the calling process and
leaves the message on the queue. If MSG_NOERROR is set, the kernel
truncates the message to msgsz and discards the truncated portion with‐
out notifying the calling process.
The msgtyp parameter specifies the message type that the process
expects to receive. Possible values and their results are as follows:
The process receives the message at the head of the queue. The process
receives the first message of the requested positive-integer type. The
process receives the first message of the lowest type on the queue. To
qualify as the lowest type, the negative-integer type must be less than
or equal to the absolute value of msgtyp.
The msgflg parameter specifies the action that the kernel should take
if the queue does not contain a message of the requested type. Either
of two kernel actions can be specified, as follows: If IPC_NOWAIT is
set, the function returns immediately with a return value of
(ssize_t)-1 and sets errno to [ENOMSG]. If IPC_NOWAIT is not set, the
kernel suspends the calling process.
The process remains suspended until one of the following occurs:
A message of the requested type appears on the queue. In this
case, the kernel wakes the process to receive the message. The
specified message queue ID is removed from the system. In this
case, the kernel sets errno to [EIDRM] and returns (ssize_t)-1
to the calling process. The process catches a signal. In this
case, the process does not receive the message and, instead,
resumes execution as directed by the sigaction() call.
NOTES
The parameter msgp points to a user-specified buffer that must contain
first a field of type long int that specifies the type of the message,
and then a data portion that holds the data bytes of the message. For
example:
struct msgbuf {
long int mtype;
char mtext[]; }
The mytpe field is set to the message type supplied by the sender.
The mtext field is set to the message text. Unless MSG_NOERROR is set,
the message size will be less than or equal to the msgsz specified on
the call to msgrcv().
RETURN VALUES
Upon successful completion, the msgrcv() function returns a value equal
to the number of bytes actually stored in mtext. Also, the kernel
updates the msqid_ds structure associated with the message queue ID as
follows: Decrements msg_qnum by 1. Sets msg_lrpid equal to the process
ID of the calling process. Sets msg_rtime equal to the current time.
[Tru64 UNIX] Decrements msg_cbytes by the message text size.
When the msgrcv() function fails, it returns a value of (ssize_t)-1 and
sets errno to indicate the error.
ERRORS
The msgrcv() function sets errno to the specified values for the fol‐
lowing conditions: The number of bytes to be received in mtext is
greater than msgsz and the MSG_NOERROR flag is not set. The calling
process does not have permission for the operation. Indicates that the
requested address is in some way invalid, for example, out of bounds.
The message queue identified by the msqid parameter has been removed
from the system. The operation was interrupted by a signal. The msqid
parameter is not a valid message queue ID. The queue does not contain
a message of the requested type and the IPC_NOWAIT flag is set.
SEE ALSO
Functions: msgctl(2), msgget(2), msgsnd(2), sigaction(2)
Data Structures: msqid_ds(4)
Standards: standards(5)msgrcv(2)