MSGRCV(2) System Calls MSGRCV(2)
NAME
msgrcv - message receive operation
SYNOPSIS
#include <sys/msg.h>
ssize_t msgrcv(
int msqid,
void *msgp,
size_t msgsz,
long int msgtyp,
int msgflg);
DESCRIPTION
The
msgrcv() function reads a message from the queue associated with
the message queue identifier specified by
msqid and places it in the
user-defined buffer pointed to by
msgp.
The
msgp argument points to a user-defined buffer that must contain
first a field of type
long int that will specify the type of the
message, and then 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:
struct mymsg {
long int mtype; /* message type */
char mtext[1]; /* message text */
}
The
mtype member is the received message's type as specified by the
sending process.
The
mtext member is the text of the message.
The
msgsz argument 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 non-zero. The truncated part of the
message is lost and no indication of the truncation is given to the
calling process.
The
msgtyp argument specifies the type of message requested as
follows:
o If
msgtyp is 0, the first message on the queue is
received.
o If
msgtyp is greater than 0, the first message of type
msgtyp is received.
o 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.
The
msgflg argument specifies which of the following actions is to be
taken if a message of the desired type is not on the queue:
o If (
msgflg&IPC_NOWAIT) is non-zero, the calling process
will return immediately with a return value of -1 and
errno set to
ENOMSG.
o If (
msgflg&IPC_NOWAIT) is 0, the calling process will
suspend execution until one of the following occurs:
o A message of the desired type is placed on the queue.
o The message queue identifier
msqid is removed from the
system (see
msgctl(2)); when this occurs,
errno is set
equal to
EIDRM and
-1 is returned.
o 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
sigaction(2).
Upon successful completion, the following actions are taken with
respect to the data structure associated with
msqid (see
Intro(2)):
o
msg_qnum is decremented by 1.
o
msg_lrpid is set equal to the process
ID of the calling
process.
o
msg_rtime is set equal to the current time.
RETURN VALUES
Upon successful completion,
msgrcv() returns a value equal to the
number of bytes actually placed into the buffer
mtext. Otherwise,
-1 is returned, no message is received, and
errno is set to indicate the
error.
ERRORS
The
msgrcv() function will fail if:
E2BIG The value of
mtext is greater than
msgsz and
(
msgflg&MSG_NOERROR) is 0.
EACCES Operation permission is denied to the calling process. See
Intro(2).
EIDRM The message queue identifier
msqid is removed from the
system.
EINTR The
msgrcv() function was interrupted by a signal.
EINVAL The
msqid argument is not a valid message queue identifier.
ENOMSG The queue does not contain a message of the desired type
and (
msgflg&IPC_NOWAIT) is non-zero.
The
msgrcv() function may fail if:
EFAULT The
msgp argument points to an illegal address.
USAGE
The value passed as the
msgp argument should be converted to type
void *.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
SEE ALSO
Intro(2),
msgctl(2),
msgget(2),
msgsnd(2),
sigaction(2),
attributes(7),
standards(7) May 19, 1999 MSGRCV(2)
