NAME
msgrcv
—
receive a message from a message
queue
LIBRARY
library “libc”
SYNOPSIS
#include
<sys/msg.h>
ssize_t
msgrcv
(int
msqid, void *msgp,
size_t msgsz,
long msgtyp,
int msgflg);
DESCRIPTION
Themsgrcv
()
function receives a message from the message queue specified in
msqid, and places it into the user-defined structure
pointed to by msgp. This structure must contain a first
field of type
long that
will indicate the user-defined type of the message. The remaining fields will
contain the contents of the message. The following is an example of what this
user-defined structure might look like:
struct mymsg { long mtype; /* message type */ char mtext[1]; /* body of message */ };
mtype is an integer greater than 0 that can
be used to select messages. mtext is an array of
bytes, with size up to the system limit MSGMAX
.
The value of msgtyp has one of the following meanings:
- msgtyp is greater than 0. The first message of type msgtyp will be received.
- msgtyp is equal to 0. The first message on the queue will be received.
- msgtyp 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 argument msgsz specifies the size in
bytes of mtext. 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 behaviour of
msgrcv
()
depends on whether the IPC_NOWAIT
flag is set in
msgflg or not. If IPC_NOWAIT
is set, then msgrcv
() will immediately return a
value of -1 and set errno to
EAGAIN
. If IPC_NOWAIT
is not
set, the calling process will block 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
EIDRM
. - A signal is received and caught. -1 is returned and
errno is set to
EINTR
.
If a message is successfully received, the data structure associated with msqid is updated as follows:
- 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 and placed into the
mtext field of the structure pointed to by
msgp. Otherwise, -1 is returned, and
errno set to indicate the error.
ERRORS
msgrcv
() will fail if:
- [
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.
- [
EAGAIN
] - There is no message of the requested type available on the message queue,
and
IPC_NOWAIT
is set in msgflg. - [
EFAULT
] - msgp points to an invalid address.
- [
EIDRM
] - The message queue identifier msqid is removed from the system.
- [
EINTR
] - The system call was interrupted by the delivery of a signal.
- [
EINVAL
] - msqid 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 in it.msgsz is greater than
SSIZE_MAX
. - [
ENOMSG
] - The queue does not contain a message of the desired type and
IPC_NOWAIT
is set.
SEE ALSO
STANDARDS
The msgrcv
system call conforms to
X/Open System Interfaces and Headers Issue 5
(“XSH5”).
HISTORY
Message queues appeared in the first release of AT&T System V UNIX.