MSGRCV(2) BSD System Calls Manual MSGRCV(2)NAMEmsgrcv — receive a message from a message queue
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/msg.h>
ssize_t
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 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.
msgsz 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 behaviour of msgrcv() depends on whether the IPC_NOWAIT flag
is set in msgflg or not. If IPC_NOWAIT is set, then msgrcv() will imme‐
diately 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
into the mtext field of the structure pointed to by msgp. Otherwise, -1
is returned, and errno set to indicate the error.
ERRORSmsgrcv() 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 wait‐
ing 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 ALSOmsgctl(2), msgget(2), msgsnd(2)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.
BSD January 26, 2009 BSD
