MSGRCV(2)

NAME
msgrcv - receive a message from a message queue

LIBRARY
libc

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.

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
msgctl(2), msgget(2), msgsnd(2)

STANDARDS
The msgrcv system call conforms to xsh5.

HISTORY
Message queues appeared in the first release of AT&T UNIX.