| MSGRCV(2) | System Calls Manual | MSGRCV(2) | 
msgrcv —
#include <sys/msg.h>
ssize_t
  
  msgrcv(int
    msqid, void *msgp,
    size_t msgsz,
    long msgtyp,
    int msgflg);
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:
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:
EIDRM.EINTR.If a message is successfully received, the data structure associated with msqid is updated as follows:
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.
msgrcv() will fail if:
E2BIG]MSG_NOERROR
      flag was not set in msgflg.EACCES]EAGAIN]IPC_NOWAIT is set in
      msgflg.EFAULT]EIDRM]EINTR]EINVAL]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]IPC_NOWAIT is set.msgrcv system call conforms to
  X/Open System Interfaces and Headers Issue 5
  (“XSH5”).
| July 24, 2013 | NetBSD 9.3 |