MsgReplyv(), MsgReplyv_r()

Reply with a message


#include <sys/neutrino.h>

int MsgReplyv( int rcvid,
               int status,
               const iov_t* riov,
               int rparts );

int MsgReplyv_r( int rcvid,
                 int status,
                 const iov_t* riov,
                 int rparts );


The receive ID that MsgReceive*() returned when you received the message.
The status to use when unblocking the client's MsgSend*() call in the rcvid thread.

Note: The MsgSend*_r() functions return negative errno values to indicate failure, so you shouldn't pass a negative value for the status to MsgReply*(), because the MsgSend*_r() functions could interpret it as an error code. If you wish to indicate an error, call MsgError().

An array of buffers that contains the message that you want to reply with.
The number of elements in the array.



Use the -l c option to qcc to link against this library. This library is usually included automatically.


The MsgReplyv() and MsgReplyv_r() kernel calls reply with a message to the thread identified by rcvid. The thread being replied to must be in the REPLY-blocked state. Any thread in the receiving process is free to reply to the message, however, it may be replied to only once for each receive.

These functions are identical except in the way they indicate errors. See the Returns section for details.

The MsgSend*() in the rcvid thread unblocks with a return value of status.

The data is taken from the array of message buffers pointed to by riov. The number of elements in this array is given by rparts. The size of the message is the sum of the sizes of each buffer.

The number of bytes transferred is the minimum of that specified by both the replier and the sender. The reply data isn't allowed to overflow the reply buffer area provided by the sender.

The data transfer occurs immediately, and the replying task doesn't block. There's no need to reply to received messages in any particular order, but you must eventually reply to each message to allow the sending thread(s) to continue execution.

It's quite common to reply with two-part messages consisting of a fixed header and a buffer of data. The MsgReplyv() function gathers the data from the buffer list into a logical contiguous message and transfers it to the sender's reply buffer(s). The sender doesn't need to specify the same number or size of buffers. The data is laid down filling each buffer as required. The filesystem, for example, builds a reply list pointing into its cache in order to reply with what appears to be one contiguous piece of data.

Blocking states

None. In the network case, lower priority threads may run.


The only difference between the MsgReplyv() and MsgReplyv_r() functions is the way they indicate errors:

If an error occurs, -1 is returned and errno is set.
This function does NOT set errno. If an error occurs, the negative of a value from the Errors section is returned.


A fault occurred in the sender's address space when a server tried to access the sender's return message buffers.
The sum of the IOV lengths exceeds INT_MAX, or the number of parts exceeds 524288.
The thread indicated by rcvid doesn't exist, or is no longer REPLY-blocked on the channel, or the connection was detached.
A fault occurred when the kernel tried to access the buffers provided.


QNX Neutrino

Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes

See also:

MsgError(), MsgReceive(), MsgReceivev(), MsgReply(), MsgSend(), MsgSendv(), MsgWrite(), MsgWritev()

Message Passing chapter of Getting Started with QNX Neutrino