MsgReadv(), MsgReadv_r()

Read data from a message


#include <sys/neutrino.h>

int MsgReadv( int rcvid,
              const iov_t* riov,
              int rparts,
              int offset );

int MsgReadv_r( int rcvid,
                const iov_t* riov,
                int rparts,
                int offset );


The value returned by MsgReceive*() when you received the message.
An array of buffers where the functions can store the data.
The number of elements in the riov array.
An offset into the thread's send message that indicates where you want to start reading the data.



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


The MsgReadv() and MsgReadv_r() kernel calls read data from a message sent by a thread identified by rcvid. The thread being read from must not have been replied to and will be in the REPLY-blocked state. The state of the sending thread doesn't change. Any thread in the receiving process is free to read the message.

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

If the sending and calling threads are on the same node, the data transfer occurs immediately and the calling thread doesn't block (see "Blocking states," below). The state of the sending thread doesn't change.

An attempt to read past the end of the thread's message results in fewer bytes returned than requested.

You'll use these functions in these situations:

When you're finished using MsgReadv(), you must use MsgReply*() to ready the REPLY-blocked process and complete the message exchange.

Blocking states

None for the local case. In the network case:

The calling thread is waiting for a network operation to complete. The calling thread is marked as REPLY-blocked on itself (the same process ID as the thread making the call).


The number of bytes read. This may differ from the total number of requested bytes, which is the sum of the lengths of the iov_t entries of the riov array. If this sum is larger than the thread's message (and hence, the caller requested to read past the end of the message), the functions return the message size. If this sum is not larger than the message, the functions return the requested number of bytes.

If an error occurs:


A deadlock occurred. You can avoid a deadlock by setting the _NTO_CHF_MSG_PAUSING flag when you create a channel; for more information, see ChannelCreate() and MsgPause().
A fault occurred in a server's address space when the kernel tried to access the server's 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, 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.
A kernel timeout unblocked the call. See TimerTimeout().


QNX Neutrino

Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes