Question

I’m sending data asynchronously to TCP socket. Is it valid to send the next data piece before the previous one was reported as sent by completion handler?

As I know it’s not allowed when sending is done from different threads. In my case all sending are done from the same thread.

Different modules of my client send data to the same socket. E.g. module1 sent some data and will continue when corresponding completion handler is invoked. Before this io_service invoked deadline_timer handler of module2 which leads to another async_write call. Should I expect any problems here?

Answer 1

Is it valid to send the next data piece before the previous one was
reported as sent by completion handler?

No it is not valid to interleave write operations. This is very clear in the documentation

This operation is implemented in terms of zero or more calls to the
stream’s async_write_some function, and is known as a composed
operation. The program must ensure that the stream performs no other
write operations (such as async_write, the stream’s async_write_some
function, or any other composed operations that perform writes) until
this operation completes.

emphasis added by me.

As I know it’s not allowed when sending is done from different
threads. In my case all sending are done from the same thread.

Your problem has nothing to do with threads.