This is a relatively complex piece of code, and it would be hard to explain
what each and every line of code does on a forum. Tracing the code would be
the fastest way to understand.
For the second question, tcp_write is used through the
grpc_endpoint_vtable. Effectively, grpc_endpoint_write calls would
translate to tcp_write.
For the third question, those functions ARE invoked through closures.
On Tuesday, August 13, 2019 at 2:05:45 AM UTC-7 xia rui wrote:
> Hello, everyone.
> I am trying to learn the code of `tcp_posix.cc`. There are some functions
> with similar names. I am confused about the purposes/use cases of these
> functions.
>
> I write a simple grpc app, and use GRPC_TRACE=tcp and GRPC_VERBOSITY=ERROR
> (only show the breakpoints I am interested in).
>
> I try to hack the code from the client, so I only focus on write
> operations.
>
> Things start from `tcp_write()`, after preparing writing buffer with:
> tcp->outgoing_buffer = buf;
> tcp->outgoing_byte_idx = 0;
> it goes to `tcp_flush()`.
>
> In `tcp_flush`, the writing operation is recursively invoked in a for
> loop. It first pushes `tcp->outgoing_buffer` to `iov` (It may be simple
> pointer copy, not memory copy). Then, the `tcp_send()` is invoked, which in
> fact sends buffer using socket. Finally, `tcp_flush` function counts the
> `sending_length` and `sent_length`.
> *My first question is: what is the purpose of `sending_length`?*
> The `sent_length` is the bytes sent by `tcp_send()` function. There is a
> `trailing` variable, which is `trailing = sending_length -
> static_cast<size_t>(sent_length);` and I don't know what is the purpose of
> the following check in `trailing`:
>
> while (trailing > 0) {
> size_t slice_length;
>
>
> outgoing_slice_idx--;
> slice_length =
> GRPC_SLICE_LENGTH(tcp->outgoing_buffer->slices[
> outgoing_slice_idx]);
> if (slice_length > trailing) {
> tcp->outgoing_byte_idx = slice_length - trailing;
> break;
> } else {
> trailing -= slice_length;
> }
> }
>
> When `tcp_flush()` returns, the writing opertion is finished.
> *My second question is: Who invoke `tcp_write()`?*
> There is no explicit call on `tcp_write()` in `tcp_posix.cc`. And at the
> end of `tcp_write()`, there is a callback:
>
> if (!tcp_flush(tcp, &error)) {
> TCP_REF(tcp, "write");
> tcp->write_cb = cb;
> if (GRPC_TRACE_FLAG_ENABLED(grpc_tcp_trace)) {
> gpr_log(GPR_INFO, "write: delayed");
> }
> notify_on_write(tcp);
> } else {
> if (GRPC_TRACE_FLAG_ENABLED(grpc_tcp_trace)) {
> const char* str = grpc_error_string(error);
> gpr_log(GPR_INFO, "write: %s", str);
> gpr_log(GPR_ERROR, "in tcp_write(), maybe finish writing");
> }
>
> *GRPC_CLOSURE_SCHED**(cb, error);*
> }
> the callback `cb` is a argument of `tcp_write()`, and I was wondering who
> calls `tcp_write()` and what is the callback `cb`.
>
> There are some functions that related to writing operations, and they are
> not invoked in my tracing log.
> *My third question is: what are the purposes of these uninvoked functions?*
>
> 1. static void notify_on_write(grpc_tcp* tcp)
> 2. static void tcp_handle_write(void* arg /* grpc_tcp */, grpc_error*
> error)
>
>
> Thank you for your time.
>
> Best wishes,
> Xia Rui
>
>
>
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