how can i get the number of bytes available to read on async socket on linux?

Question

I have a simple tcp/ip server written in c++ on linux. I'm using asynchronous sockets and epoll. Is it possible to find out how many bytes are available for reading, when i get the EPOLLIN event?

Answer 1

From man 7 tcp:

int value;
error = ioctl(sock, FIONREAD, &value);

Or alternatively SIOCINQ, which is a synonym of FIONREAD.

Anyway, I'd recommend just to use recv in non-blocking mode in a loop until it returns EWOULDBLOCK.

UPDATE:

From your comments below I think that this is not the appropriate solution for your problem.

Imagine that your header is 8 bytes and you receive just 4; then your poll/select will return EPOLLIN, you will check the FIONREAD, see that the header is not yet complete and wayt for more bytes. But these bytes never arrive, so you keep on getting EPOLLIN on every call to poll/select and you have a no-op busy-loop. That is, poll/select are level-triggered. Not that an edge triggered function solves your problem either.

At the end you are far better doing a bit of work, adding a buffer per connection, and queuing the bytes until you have enough. It is not as difficult as it seems and it works far better. For example, something like that:

struct ConnectionData
{
    int sck;
    std::vector<uint8_t> buffer;
    size_t offset, pending;
};

void OnPollIn(ConnectionData *d)
{
    int res = recv(d->sck, d->buffer.data() + offset, d->pending);
    if (res < 0) 
        handle_error();
    d->offset += res;
    d->pending -= res;

    if (d->pending == 0)
        DoSomethingUseful(d);
}

And whenever you want to get a number of bytes:

void PrepareToRecv(ConnectionData *d, size_t size)
{
    d->buffer.resize(size);
    d->offset = 0;
    d->pending = size;
}