Question

How to determine if I can write the given number of bytes to a filehandle (socket actually)? (Alternatively, how to "unread" the data I had read from other filehandle?)

I want something like:

n = how_much_can_I_write(w_handle);
n = read(r_handle, buf, n);
assert(n==write(w_handle, buf, n));

Both filehandles (r_handle and w_handle) have received ready status from epoll_wait.

I want all data from r_handle to be copied to w_handle without using a "write debt" buffer.

In general, how to copy the data from one filehandle to the other simply and reliably?

@related How can I "interconnect" two sockets in Linux?

Answer 1

You can t do that - once the data is written, it s written - the operation is not reversible or predictable in advance. You need to rethink your program logic.

Answer 2

I don t think there s any interface that allows you access to that information, and it would be stale as soon as you got it anyway.

I d suggest setting both file descriptors to non-blocking, then reading/writing 1K (maybe larger) blocks until you get EAGAIN/EWOULDBLOCK, when you should cache one block until the next time the write fd is ready.

You need to have a buffer for doing the read/write cycle anyway, so keeping the buffer for the write-debt should be too much of a problem?

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