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?