What does golang's func (c *IPConn) Write(b []byte) (int, error)) return?

I guess that the int is the number of bytes written.

I think the function blocks until the buffer is fully written to the socket or the socket is closed so I think there's nothing to do be done with this number (unlike in a c socket where I would need to retry the write with the unwritten bytes).

I guess the only error that can be returned is in the event that the write fails because the socket is closed?

None of this seems to be in the documentation at https://golang.org/pkg/net/#IPConn.Write or am I looking in the wrong place?

> Package io
>
> import "io"
>
> type Writer
>
> type Writer interface {
> Write(p []byte) (n int, err error)
> }
>
> Writer is the interface that wraps the basic Write method.
>
> Write writes len(p) bytes from p to the underlying data stream. It
> returns the number of bytes written from p (0 <= n <= len(p)) and any
> error encountered that caused the write to stop early. Write must
> return a non-nil error if it returns n < len(p). Write must not modify
> the slice data, even temporarily.
>
> Implementations must not retain p.
>
> Package net
>
> import "net"
>
> type Conn
>
> type Conn interface {
> // Read reads data from the connection.
> // Read can be made to time out and return a Error with Timeout() == true
> // after a fixed time limit; see SetDeadline and SetReadDeadline.
> Read(b []byte) (n int, err error)
>
> // Write writes data to the connection.
> // Write can be made to time out and return a Error with Timeout() == true
> // after a fixed time limit; see SetDeadline and SetWriteDeadline.
> Write(b []byte) (n int, err error)
>
> // Close closes the connection.
> // Any blocked Read or Write operations will be unblocked and return errors.
> Close() error
>
> // LocalAddr returns the local network address.
> LocalAddr() Addr
>
> // RemoteAddr returns the remote network address.
> RemoteAddr() Addr
>
> // SetDeadline sets the read and write deadlines associated
> // with the connection. It is equivalent to calling both
> // SetReadDeadline and SetWriteDeadline.
> //
> // A deadline is an absolute time after which I/O operations
> // fail with a timeout (see type Error) instead of
> // blocking. The deadline applies to all future I/O, not just
> // the immediately following call to Read or Write.
> //
> // An idle timeout can be implemented by repeatedly extending
> // the deadline after successful Read or Write calls.
> //
> // A zero value for t means I/O operations will not time out.
> SetDeadline(t time.Time) error
>
> // SetReadDeadline sets the deadline for future Read calls.
> // A zero value for t means Read will not time out.
> SetReadDeadline(t time.Time) error
>
> // SetWriteDeadline sets the deadline for future Write calls.
> // Even if write times out, it may return n > 0, indicating that
> // some of the data was successfully written.
> // A zero value for t means Write will not time out.
> SetWriteDeadline(t time.Time) error
> }
>
> Conn is a generic stream-oriented network connection.
>
> Multiple goroutines may invoke methods on a Conn simultaneously.
>
> func (*IPConn) Write
>
> func (c *IPConn) Write(b []byte) (int, error)
>
> Write implements the Conn Write method.

It's an implementation of the io.Writer interface. Write writes len(p) bytes from p to the underlying data stream. It returns the number of bytes (n) written from p (0 <= n <= len(p)) and any > error (err) encountered that caused the write to stop early.

In particular, for the net.Conn interface, func (*IPConn) Write writes data to the connection. Write can be made to time out and return a Error with Timeout() == true after a fixed time limit; see SetDeadline and SetWriteDeadline.

It is for the timeout based on SetWriteDeadline. If it times out and had written some bytes you know how many were written.