syscall: errno handling for write call may inappropriate

[changkun]

syscall write returns -1 and set errno is a trivial case. I am interested in the status of errno if write call returning zero or positive. The wrapper of Go simply returns err if errno is not zero, which also includes the case of write call returns positive.

go/src/syscall/zsyscall_linux_386.go

Line 1007 in 3cb64ea

if e1 != 0 {

However, the man page of write call roughly describes errno may also be set but unspecified if returns zero without explaining any detail.

Thus, the following cases seem unclear:

1. What is the status of errno if write call returning 0?
2. When and how write call returning 0 and errno is not 0?
3. What is the status of errno if write call returning positive?
4. Is there any other syscall may encounter the same situation?
5. How do above influences Go syscall wrapper?

[ianlancetaylor]

We don't use the issue tracker for questions. Please see https://golang.org/wiki/Questions . Thanks.

In reading the man page, you are confusing the behavior of the C library function write with the system call write. Go's syscall package invokes the system call directly. The system call in effect returns two values: the number of bytes written, and an error value. Those translate directly into the values returned by syscall.Write.

Closing because there is no action to take.

[changkun]

Hi @ianlancetaylor, thanks for your clarification. I think this issue points to the difference between C write call and Go syscall.Write, which is unclear for developers.

According to the man page, the 0 return is clearly defined in C write call for to files and for non-blocking sockets, but it's unclear whether there are non-error conditions for a blocking socket which would result in a write() not blocking, returning 0, and (presumably) possibly succeeding later if retried.

Indeed Go directly wraps system call write. However, the following code snippet seems not safe because written equals to zero is a case that may trigger err but we don't want to break the loop:

func writeAll(fd int, buffer []byte) bool {
	length := len(buffer)
	for length > 0 {
		written, err := syscall.Write(fd, buffer)
		if err != nil { // here
			return false
		}
		length -= written
		buffer = buffer[written:]
	}
	return true
}

[ianlancetaylor]

If you are speculating, please use a forum; see https://golang.org/wiki/Questions . If you are reporting a bug, please include a complete test program that we can use to reproduce the problem. Thanks.