go mismatched types uint64 and int32

I don't see what I'm doing wrong here with this error, both are of type syscall.Timeval Usec

Thanks

package common

import (
	"syscall"
)

func getUsecSince(oldTime syscall.Timeval) (result uint64) {
	now := syscall.Timeval{}
	syscall.Gettimeofday(&now)

	for now.Sec > oldTime.Sec {
		result += 1000000
		now.Sec--
	}
	return result + (now.Usec - oldTime.Usec)
}

> ./common.go:15: invalid operation: result + (now.Usec - oldTime.Usec) (mismatched types uint64 and int32)

Use a signed return value (int64), like Timeval.Sec and Timeval.Usec. Use TimevalToNsec for portability across operating systems. For example, Timeval fields may be int32 or int64. For a correct result, use,

package main

import (
	"fmt"
	"syscall"
	"time"
)

func getUsecSince(old syscall.Timeval) int64 {
	var now syscall.Timeval
	syscall.Gettimeofday(&now)
	nsecs := syscall.TimevalToNsec(now) - syscall.TimevalToNsec(old)
	return nsecs / int64(time.Microsecond)
}

func main() {
	old := syscall.Timeval{}
	syscall.Gettimeofday(&old)
	time.Sleep(2*time.Second + 42*time.Microsecond)
	fmt.Println(getUsecSince(old))
}

Output:

2000377

The simplest solution to this is:

func getUsecSince(oldTime syscall.Timeval) (result uint64) {
    now := syscall.Timeval{}
    syscall.Gettimeofday(&now)

    // Automatically ignore cases like 12.5 - 11.8
    result = uint64((now.Sec - oldTime.Sec) * 1000000 + int64(now.Usec - oldTime.Usec))
    return result
}

By converting to the smallest unit you can ignore the boundary conditions easily as long as there is no overflow during conversion.

Note that if you purposely test the above by using an oldTime in the future, the function will fail. You will need to do a check (with both times converted to Usec) if you want to cover such cases.

Timeval.Usec is defined as int32. Maybe result should be also int32? Alternatively if you want to use uint64 you can cast it by uint64(now.Usec - oldTime.Usec).

result is uint64.
The other operands are int32
Running this

now := syscall.Timeval{}
syscall.Gettimeofday(&now)
typeUsec := reflect.TypeOf(now.Usec)
fmt.Printf("type %s, value %d \n", typeUsec, now.Usec)

will print

type int32, value 238376 

Strangely, the go documents have the following

type Timeval struct {
    Sec  int64
    Usec int64

}