Why passing Go bytes to C, the bytes length doesn't match in cgo in this code below?

When passing golang bytes to C, the bytes length doesn't match.

The resulting strlen(key) and keylen doesnt match up.

Build with "go build file.go"

You can download the go file below here:
https://pastebin.com/raw/hnMfXJKq
<- does anyone know why my cgo call the []bytes to c has error? why the strlen doesnt match?

Expected output is supposed to be the same key length. Sometimes work, sometimes doesn't.

package main
/*
#include <stddef.h>
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <assert.h>
void replaceK(void *key, size_t keylen, void *value, size_t valuelen);
void replaceK(void *key, size_t keylen, void *value, size_t valuelen)
{
printf("replaceK : key = %s = %lu = %lu = %s = %lu\n",(char*) key,keylen,strlen(key),(char*) value,valuelen);
if (keylen != strlen(key)){
printf("ERROR!!! keylen : %lu != strlen(key) : %lu!!!\n",keylen,strlen(key));
exit(1);
}
}
*/
import "C"
import (
"fmt"
"unsafe"
"math/rand"
"time"
)
func Set(key,value []byte) {
cKey := cByteSlice(key)
cValue := cByteSlice(value)
C.replaceK(unsafe.Pointer(cKey),C.ulong(len(key)),unsafe.Pointer(cValue),C.ulong(len(value)))
C.free(unsafe.Pointer(cKey))
C.free(unsafe.Pointer(cValue))
}
func byteToChar(b []byte) *C.char {
var c *C.char
if len(b) > 0 {
c = (*C.char)(unsafe.Pointer(&b[0]))
}
return c
}
var letterRunes = []byte("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ")
func RandStringRunes(n int) []byte {
randNum := rand.Intn(n)+1
b := make([]byte, randNum)
for i := range b {
b[i] = letterRunes[rand.Intn(len(letterRunes))]
}
return b
}
func cByteSlice(b []byte) *C.char {
var c *C.char
if len(b) > 0 {
c = (*C.char)(C.CBytes(b))
}
return c
}
func main() {
rand.Seed(time.Now().UnixNano())
var key []byte
var value []byte
for i:=0;i<10000000;i++ {
key = RandStringRunes(10)
value = RandStringRunes(20)
randnum := 1
if randnum == 1 {
fmt.Printf(">>> = %s = %s\n",key, value)
Set(key,value)
}
}
}                                                                                  

The C strlen function is for use with null-terminated strings, not pointer+length strings.

You can print a pointer+length string with the C printf function using %.*s instead of %s. Since Go string and []byte variables both use the pointer+length encoding, that is probably the right way to print a string obtained from Go.