lua_tostring adding a hyphen to the end of a string?

Question

This function:

static int function_name(lua_State *L) {
  const char* str = lua_tostring(L, 1);
  const char* substr = lua_tostring(L, 2); // passing "how" results in "how-" ???

  char *pch = strstr(str, substr);

  if (pch != NULL)
    lua_pushinteger(L, pch - str);
  else
    lua_pushnil(L);

  return 1;
}

Invoking the function (i.e, function_name("hello how world", "how") produces this result. Why would lua_tostring return such an obfuscated result? This issue only occurs when I pass a string equal to the length of the substring I want to match to. For example

"hello how world", "hell" -> works fine
"hello how world", "hello" -> fails, thinks "hello-" is passed. 
"hello how world", "ho" -> works fine
"hello how world", "how" -> fails, thinks "how-" is passed.

I'm printing substr immediately after I pull it from the stack to observe results. I don't believe any middle-men could mess it up.

Answer 1

Using PUC Lua version lua-5.4.0 compiled for X86-64 with MinGW, I registered the C function function_name to the Lua function TEST.

print(TEST("hello how world", "hell"))
print(TEST("hello how world", "hello")) 
print(TEST("hello how world", "ho"))
print(TEST("hello how world", "how"))

The results seems correct:

0
0
6
6

If you are facing strange data corruption during the runtime of your program, you might have an pointer issue in some other places of your program, possibility in another thread.