startwith.c

#include <stdio.h>
#include <string.h>
#include <assert.h>

int start_with(char const *str, char const *prefix){
  if (strlen(str) < strlen(prefix))
    return 0;
  return strstr(str, prefix) == str;
}

int main(int argc, char const* argv[])
{
  assert(start_with("abcdefg", "abc"));
  assert(!start_with("abcdefg", "123"));
  assert(!start_with("abc", "abcdefg"));
  assert(!start_with("123", "abcdefg"));
  return 0;
}