How to test input is sane

Question

Consider the following simple C program.

//C test

#include

int main()
{
   int a, b, c;

   printf(\"Enter two numbers to add\\n\");
   scan         


        

Answer 1

User input is evil. Parse per:

(optional whitespace)[decimal int][whitespace][decimal int](optional whitespace)

strtol() and family have better error handling than scanf().
Coda: Best to handle user input in a helper function. Break into 2 parts: I/O and parsing.

#include 
#include 
#include 
#include 
#include 

// return 1 (success), -1 (EOF/IOError) or 0 (conversion failure)
int Readint(const char *prompt, int *dest, size_t n) {
  char buf[n * 21 * 2]; // big enough for `n` 64-bit int and then 2x

  fputs(prompt, stdout);  // do not use printf here to avoid UB
  fflush(stdout); // per @OP suggestion
  if (fgets(buf, sizeof buf, stdin) == NULL) {
    return -1;
  }
  const char *p = buf;

  while (n-- > 0) {
    char *endptr;
    errno = 0;
    long l = strtol(p, &endptr, 10);
    if (errno || (p == endptr) || (l < INT_MIN) || (l > INT_MAX)) {
      return 0;
    }
    *dest++ = (int) l;
    p = endptr;
  }

  // Trailing whitespace OK
  while (isspace((unsigned char) *p)) p++;
  // Still more text
  if (*p) return 0;
  return 1;
}

int main() {  // for testing
  int Result;
  do {
    int dest[2] = { -1 };
    Result = Readint("Enter two numbers to add\n", dest, 2);
    printf("%d %d %d\n", Result, dest[0], dest[1]);
  } while (Result >= 0);
  return 0;
}