Input validation of an Integer using atoi()

Question

#include \"stdafx.h\"
#include 

void main()
{
    char buffer[20];
    int num;

    printf(\"Please enter a number\\n\");
    fgets(buffer, 20, std         


        

Answer 1

atoi is not suitable for error checking. Use strtol or strtoul instead.

#include <errno.h>
#include <limits.h>
#include <stdlib.h>
#include <string.h>

long int result;
char *pend;

errno = 0;
result = strtol (buffer, &pend, 10);

if (result == LONG_MIN && errno != 0) 
{
  /* Underflow. */
}

if (result == LONG_MAX && errno != 0) 
{
  /* Overflow. */
}

if (*pend != '\0') 
{
    /* Integer followed by some stuff (floating-point number for instance). */
}

Answer 2

There is the isdigit function that can help you check each character:

#include <ctype.h>

/* ... */

for (i=0; buffer[i]; i++) {
        if (!isdigit(buffer[i])) {
            printf("Bad\n");
            break;
        }
}