Validate max integer in scanf

Question

I want to read a int from stdin but I want to validate if the user exceeds the int max value. How can I do it?

int n;
scanf(\"%d\", &n);

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Answer 1

Two methods come to mind.

The first is useful if you know the input is integer-like only input, but suffers from "slightly" exceeding integer range:

long x;
if (1 != scanf("%ld", &x))
         error "not a number or other i/o error"
else
if (x < -MAXINT ||  x > MAXINT)
        error "exceeds integer range"
else    printf "it is an integer";

This has a lot of dependency on the compiler and platform. Note that the C language guarantees only that long is greater than or equal to the size of int. If this is run on a platform where long and int are the same size, it is a useless approach.

The second approach is to read the input as a string and parse straightforwardly:

char buf [1000];
if (1 != scanf ("%1000s", buf))
        error "i/o error";
size_t len = strspn (buf, "0123456789+-");
if (len != strlen (buf))
        error "contains invalid characters";
long  number = 0;
int   sign = 1;
for (char *bp = buf;  *bp;  ++bp)
{
          if (*bp == '-')
          {
               sign = -sign;
               continue;
          }
          if (*bp == '+')
               continue;
          number = number * 10  +  *bp - '0';
          if (number > MAXINT)
               error "number too large";
}
if (sign < 0)
         number = -number;

This code has several weaknesses: it depends on long being larger than int; it allows plus and minus to appear anywhere in the string. It could be easily extended to allow other than base ten numbers.

A possible third approach might be to input a string check the character set and use a double conversion to range check.