To check for integer overflow, you can't simply check the *outcome* of an operation, you need to check *before* the operation. So how do you do that? This example checks for overflow with multiplication.

```
#include<stdio.h>
#include<limits.h>
int foo(int a, int b, int *result)
{
if ( a > 0 && b > 0 )
{
/*
* 'a' is positive, 'b' is positive, product will be positive.
* See if one positive value 'b' is more than the most positive int
* divided by the other positive value 'a' (positive quotient).
*/
if ( b > INT_MAX / a )
{
return -1;
}
}
else if ( a > 0 && b < 0 )
{
/*
* 'a' is positive, 'b' is negative, product will be negative.
* See if the negative value 'b' is less than the most negative int
* divided by the positive value 'a' (negative quotient).
*/
if ( b < INT_MIN / a )
{
return -1;
}
}
else if ( a < 0 && b > 0 )
{
/*
* 'a' is positive, 'b' is negative, product will be negative.
* See if the negative value 'a' is less than the most negative int
* divided by the positive value 'b' (negative quotient).
*/
if ( a < INT_MIN / b )
{
return -1;
}
}
else /* if (a < 0 && b < 0) */
{
/*
* 'a' is negative, 'b' is negative, product will be positive.
* See if one negative value 'b' is less than the most positive int
* divided by the other negative value 'a' (negative quotient).
*/
if ( b < INT_MAX / a )
{
return -1;
}
}
*result = a * b;
return 0;
}
void bar(int a, int b)
{
int product;
printf("a = %d, b = %d, ", a, b);
if ( foo(a, b, &product) < 0 )
{
puts("overflow warning");
}
else
{
printf("a * b = %d\n", a * b);
}
}
int main(void)
{
int a, b;
printf("INT_MAX = %d\n", INT_MAX);
printf("INT_MIN = %d\n", INT_MIN);
for ( a = 10, b = 5; a < INT_MAX / 10; a *= 10, b *= 10 )
{
bar(a,b);
}
for ( a = 10, b = -5; a < INT_MAX / 10; a *= 10, b *= 10 )
{
bar(a,b);
}
for ( a = -10, b = 5; a > INT_MIN / 10; a *= 10, b *= 10 )
{
bar(a,b);
}
for ( a = -10, b = -5; a > INT_MIN / 10; a *= 10, b *= 10 )
{
bar(a,b);
}
return 0;
}
/* my output
INT_MAX = 2147483647
INT_MIN = -2147483648
a = 10, b = 5, a * b = 50
a = 100, b = 50, a * b = 5000
a = 1000, b = 500, a * b = 500000
a = 10000, b = 5000, a * b = 50000000
a = 100000, b = 50000, overflow warning
a = 1000000, b = 500000, overflow warning
a = 10000000, b = 5000000, overflow warning
a = 100000000, b = 50000000, overflow warning
a = 10, b = -5, a * b = -50
a = 100, b = -50, a * b = -5000
a = 1000, b = -500, a * b = -500000
a = 10000, b = -5000, a * b = -50000000
a = 100000, b = -50000, overflow warning
a = 1000000, b = -500000, overflow warning
a = 10000000, b = -5000000, overflow warning
a = 100000000, b = -50000000, overflow warning
a = -10, b = 5, a * b = -50
a = -100, b = 50, a * b = -5000
a = -1000, b = 500, a * b = -500000
a = -10000, b = 5000, a * b = -50000000
a = -100000, b = 50000, overflow warning
a = -1000000, b = 500000, overflow warning
a = -10000000, b = 5000000, overflow warning
a = -100000000, b = 50000000, overflow warning
a = -10, b = -5, a * b = 50
a = -100, b = -50, a * b = 5000
a = -1000, b = -500, a * b = 500000
a = -10000, b = -5000, a * b = 50000000
a = -100000, b = -50000, overflow warning
a = -1000000, b = -500000, overflow warning
a = -10000000, b = -5000000, overflow warning
a = -100000000, b = -50000000, overflow warning
*/
```