Integer overflow behavior in C

Thursday, Jan 2, 2020
C Security

As I continue to explore the C language, I realize that this is way different than anything else I have come across. When it comes down to what the rules are, many behaviours are machine dependent (size of data types for example). Surprisingly often, I also see the phrase undefined behavior. This means that the language does not define what happens should a specific situation occurs and it is upto the compiler to decide what should happen! This is very unsettling to me (you say there are no rules for this??). One such thing I came across today is integer overflow.

What it Integer Overflow?

Regardless of what type of data we deal with, it is represented in computer memory using finite number of bits. This means there is a maximum limit of a quantity for each type that can be stored. For any system, these limits can be seen by looking at the constants defined in limits.h header file as shown in the example below. The question is what happens when an arithmatic operation on a data pushes the result beyond what a specific type can accomodate. Guess what? We don't see any error. Most compilers apparently do a wrap around behavior. In the example below for intance, if UCHAR_MAX is 255 and you add 1 to it, you get 0 as result. Similarly if we subreact 1 from 0 (underflow?), we get 255.

Since there are no errors generated (I did not see any warnings either), the behavior can lead to bugs or vulnerabilities that might be difficult to pin down.

What I don't know (yet)

An interesting topic that needs some further thought!

A Sample Program

// int_overflow.c -- overflow, underflow with some typical integer types
#include <stdio.h>
#include <limits.h>
int main(void)
    printf("int min and max values are: %d, %d\n\n", INT_MIN, INT_MAX);
    printf("short min and max values are: %d, %d\n\n", SHRT_MIN, SHRT_MAX);
    printf("char min and max values are: %d, %d\n\n", CHAR_MIN, CHAR_MAX);
    printf("unsigned max value is: %d\n\n", UCHAR_MAX);

    int a = INT_MAX;
    printf("a is %14d\n", a);
    printf("a is %14d\n", a);
    printf("a is %14d\n\n", a);

    short b = SHRT_MAX;
    printf("b is %14hd\n", b);
    printf("b is %14hd\n", b);
    printf("b is %14hd\n\n", b);

    char c = CHAR_MAX;
    printf("c is %14hd\n", c);
    printf("c is %14hd\n", c);
    printf("c is %14hd\n\n", c);

    unsigned char d = UCHAR_MAX;
    printf("d is %14hd\n", d);
    printf("d is %14hd\n", d);
    printf("d is %14hd\n\n", d);

    return 0;


int min and max values are: -2147483648, 2147483647

short min and max values are: -32768, 32767

char min and max values are: -128, 127

unsigned max value is: 255

a is     2147483647
a is    -2147483648
a is     2147483647

b is          32767
b is         -32768
b is          32767

c is            127
c is           -128
c is            127

d is            255
d is              0
d is            255