Is unsigned integer subtraction defined behavior?

by

When you work with unsigned types, modular arithmetic (also known as “wrap around” behavior) is taking place. To understand this modular arithmetic, just have a look at these clocks:

enter image description here

9 + 4 = 1 (13 mod 12), so to the other direction it is: 1 – 4 = 9 (-3 mod 12). The same principle is applied while working with unsigned types. If the result type is unsigned, then modular arithmetic takes place.

Now look at the following operations storing the result as an unsigned int:

unsigned int five = 5, seven = 7;
unsigned int a = five - seven;      // a = (-2 % 2^32) = 4294967294 

int one = 1, six = 6;
unsigned int b = one - six;         // b = (-5 % 2^32) = 4294967291

When you want to make sure that the result is signed, then stored it into signed variable or cast it to signed. When you want to get the difference between numbers and make sure that the modular arithmetic will not be applied, then you should consider using abs() function defined in stdlib.h:

int c = five - seven;       // c = -2
int d = abs(five - seven);  // d =  2

Be very careful, especially while writing conditions, because:

if (abs(five - seven) < seven)  // = if (2 < 7)
    // ...

if (five - seven < -1)          // = if (-2 < -1)
    // ...

if (one - six < 1)              // = if (-5 < 1)
    // ...

if ((int)(five - seven) < 1)    // = if (-2 < 1)
    // ...

but

if (five - seven < 1)   // = if ((unsigned int)-2 < 1) = if (4294967294 < 1)
    // ...

if (one - six < five)   // = if ((unsigned int)-5 < 5) = if (4294967291 < 5)
    // ...

More Related Contents:

  1. C – is any use of unsigned int just terrible coding practice? [closed]
  2. Are there machines, where sizeof(char) != 1, or at least CHAR_BIT > 8?
  3. Are compound statements (blocks) surrounded by parens expressions in ANSI C?
  4. Effects of the extern keyword on C functions
  5. Is an empty initializer list valid C code?
  6. Does printf(“%x”,1) invoke undefined behavior?
  7. Can a conforming C implementation #define NULL to be something wacky
  8. bit shifting with unsigned long type produces wrong results
  9. How do you store an arbitrarily large integer value in memory?
  10. Why is a negative int greater than unsigned int? [duplicate]
  11. Declare main prototype
  12. How to printf “unsigned long” in C?
  13. Difference between unsigned and unsigned int in C
  14. Difference between size_t and unsigned int?
  15. Unsigned hexadecimal constant in C?
  16. Unsigned values in C
  17. what is the unsigned datatype?
  18. What does -fwrapv do?
  19. signed and unsigned arithmetic implementation on x86
  20. Bitshift and integer promotion?
  21. C Unsigned int providing a negative value?
  22. C reverse bits in unsigned integer
  23. Standard C library in mingW
  24. No defined type of a function parameter defaults to int? Am I insane?
  25. Why is int rather than unsigned int used for C and C++ for loops?
  26. List of platforms supported by the C standard
  27. Difference between scanf() and strtol() / strtod() in parsing numbers
  28. Why both clang and gcc only give a warning when there is a space after backslash if C standard says that whitespace is forbidden?
  29. double negation in C : is it guaranteed to return 0/1?
  30. Is a compiler allowed to add functions to standard headers?

Leave a Comment