Integer types in C have both a size and a precision. The size indicates the number of bytes used by an object, and can be retrieved for any object or type using the sizeof
operator. The precision of an integer type is the number of bits it uses to represent values, excluding any sign and padding bits.
Padding bits contribute to the integer's size, but not to its precision. Consequently, inferring the precision of an integer type from its size may result in too large a value, which can then lead to incorrect assumptions about the numeric range of these types. Make sure you use correct integer precisions in your code, and in particular, do not use the sizeof
operator to compute the precision of an integer type on architectures that use padding bits or in strictly conforming (that is, portable) programs.
Noncompliant Code Example
This noncompliant code example illustrates a function that produces 2 raised to the power of the function argument. To prevent undefined behavior, in compliance with INT34-C. Do not shift a negative number of bits or more bits than exist in the operand, the function ensures that the argument is less than the number of bits used to store an unsigned int
.
#include <limits.h> unsigned int pow2(unsigned int exp) { if (exp >= sizeof(unsigned int) * CHAR_BIT) { /* Handle error */ } return 1 << exp; }
However, if this code runs on a platform where unsigned int
has one or more padding bits, it can still accept values for exp
that are too large. For example, a platform that stores unsigned int
in 64 bits, but uses only 48 bits to represent the value, could perform a left shift on an illegal value of 56.
Compliant Solution
This compliant solution uses a popcount()
function, which counts the number of bits set on any unsigned integer. This allows this code to determine the precision of any integer type, signed or unsigned.
#include <stddef.h> #include <stdint.h> /* Returns the number of set bits */ size_t popcount(uintmax_t num) { size_t precision = 0; while (num != 0) { if (num % 2 == 1) { precision++; } num >>= 1; } return precision; } #define PRECISION(umax_value) popcount(umax_value)
Actual implementations can replace the PRECISION()
macro with a type-generic macro that returns an integer constant expression that is the precision of the specified type for that implementation. This return value can then be used anywhere an integer constant expression can be used, such as in a static assertion (see DCL03-C. Use a static assertion to test the value of a constant expression). The following type generic macro. for example, might be used for a specific implementation targeting the IA-32 architecture:
#define PRECISION(value) _Generic(value, \ unsigned char : 8, \ unsigned short: 16, \ unsigned int : 32, \ unsigned long : 32, \ unsigned long long : 64, \ signed char : 7, \ signed short : 15, \ signed int : 31, \ signed long : 31, \ signed long long : 63)
The revised version of the pow2()
function uses the PRECISION()
macro to determine the precision of the unsigned type.
#include <stddef.h> #include <stdint.h> #include <limits.h> extern size_t popcount(uintmax_t); #define PRECISION(umax_value) popcount(umax_value) unsigned int pow2(unsigned int exp) { if (exp >= PRECISION(UINT_MAX)) { /* handle error */ } return 1 << exp; }
Implementation Details
Some platforms, such as the Cray Linux Environment (CLE) provide a _popcnt
instruction which can substitute for our popcount()
function.
#define PRECISION(umax_value) _popcnt(umax_value)
Risk Assessment
Mistaking an integer size for its precision can permit invalid precision arguments to operations such as bitwise shifts, resulting in undefined behavior.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
INT35-C | low | unlikely | medium | P2 | L3 |
Bibliography
[Dowd 2006] | Chapter 6, "C Language Issues" |
[C99 Rationale 2003] | Subclause 6.5.7, "Bitwise Shift Operators" |
Cray Linux Environment (CLE): supported on Cray XT CNL compute nodes |