Bit Manipulation (The GNU C Library)

## 21 Bit Manipulation ¶

This chapter contains information about functions and macros for determining the endianness of integer types and manipulating the bits of unsigned integers. These functions and macros are from ISO C23 and are declared in the header file `stdbit.h`.

The following macros describe the endianness of integer types. They have values that are integer constant expressions.

Macro: __STDC_ENDIAN_LITTLE__ ¶

  This macro represents little-endian storage.

Macro: __STDC_ENDIAN_BIG__ ¶

  This macro represents big-endian storage.

Macro: __STDC_ENDIAN_NATIVE__ ¶

  This macro equals `__STDC_ENDIAN_LITTLE__` if integer types are stored in memory in little-endian format, and equals `__STDC_ENDIAN_BIG__` if integer types are stored in memory in big-endian format.

The following functions manipulate the bits of unsigned integers. Each function family has functions for the types `unsigned char`, `unsigned short`, `unsigned int`, `unsigned long int` and `unsigned long long int`. In addition, there is a corresponding type-generic macro (not listed below), named the same as the functions but without any suffix such as ‘`_uc`’. The type-generic macro can only be used with an argument of an unsigned integer type with a width of 8, 16, 32 or 64 bits, or when using a compiler with support for `__builtin_stdc_bit_ceil`, etc., built-in functions such as GCC 14.1 or later any unsigned integer type those built-in functions support. In GCC 14.1 that includes support for `unsigned __int128` and `unsigned _BitInt(n)` if supported by the target.

Function: `unsigned int` stdc_leading_zeros_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_leading_zeros_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_leading_zeros_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_leading_zeros_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_leading_zeros_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_leading_zeros` functions count the number of leading (most significant) zero bits in x, starting from the most significant bit of the argument type. If x is zero, they return the width of x in bits.

Function: `unsigned int` stdc_leading_ones_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_leading_ones_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_leading_ones_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_leading_ones_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_leading_ones_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_leading_ones` functions count the number of leading (most significant) one bits in x, starting from the most significant bit of the argument type.

Function: `unsigned int` stdc_trailing_zeros_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_trailing_zeros_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_trailing_zeros_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_trailing_zeros_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_trailing_zeros_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_trailing_zeros` functions count the number of trailing (least significant) zero bits in x, starting from the least significant bit of the argument type. If x is zero, they return the width of x in bits.

Function: `unsigned int` stdc_trailing_ones_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_trailing_ones_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_trailing_ones_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_trailing_ones_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_trailing_ones_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_trailing_ones` functions count the number of trailing (least significant) one bits in x, starting from the least significant bit of the argument type.

Function: `unsigned int` stdc_first_leading_zero_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_first_leading_zero_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_first_leading_zero_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_first_leading_zero_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_first_leading_zero_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_first_leading_zero` functions return the position of the most significant zero bit in x, counting from the most significant bit of x as 1, or zero if there is no zero bit in x.

Function: `unsigned int` stdc_first_leading_one_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_first_leading_one_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_first_leading_one_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_first_leading_one_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_first_leading_one_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_first_leading_one` functions return the position of the most significant one bit in x, counting from the most significant bit of x as 1, or zero if there is no one bit in x.

Function: `unsigned int` stdc_first_trailing_zero_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_first_trailing_zero_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_first_trailing_zero_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_first_trailing_zero_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_first_trailing_zero_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_first_trailing_zero` functions return the position of the least significant zero bit in x, counting from the least significant bit of x as 1, or zero if there is no zero bit in x.

Function: `unsigned int` stdc_first_trailing_one_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_first_trailing_one_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_first_trailing_one_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_first_trailing_one_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_first_trailing_one_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_first_trailing_one` functions return the position of the least significant one bit in x, counting from the least significant bit of x as 1, or zero if there is no one bit in x.

Function: `unsigned int` stdc_count_zeros_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_count_zeros_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_count_zeros_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_count_zeros_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_count_zeros_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_count_zeros` functions count the number of zero bits in x.

Function: `unsigned int` stdc_count_ones_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_count_ones_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_count_ones_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_count_ones_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_count_ones_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_count_ones` functions count the number of one bits in x.

Function: `_Bool` stdc_has_single_bit_uc `(unsigned char x)` ¶
Function: `_Bool` stdc_has_single_bit_us `(unsigned short x)` ¶
Function: `_Bool` stdc_has_single_bit_ui `(unsigned int x)` ¶
Function: `_Bool` stdc_has_single_bit_ul `(unsigned long int x)` ¶
Function: `_Bool` stdc_has_single_bit_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_has_single_bit` functions return whether x has exactly one bit set to one.

Function: `unsigned int` stdc_bit_width_uc `(unsigned char x)` ¶
Function: `unsigned int` stdc_bit_width_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_bit_width_ui `(unsigned int x)` ¶
Function: `unsigned int` stdc_bit_width_ul `(unsigned long int x)` ¶
Function: `unsigned int` stdc_bit_width_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_bit_width` functions return the minimum number of bits needed to store x, not counting leading zero bits. If x is zero, they return zero.

Function: `unsigned char` stdc_bit_floor_uc `(unsigned char x)` ¶
Function: `unsigned short` stdc_bit_floor_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_bit_floor_ui `(unsigned int x)` ¶
Function: `unsigned long int` stdc_bit_floor_ul `(unsigned long int x)` ¶
Function: `unsigned long long int` stdc_bit_floor_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_bit_floor` functions return the largest integer power of two that is less than or equal to x. If x is zero, they return zero.

Function: `unsigned char` stdc_bit_ceil_uc `(unsigned char x)` ¶
Function: `unsigned short` stdc_bit_ceil_us `(unsigned short x)` ¶
Function: `unsigned int` stdc_bit_ceil_ui `(unsigned int x)` ¶
Function: `unsigned long int` stdc_bit_ceil_ul `(unsigned long int x)` ¶
Function: `unsigned long long int` stdc_bit_ceil_ull `(unsigned long long int x)` ¶

  Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `stdc_bit_ceil` functions return the smallest integer power of two that is greater than or equal to x. If this cannot be represented in the return type, they return zero.