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stdc_bit_width (3) - glibc

Split from the glibc Bit Manipulation chapter (stdbit.h) into the per-function layout; see the method page and the glibc ledger for coverage status.

FieldValue
Headerstdbit.h (ISO C23 7.18, /usr/include/stdbit.h; __STDC_VERSION_STDBIT_H__ 202311L at line 36)
StandardsISO C23 (7.18); out-of-line symbols since glibc 2.39
Exportedstdc_bit_width_uc/us/ui/ul/ull all @@GLIBC_2.39 (nm -D libc.so.6, host glibc 2.41); the unsuffixed generic name is a header macro, not a symbol
Thread-safetyMT-Safe, AS-Safe, AC-Safe (manual stdbit.texi safety annotation; no man3 page exists for stdbit)
Ledger IDGLIBC-stdc_bit_width_uc, GLIBC-stdc_bit_width_us, GLIBC-stdc_bit_width_ui, GLIBC-stdc_bit_width_ul, GLIBC-stdc_bit_width_ull

Declaration and gates

#include <stdbit.h>

unsigned int stdc_bit_width_uc (unsigned char x);
unsigned int stdc_bit_width_us (unsigned short x);
unsigned int stdc_bit_width_ui (unsigned int x);
unsigned int stdc_bit_width_ul (unsigned long int x);
unsigned int stdc_bit_width_ull (unsigned long long int x);
unsigned int stdc_bit_width (x);  /* type-generic macro */

No feature-test gate: C23 7.18 header (include guard lines 19-20). The five out-of-line declarations sit at lines 676-687 of /usr/include/stdbit.h. The generic macro (lines 688-692) expands to __builtin_stdc_bit_width when present, otherwise it simply calls the _ull variant. With GCC >= 3.4 the type-specific names are redefined as __always_inline macros over __bw*_inline, each width - __clz*_inline (x) (lines 694-728).

Semantics

The functions return the minimum number of bits needed to store x, not counting leading zero bits; if x is zero they return zero (manual stdbit.texi, stdc_bit_width node). All variants return unsigned int; all are __THROW __attribute_const__ (pure, never set errno). Equivalently the result is the position of the most significant set bit counted from 1, i.e. 64 - clz for the 64-bit shim (__bw64_inline, lines 695-699). The generic macro accepts unsigned operands of width 8, 16, 32 or 64, or with GCC 14.1+ builtins also unsigned __int128 and unsigned _BitInt(n) (manual chapter intro).

Gotchas and portability

  • Zero returns 0, so the result doubles as a “is nonzero” test; but converting to a shift count needs care because 0 is not a valid position for 1 << width.
  • stdc_bit_width(x) equals stdc_first_leading_one(x) for nonzero x (both are 1 + clz in the inline layer) and differs only at 0, where both return 0.
  • It is the inverse of stdc_bit_floor: bit_floor returns 1 << (bit_width - 1) for nonzero x (header __bf64_inline, lines 749-753).
  • Signed operands are outside the C23 contract; the fallback macro does not diagnose. musl 1.2.5 ships no stdbit.h (absence verified).

References

  • glibc manual: stdbit.texi, chapter Bit Manipulation
  • header: /usr/include/stdbit.h lines 19-20, 676-728
  • symbols: nm -D --with-symbol-versions libc.so.6 (70 stdc_*, all GLIBC_2.39)
  • musl 1.2.5: no stdbit.h in include/ (absence verified)