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stdc_trailing_zeros (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_trailing_zeros_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_trailing_zeros_uc, GLIBC-stdc_trailing_zeros_us, GLIBC-stdc_trailing_zeros_ui, GLIBC-stdc_trailing_zeros_ul, GLIBC-stdc_trailing_zeros_ull

Declaration and gates

#include <stdbit.h>

unsigned int stdc_trailing_zeros_uc (unsigned char x);
unsigned int stdc_trailing_zeros_us (unsigned short x);
unsigned int stdc_trailing_zeros_ui (unsigned int x);
unsigned int stdc_trailing_zeros_ul (unsigned long int x);
unsigned int stdc_trailing_zeros_ull (unsigned long long int x);
unsigned int stdc_trailing_zeros (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 167-178 of /usr/include/stdbit.h. The generic macro (lines 179-187) expands to __builtin_stdc_trailing_zeros when present, otherwise to a sizeof-switch that dispatches to the matching _uc/_us/_ui/_ull function. With GCC >= 3.4 the type-specific names are redefined as __always_inline macros over __ctz8/16/32/64_inline (lines 189-223).

Semantics

The 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 the type in bits (manual stdbit.texi, stdc_trailing_zeros node). All variants return unsigned int; all are __THROW __attribute_const__ (pure, never set errno). 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

  • stdc_trailing_zeros(0) is the full type width, unlike __builtin_ctz which is undefined for 0; each inline shim handles it explicitly (__ctz64_inline returns 64 for 0, line 193).
  • The dispatch-by-sizeof fallback rounds any unusual type down to the matching width function; operand types other than the five unsigned integer types of width 8/16/32/64 are outside the C23 contract.
  • Because the trailing end needs no widening, this family’s inline shims are the only ones where _uc through _ull all keep an explicit zero check rather than reusing a wider function.
  • musl 1.2.5 ships no stdbit.h (absence verified in musl/include/); pair with a feature check before using in portable code.

References

  • glibc manual: stdbit.texi, chapter Bit Manipulation
  • header: /usr/include/stdbit.h lines 19-20, 167-223
  • 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)