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stdc_first_trailing_one (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_first_trailing_one_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_first_trailing_one_uc, GLIBC-stdc_first_trailing_one_us, GLIBC-stdc_first_trailing_one_ui, GLIBC-stdc_first_trailing_one_ul, GLIBC-stdc_first_trailing_one_ull

Declaration and gates

#include <stdbit.h>

unsigned int stdc_first_trailing_one_uc (unsigned char x);
unsigned int stdc_first_trailing_one_us (unsigned short x);
unsigned int stdc_first_trailing_one_ui (unsigned int x);
unsigned int stdc_first_trailing_one_ul (unsigned long int x);
unsigned int stdc_first_trailing_one_ull (unsigned long long int x);
unsigned int stdc_first_trailing_one (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 453-464 of /usr/include/stdbit.h. The generic macro (lines 465-473) expands to __builtin_stdc_first_trailing_one when present, otherwise to a sizeof-switch dispatch. With GCC >= 3.4 the type-specific names are redefined as __always_inline macros over __fto*_inline, each of which is x == 0 ? 0 : 1 + __ctz*_inline (x) (lines 475-509).

Semantics

The functions return the position of the least significant one bit in x, counting from the least significant bit of the argument type as 1; they return 0 if there is no one bit in x (manual stdbit.texi, stdc_first_trailing_one 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

  • Positions are 1-based and 0 means “no one bit” (i.e. the operand was 0), unlike __builtin_ctz which is merely undefined for 0 (__fto64_inline, lines 477-480).
  • For nonzero x the relation to the count family is stdc_first_trailing_one(x) == stdc_trailing_zeros(x) + 1; both come from the same ctz builtin in the inline layer.
  • The result is an ordinal, not a bitmask; mask it with 1u << (stdc_first_trailing_one(x) - 1) to use it as one.
  • 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, 453-509
  • 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)