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stdc_count_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_count_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_count_zeros_uc, GLIBC-stdc_count_zeros_us, GLIBC-stdc_count_zeros_ui, GLIBC-stdc_count_zeros_ul, GLIBC-stdc_count_zeros_ull

Declaration and gates

#include <stdbit.h>

unsigned int stdc_count_zeros_uc (unsigned char x);
unsigned int stdc_count_zeros_us (unsigned short x);
unsigned int stdc_count_zeros_ui (unsigned int x);
unsigned int stdc_count_zeros_ul (unsigned long int x);
unsigned int stdc_count_zeros_ull (unsigned long long int x);
unsigned int stdc_count_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 511-522 of /usr/include/stdbit.h. The generic macro (lines 523-529) expands to __builtin_stdc_count_zeros when present, otherwise it widens to _ull and subtracts the unused high bytes. With GCC >= 3.4 the type-specific names are redefined as __always_inline macros over __cz*_inline, each width - __builtin_popcount (lines 531-565).

Semantics

The functions count the number of zero bits in x (manual stdbit.texi, stdc_count_zeros node). All variants return unsigned int; all are __THROW __attribute_const__ (pure, never set errno). There is no special case at zero: a zero operand returns the full type width, an all-ones operand returns 0. 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

  • Counting zeros of the operand equals counting ones of the complement, but the width matters: __cz8_inline computes 8 - popcount(x) (lines 551-553), so the answer is always relative to the argument type, not the promoted type.
  • The ul inline again branches on __WORDSIZE (lines 559-563); on 64-bit targets unsigned long and unsigned long long share the 64-bit shim.
  • Do not widen by hand before calling: stdc_count_zeros((unsigned long long) x) counts the newly added high zeros too and differs from the _uc result.
  • 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, 511-565
  • 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)