wmemchr / wmemcmp / wmemcpy / wmemmove / wmemset / wmempcpy (3) - glibc

Split from the chapter glibc: strings, ctype and stdlib arithmetic into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <wchar.h>

wchar_t *wmemchr(const wchar_t *s, wchar_t c, size_t n);
int      wmemcmp(const wchar_t *s1, const wchar_t *s2, size_t n);
wchar_t *wmemcpy(wchar_t *restrict dest, const wchar_t *restrict src,
                 size_t n);
wchar_t *wmemmove(wchar_t *dest, const wchar_t *src, size_t n);
wchar_t *wmemset(wchar_t *wcs, wchar_t wc, size_t n);

wchar_t *wmempcpy(wchar_t *restrict dest, const wchar_t *restrict src,
                  size_t n);            /* __USE_GNU */

The five ISO functions are unconditional in /usr/include/wchar.h (wmemchr around lines 262-270, wmemcmp 272, wmemcpy 287-288, wmemmove 292, wmemset 296); wmempcpy sits under __USE_GNU at lines 298-303. wmempcpy(3) documents _GNU_SOURCE plus <wchar.h> for the wide spelling.

Semantics

The wmem family works on wide-character arrays with an explicit count - not null-terminated strings; the manual’s String/Array Conventions node reserves the wmem prefix exactly for this (wmemcpy etc. take a size and may hold embedded L’\0’). wmemcpy copies n wide characters and requires non-overlapping arrays; wmemmove handles overlap like memmove. wmemset fills n slots with wc; wmemchr finds the first c among n characters, returning NULL when absent; wmemcmp compares n wide characters and returns the sign of the first difference. wmempcpy is wmemcpy plus the mempcpy twist: it returns dest + n, so successive copies chain without re-computing offsets (mempcpy(3)).

Gotchas and portability

• Counted in wide characters, never bytes: passing a sizeof-style byte count multiplies the copy by sizeof(wchar_t) and runs off the buffer. The wcsnlen(3) man page notes the same confusion for lengths.
• Overlap: wmemcpy with overlapping arrays is undefined - switch to wmemmove (wmemcpy(3) DESCRIPTION).
• wmempcpy is GNU-only and musl 1.2.5 does not have it (no wmempcpy.c, no declaration in musl include/wchar.h); the other five are all present in musl (musl src/string/wmem*.c).

References

• glibc manual: string.texi, nodes String/Array Conventions, Copying Strings and Arrays (wmem functions inline)
• man3: wmemchr(3), wmemcmp(3), wmemcpy(3), wmemmove(3), wmemset(3), mempcpy(3) (covers wmempcpy)
• header: /usr/include/wchar.h
• symbols: nm -D --with-symbol-versions libc.so.6