strlcpy / strlcat / wcslcpy / wcslcat (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 <string.h>
size_t strlcpy(char *restrict dest, const char *restrict src, size_t n);
size_t strlcat(char *restrict dest, const char *restrict src, size_t n);

#include <wchar.h>
size_t wcslcpy(wchar_t *restrict dest, const wchar_t *restrict src,
               size_t n);
size_t wcslcat(wchar_t *restrict dest, const wchar_t *restrict src,
               size_t n);

The narrow pair sits in /usr/include/string.h lines 504-515 under __USE_MISC; the wide pair in /usr/include/wchar.h lines 107-119 under the same gate, with the mirrored comments “Copy at most N - 1 characters” / “Append SRC to DEST, possibly with truncation to keep the total size below N”. Both are glibc 2.38 additions (nm version GLIBC_2.38, the only non-2.2.5 versions in this batch).

Semantics

strlcpy copies src into a destination of total size n: when the string fits, it copies and null-terminates, leaving any remaining destination bytes unchanged (unlike strncpy’s padding); when it does not fit, it copies exactly n-1 bytes plus a terminator. It returns strlen(src), so truncation happened iff the return value is >= n (manual, Truncating Strings). strlcat appends at the existing terminator under the same size cap and returns the sum of the original to length and strlen(src). The wide variants are identical with n counting wide characters (wcslcpy(3) story per manual).

Gotchas and portability

• The manual is blunt: these “are generally poor choices for processing” strings, and strlcpy’s cost is proportional to strlen(src) even when n is tiny - it always walks the source to size the return value.
• GLIBC_2.38 versioning means glibc < 2.38 links fail or need a local shim; the sibling page strcpy-strncpy-stpcpy.md records the 2.38 NEWS note (“derived from OpenBSD, expected to be added to a future POSIX version”) - that future is POSIX.1-2024 per the manual.
• musl 1.2.5 has the narrow pair unconditionally (musl include/string.h lines 86-87) but neither wide variant - no wcslcpy.c/wcslcat.c, no declaration in musl include/wchar.h.

References

• glibc manual: string.texi, node Truncating Strings (strlcpy/strlcat/wcslcpy/wcslcat, POSIX.1-2024)
• man3: none cached for these four names in source-cache/man-pages-sparse/man3/
• headers: /usr/include/string.h, /usr/include/wchar.h
• symbols: nm -D --with-symbol-versions libc.so.6