memccpy / mempcpy / memrchr / rawmemchr (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>

void *memccpy(void *restrict dest, const void *restrict src,
              int c, size_t n);          /* __USE_MISC || __USE_XOPEN || ISOC23 */
void *mempcpy(void *restrict dest, const void *restrict src, size_t n);
void *memrchr(const void *s, int c, size_t n);       /* __USE_GNU */
void *rawmemchr(const void *s, int c);               /* __USE_GNU */

memccpy is gated __USE_MISC || __USE_XOPEN || __GLIBC_USE(ISOC23) (/usr/include/string.h lines 53-58). mempcpy (with private __mempcpy) sits under __USE_MISC at lines 395-402, although mempcpy(3) documents the _GNU_SOURCE spelling. memrchr and rawmemchr share the __USE_GNU block at lines 111-135; memchr(3) records their macros as plain _GNU_SOURCE.

Semantics

memccpy copies at most n bytes, stopping after byte c is copied, and returns a pointer just past that byte in dest - or NULL if c never appeared in the first n bytes of src; overlapping areas are undefined (memccpy(3)). mempcpy is memcpy that returns dest + n, so chained copies fill consecutive regions without arithmetic; the man page’s combine example builds one buffer from two objects with nested calls (mempcpy(3)). memrchr scans the n bytes backward from the end; rawmemchr drops the count entirely and assumes an occurrence exists - the documented idiom rawmemchr(s, '\0') is the fast strlen endpoint finder - with unspecified results when c is absent (memchr(3), which also documents both GNU names). Versions per memchr(3)/mempcpy(3): rawmemchr glibc 2.1, memrchr 2.2, mempcpy 2.1.

Gotchas and portability

• memccpy’s return is “one past the copied c”, not the copied byte: *memccpy(buf, src, '\n', n) = '\0' writes the terminator after the newline - check NULL before dereferencing.
• rawmemchr without a guaranteed match is undefined behavior; it is the one member of this batch that cannot be used defensively.
• musl 1.2.5: memccpy, mempcpy, memrchr present (musl include/string.h lines 81, 96-97); rawmemchr absent.

References

• glibc manual: string.texi, nodes Copying Strings and Arrays, Search Functions (rawmemchr, mempcpy documented inline)
• man3: memccpy(3), mempcpy(3) (covers wmempcpy), memchr(3) (covers memrchr, rawmemchr)
• header: /usr/include/string.h
• symbols: nm -D --with-symbol-versions libc.so.6