bcmp (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 <strings.h>

int bcmp(const void *s1, const void *s2, size_t n);

Declared in strings.h at line 34 under the same __USE_MISC || !__USE_XOPEN2K8 gate as the rest of the BSD byte-string family (/usr/include/strings.h; see the sibling page bcopy-bzero-index-rindex.md, which covers bcopy, bzero, index and rindex but only mentions bcmp in passing). Strict _POSIX_C_SOURCE >= 200809L builds hide the prototype; legacy code needs _DEFAULT_SOURCE.

Semantics

bcmp compares the two byte sequences s1 and s2, each of length n. It returns 0 when they are equal - in particular when n is zero - and some nonzero value otherwise (bcmp(3) DESCRIPTION and RETURN VALUE). It is the BSD spelling of memcmp; glibc implements it as an IFUNC-resolved alias in the same object family as memcmp.

Gotchas and portability

• Only the zero/nonzero distinction is documented, not the sign: bcmp(3) never promises s1 < s2 ordering the way memcmp(3) does (return sign of the first difference). Code that relies on the sign must use memcmp.
• Obsolescence is documented, not folkloric: bcmp(3) CONFORMING TO marks it LEGACY in POSIX.1-2001 and removed in POSIX.1-2008, directing new programs to memcmp(3).
• Unlike index/rindex, which nm shows as literal address aliases of strchr/strrchr, bcmp is an ifunc-dispatched symbol (nm type i at address 0x…a66d0 on this host) - still semantically memcmp.
• musl 1.2.5 keeps it for compatibility (musl src/string/bcmp.c), though its include/string.h does not declare it - a prototype is needed in user code.

References

• glibc manual: string.texi, node String/Array Comparison (memcmp context; the BSD names are not documented as a family there)
• man3: bcmp(3) (also bstring(3) overview)
• header: /usr/include/strings.h
• symbols: nm -D --with-symbol-versions libc.so.6