strcmp / strncmp / strcasecmp / strncasecmp (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>
int strcmp(const char *s1, const char *s2);
int strncmp(const char *s1, const char *s2, size_t n);

#include <strings.h>       /* POSIX home of the case-insensitive pair */
int strcasecmp(const char *s1, const char *s2);
int strncasecmp(const char *s1, const char *s2, size_t n);

strings.h declares the case-insensitive pair unconditionally (/usr/include/strings.h lines 115-121); string.h re-declares them only when _DEFAULT_SOURCE is set for historical 4.4BSD compatibility (strcasecmp(3) NOTES). Locale-taking twins strcasecmp_l/strncasecmp_l live in strings.h under __USE_XOPEN2K8.

Semantics

strcmp compares s1 and s2 byte-by-byte as unsigned char values and ignores the locale (strcmp(3)); strncmp compares at most the first n bytes. All four return an integer less than, equal to, or greater than zero as s1 orders before, equal to, or after s2. strcasecmp performs the same comparison ignoring case; strncasecmp bounds it to n bytes (strcasecmp(3)).

Gotchas and portability

• The magnitude of a nonzero result is unspecified by the standards. strcmp(3) NOTES quotes POSIX: only the sign is defined, and records that glibc, like most implementations, returns the arithmetic difference of the first differing bytes (e.g. 'C' - '\0' = 67 for strcmp("ABC", "AB")). Never write abs(strcmp(a,b)) == 1 style logic.
• Unsigned comparison matters: strcmp with a high-bit byte compares as unsigned (strcmp(3) example ./string_comp $'\201' A yields 64), so sign-extended char ordering surprises do not happen, but negative char arithmetic in your own comparators can.
• Locale: strcasecmp results are unspecified outside the POSIX locale unless you use the _l variants; strcasecmp(3) NOTES quotes POSIX.1-2008 on exactly this. For locale-aware collation (not case folding), the proper pair is strcoll/strxfrm (manual Collation Functions node).
• strcasecmp lives in strings.h, not string.h, in strict POSIX mode; forgetting the include breaks _POSIX_C_SOURCE-only builds.
• Equality testing is the common case: the manual (String/Array Comparison) recommends the !strcmp(s1, s2) idiom.

References

• glibc manual: string.texi, node String/Array Comparison
• man3: strcmp(3) (covers strncmp), strcasecmp(3)
• headers: /usr/include/string.h, /usr/include/strings.h
• symbols: nm -D --with-symbol-versions libc.so.6