strcoll

strcoll is a function in the C standard library used to compare two null-terminated strings according to the collation order of the current locale. It is declared in string.h with the signature int strcoll(const char *s1, const char *s2). The function returns a negative value if s1 is before s2, zero if they are considered equivalent, and a positive value if s1 is after s2 within the locale’s sorting rules. The exact return value is implementation-defined beyond the sign.

Locale dependence is the defining characteristic of strcoll. The comparison uses the LC_COLLATE category of the process’s current locale. In the C (or POSIX) locale, strcoll typically behaves like strcmp, performing a byte-wise comparison. In other locales, collation rules may account for multibyte characters, diacritics, case folding, and locale-specific sorting orders, which can produce results that differ from a simple character-by-character comparison.

Locale scope and thread considerations are important. In many C libraries, the process-wide locale affects strcoll; changing the locale with setlocale changes behavior for all threads. Some environments provide per-thread locales via additional APIs such as newlocale and uselocale, which can be used to constrain strcoll to a specific locale in a multithreaded program.

Related functionality includes strxfrm, which produces a transformation of a string suitable for efficient comparison, enabling locale-aware sorting by comparing transformed keys rather than strings directly. strcoll is primarily used when a direct, locale-aware string comparison is needed without constructing transformation keys.