tolower - Man Page

transliterate uppercase characters to lowercase


This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.


#include <ctype.h>

int tolower(int c);
int tolower_l(int c, locale_t locale);


For tolower(): The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2017 defers to the ISO C standard.

The tolower() and tolower_l() functions have as a domain a type int, the value of which is representable as an unsigned char or the value of EOF. If the argument has any other value, the behavior is undefined. If the argument of tolower() or tolower_l() represents an uppercase letter, and there exists a corresponding lowercase letter as defined by character type information in the current locale or in the locale represented by locale, respectively (category LC_CTYPE), the result shall be the corresponding lowercase letter. All other arguments in the domain are returned unchanged.

The behavior is undefined if the locale argument to tolower_l() is the special locale object LC_GLOBAL_LOCALE or is not a valid locale object handle.

Return Value

Upon successful completion, the tolower() and tolower_l() functions shall return the lowercase letter corresponding to the argument passed; otherwise, they shall return the argument unchanged.


No errors are defined.

The following sections are informative.



Application Usage




Future Directions


See Also

setlocale(), uselocale()

The Base Definitions volume of POSIX.1-2017, Chapter 7, Locale, <ctype.h>, <locale.h>

Referenced By

ctype.h(0p), setlocale(3p), _tolower(3p).

2017 IEEE/The Open Group POSIX Programmer's Manual