Defined in header <ctype.h>
int tolower( int ch );

Converts the given character to lowercase according to the character conversion rules defined by the currently installed C locale.

In the default "C" locale, the following uppercase letters ABCDEFGHIJKLMNOPQRSTUVWXYZ are replaced with respective lowercase letters abcdefghijklmnopqrstuvwxyz.


ch - character to be converted. If the value of ch is not representable as unsigned char and does not equal EOF, the behavior is undefined.

Return value

Lowercase version of ch or unmodified ch if no lowercase version is listed in the current C locale.


#include <stdio.h>
#include <ctype.h>
#include <locale.h>
#include <limits.h>
int main(void)
    /* In the default locale: */
    for (unsigned char u = 0; u < UCHAR_MAX; u++) {
        unsigned char l = tolower(u);
        if (l != u) printf("%c%c ", u, l);
    unsigned char c = '\xb4'; // the character Ž in ISO-8859-15
                              // but ´ (acute accent) in ISO-8859-1
    setlocale(LC_ALL, "en_US.iso88591");
    printf("in iso8859-1, tolower('0x%x') gives 0x%x\n", c, tolower(c));
    setlocale(LC_ALL, "en_US.iso885915");
    printf("in iso8859-15, tolower('0x%x') gives 0x%x\n", c, tolower(c));

Possible output:

Aa Bb Cc Dd Ee Ff Gg Hh Ii Jj Kk Ll Mm Nn Oo Pp Qq Rr Ss Tt Uu Vv Ww Xx Yy Zz
in iso8859-1, tolower('0xb4') gives 0xb4
in iso8859-15, tolower('0xb4') gives 0xb8


  • C17 standard (ISO/IEC 9899:2018):
    • The tolower function (p: 147)
  • C11 standard (ISO/IEC 9899:2011):
    • The tolower function (p: 203)
  • C99 standard (ISO/IEC 9899:1999):
    • The tolower function (p: 184)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The tolower function

See also

converts a character to uppercase
converts a wide character to lowercase
C++ documentation for tolower

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