# tolower

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< c‎ | string‎ | byte

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`.

### Parameters

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.

### Example

Run this code

    #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);
        }
        printf("\n\n");

        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

### References

- C17 standard (ISO/IEC 9899:2018):

  - 7.4.2.1 The tolower function (p: 147)

- C11 standard (ISO/IEC 9899:2011):

  - 7.4.2.1 The tolower function (p: 203)

- C99 standard (ISO/IEC 9899:1999):

  - 7.4.2.1 The tolower function (p: 184)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.3.2.1 The tolower function

### See also

toupper | converts a character to uppercase (function)
towlower (C95) | converts a wide character to lowercase (function)

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