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

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

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

Uppercase version of ch or unmodified ch if no uppercase 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 l = 0; l < UCHAR_MAX; l++) {
        unsigned char u = toupper(l);
        if (u != l) printf("%c%c ", l, u);
    unsigned char c = '\xb8'; // the character Ž in ISO-8859-15
                              // but ´ (acute accent) in ISO-8859-1 
    setlocale(LC_ALL, "en_US.iso88591");
    printf("in iso8859-1, toupper('0x%x') gives 0x%x\n", c, toupper(c));
    setlocale(LC_ALL, "en_US.iso885915");
    printf("in iso8859-15, toupper('0x%x') gives 0x%x\n", c, toupper(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, toupper('0xb8') gives 0xb8
in iso8859-15, toupper('0xb8') gives 0xb4


  • C17 standard (ISO/IEC 9899:2018):
    • The toupper function (p: 147-148)
  • C11 standard (ISO/IEC 9899:2011):
    • The toupper function (p: 204)
  • C99 standard (ISO/IEC 9899:1999):
    • The toupper function (p: 185)
  • C89/C90 standard (ISO/IEC 9899:1990):
    • The toupper function

See also

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

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