C <ctype.h> - tolower() Function

The C <ctype.h> tolower() function is used to convert 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.

Syntax

int tolower ( int ch );               

Parameters

Return Value

Returns lowercase version of ch or unchanged ch if no lowercase value is listed in the current C locale.

Example:

The example below shows the usage of tolower() function.

#include <stdio.h>
#include <ctype.h>
 
int main (){
  char str[50] = "HELLO World!";

  //converting str into lower case
  int i = 0;
  while(str[i]) {
    str[i] = tolower(str[i]);
    i++;
  }

  //displaying the output
  printf("%s\n", str);  
  return 0;
}

The output of the above code will be:

hello world!