Header File 

ctype.h, mbstring.h 

Category 

Conversion Routines 

Prototype 

int tolower(int ch); 

int towlower(wint_t ch); // Unicode version 

unsigned int _mbctolower(unsigned int c); 

Description 

Translates characters to lowercase. 

tolower is a function that converts an integer ch (in the range EOF to 255) to its lowercase value (a to z; if it was uppercase, A to Z). All others are left unchanged. 

Return Value 

tolower returns the converted value of ch if it is uppercase; it returns all others unchanged. 

Example  

#include <string.h>
#include <stdio.h>
#include <ctype.h>
int main(void)
{
   int length, i;
   char *string = "THIS IS A STRING";
   length = strlen(string);
   for (i=0; i<length; i++)
   {
       string[i] = tolower(string[i]);
   }
   printf("%s\n",string);
   return 0;
}

Portability