#include<stdio.h>

#include<string.h>

int main()

{

char str[20];

int i;

  printf("Enter any string->");

  scanf("%s",&str);

  for(i=0;i<=strlen(str);i++){


      if(str[i]>=65&&str[i]<=90)

       str[i]=str[i]+32;

  }

  printf("nThe string in lower case is->%s",str);

  return 0;

}

#include <stdio.h>
#include <ctype.h>
#include <string.h>
...
if ( fgets( str, sizeof str, stdin ) != NULL )
{
  /**
   * chop off the newline if it's present
   */
  char *newline = strchr( str, 'n' );
  if ( newline )
  {
    *newline = 0;
  }

  /**
   * Convert the string to lower case.
   */
  for ( int i = 0; str[i] != 0; i++ )
    str[i] = tolower( str[i] );
}

void convertToUpperCase(char *sPtr)
{
  while(*sPtr != '')
  {
     if (islower(*sPtr))
          *sPtr = toupper(*sPtr);
   }
}

scanf("%s",&str);

scanf("%[^n]",str);