Using library function:

We can use toupper() library function present in "string.h" to convert string into uppercase.

#include <stdio.h>
#include <string.h>

int main()
{
   char str[100];

   printf("Enter a string to convert it to Uppercase\n");
   gets(str);

   printf("Input string in upper case: %s", toupper(str));

   getch();
}

Without using Library Function:

In this program we are using a function upper() which will take a string as argument and convert into uppercase character by character and will return that character.

  #include <stdio.h>;

char upper(char char1)
{
      char char2;

      if(char1 >= 'a' && char1 <= 'z'){
            char2 = ('A' + char1 - 'a'); // converting into uppercase
            return char2;                // returning uppercase character
      }
      else{                               //character are already in uppercase
            char2 = char1; 
            return char2;
      }
}

int main()
{
      char lowercase, uppercase;

      printf("Enter a string to convert it to Uppercase\n");
      scanf("%c", &lowercase);

      uppercase = upper(lowercase);

      printf("The string in  uppercase  is: %c", uppercase);
      return 0;
}