You can achieve this by extracting each bits one by one, like this
str[i] = (char)( (num % 10) - 48 )
48 has to be subtracted because an integer value changing to character goes through an ASCII conversion. Keeping the above line in a loop, that would run for each digit in the number, should do the trick..
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solved Converting an int to string without using library functions in C? [closed]