Can I turn unsigned char into char and vice versa?

Question

I want to use a function that expects data like this:

void process(char *data_in, int data_len);

So it\'s just processing some bytes really

Answer 1

You can pass a pointer to a different kind of char, but you may need to explicitly cast it. The pointers are guaranteed to be the same size and the same values. There isn't going to be any information loss during the conversion.

If you want to convert char to unsigned char inside the function, you just assign a char value to an unsigned char variable or cast the char value to unsigned char.

If you need to convert unsigned char to char without data loss, it's a bit harder, but still possible:

#include 

char uc2c(unsigned char c)
{
#if CHAR_MIN == 0
  // char is unsigned
  return c;
#else
  // char is signed
  if (c <= CHAR_MAX)
    return c;
  else
    // ASSUMPTION 1: int is larger than char
    // ASSUMPTION 2: integers are 2's complement
    return c - CHAR_MAX - 1 - CHAR_MAX - 1;
#endif
}

This function will convert unsigned char to char in such a way that the returned value can be converted back to the same unsigned char value as the parameter.