What is the deal with assigning an unsigned variable to a signed value?

Question

This code I am looking at has a lot of places where I see things like this happening:

char *functionName(char *passedVariable)
{
    unsigned char *newVariable = (char* ) passedVariable;

Why is this being done? I always try to be consistent in the use of signed/unsigned, because I know that switching between the two can cause problems, but this developer doesn't seem to care.

Answer 1

Changing the pointer type is not really an issue, this address will still be valid. However interpreting the pointed data as signed/unsigned makes a difference if and only if... the signed data is negative. So in your example if your char's are always positive, then it's ok, otherwise it is not.

Example of signed/unsigned casts:

char c = 42;
char d = -42;
unsigned char cu = c;
unsigned char du = d;

printf("c  %d\n", c);
printf("cu %d\n", cu);
printf("d  %d\n", d);
printf("du %d\n", du);

Output:

c  42
cu 42
d  -42
du 214