pointer in C function needs unsigned type array, but my array is signed

Question

I have a function as follows that processes the information contained in an array of type unsigned char:

unsigned char LRCsimple(unsigned char *p, createLRC , unsigned char length)
{

}

Works great for mostly unsigned char arrays.

Now, I have a signed array and when I use such a function and it works very well, but I have a warning when compiling the code:

> ../src/apptcpipserver.c:102:9: warning: pointer targets in passing argument 1 of 'LRCsimple' differ in signedness [-Wpointer-sign]
         if (0x01 == LRCsimple(apptcpipserverData.cRxedData,0x00,(apptcpipserverData.cRxedData[0x02] - 0x02)))

If I want to avoid this warning, I think the optimal solution is to create a function similar to the one above, but for a signed array, as follows:

unsigned char signedLRCsimple(char *p, createLRC , unsigned char length)
{

}

Or is there something else I can do to avoid that warning message?

Answer 1

Strict aliasing rule allows unsigned char and char alias. Therefore you should be able reuse LRCsimple for processing char*.

Therefore signedLRCsimple could be implemented as:

unsigned char signedLRCsimple(char *p, createLRC xxx, unsigned char length)
{
   return LRCsimple((unsigned char*)p, xxx, length);
}

To avoid forcing client to change their code to use signedLRCsimple you could use generic selection introduced in C11 in form of _Generic. Typically it is used to select a function pointer basing on the type of first argument of _Generic.

#define LRCsimple(p, xxx, length)          \
  _Generic((p), unsigned char*: LRCsimple, \
                char *: signedLRCsimple)(p, xxx, length)

Whenever LRCsimple is called the generic selection selects between LRCsimple for unsigned char* and signedLRCsimple for char*. For other types an error is raised.