#include <strings.h>

typedef unsigned char byte;

byte READBYTE(byte ** from)
{
        byte *from_tmp = *from;
        byte b;
        memcpy (&b, from_tmp, sizeof (byte));
        from_tmp++;
        *from = (void *)from_tmp;
        return b;
}

Gives:pickle@detritus:~$ gcc -c test.c
test.c: In function ‘READBYTE’:
test.c:9: warning: incompatible implicit declaration of built-in function ‘memcpy’