[C] Pointer problem

[psld]

Hi!
I have a little problem.
The struct msgPacket contains a void* named packetStart. packetStart points at a place. I want the second byte that packetStart points at to be an adress to a char.
I've tried this:

msg.packetStart is void*, points at place, contains adress
msg.packetStart[1] is void, is sizeof(void) away from packetStart (yes I know I cant use sizeof(void)
(char)msg.packetStart[1] is char, 1 byte away from packetStart
&(char)msg.packetStart[1] is &char, should work? It dont.

The error is:
"pointer of type <void *> used in arithmetic"

The goal is to pass a &char over to strcpy();

[MadCow108]

is that what you want?


#include <stdio.h>
#include <string.h>
#include <stdlib.h>

struct bla
{
void * pk;
};
int main (int argc, char *argv[])
{
struct bla test;
char c[] = "bkd";
test.pk = (void*)&c;
printf("%s\n",c);
char * n = malloc(3*sizeof(char));

//one
strcpy(n,&(((char*)test.pk)[1]));
printf("%s\n",n);
//two
char * pointer =((char*)test.pk)+1;
strcpy(n,pointer);
printf("%s\n",n);
//three
pointer =((char*)test.pk);
strcpy(n,++pointer);
printf("%s\n",n);
free(n);
return 0;
}

output


bkd
kd
kd
kd

[fr4nko]

Whay you should do is a cast to a (char *) before any pointer arithmetic:


char *ptr = msg.packetStart;

/* ptr+1 gives the address of packetStart by skipping 1 byte */
strcpy (mybuffer, ptr + 1);
or, equivalently


strcpy (mybuffer, (char *) msg.packetStart + 1);
The logic of this latter expression is


cast to (char *): consider msg.packetStart as a pointer to an array of char
add to this latter point +1. Since it is a char array +1 means +1 char

Note that it works because the cast operator has an higher precedence that addition.

Francesco