dereferencing void pointer

nkhambal · #1 24-Apr-2005, 21:20

Hi,

I have a void pointer. I am mallocing to it and cast it to appropriate type. However I am getting compiler errors about dereferecing void pointer.

Following is the code

CPP / C++ / C Code:

/*
typedef struct pptp_generic_t_
{
	CARD16 length;
	CARD16 pptp_msg_type;
	CARD32 cookie;
	CARD16 control_message_type;
	CARD16 reserved0;
} pptp_generic_t;

typedef struct ctrl_msg_sccrq_t_
{
	pptp_generic_t pptp_generic_pkt;
	CARD16 version;
	CARD16 reserved1;
	CARD32 framing;
	CARD32 bearer;
	CARD16 channels;
	CARD16 firmware;
	CARD8 hostname[64];
	CARD8 vendor[64];
} ctrl_msg_sccrq_t;
*/
void *packet;

if (ptype == START_CONTROL_CONNECTION_REQUEST)
{
	packet = (ctrl_msg_sccrq_t *)malloc(sizeof(ctrl_msg_sccrq_t));
	packet->pptp_generic_pkt.pptp_msg_type=CONTROL_MESSAGE;
	packet->pptp_generic_pkt.cookie=MAGIC_COOKIE;
   
packet->pptp_generic_pkt.control_message_type=START_CONTROL_CONNECTION_REQUEST;
		packet->pptp_generic_pkt.reserved=0;


}

Compiler errors

Quote:

pclientfunction.c: In function `sendPpacket':
pclientfunction.c:116: warning: dereferencing `void *' pointer
pclientfunction.c:116: request for member `pptp_generic_pkt' in something not a structure or union
pclientfunction.c:117: warning: dereferencing `void *' pointer
pclientfunction.c:117: request for member `pptp_generic_pkt' in something not a structure or union
pclientfunction.c:118: warning: dereferencing `void *' pointer
pclientfunction.c:118: request for member `pptp_generic_pkt' in something not a structure or union
pclientfunction.c:119: warning: dereferencing `void *' pointer
pclientfunction.c:119: request for member `pptp_generic_pkt' in something not a structure or union
make: *** [pptpclient] Error 1

Can anybody help me whats going wrong here?

Thanks,

LuciWiz · #2 25-Apr-2005, 02:31

You can't dereference a void pointer, because it has an undetermined size. I only use those for returning undetermined types from functions, or storing undetermined data into a map (other people might make better use of them). To accomplish the task you set out for, you need something like this:

CPP / C++ / C Code:

if (ptype == START_CONTROL_CONNECTION_REQUEST)
{
	packet = (ctrl_msg_sccrq_t *)malloc(sizeof(ctrl_msg_sccrq_t));
	((ctrl_msg_sccrq_t *)packet)->pptp_generic_pkt.pptp_msg_type=CONTROL_MESSAGE;
	//...

Best regards,
Lucian

nkhambal · #3 25-Apr-2005, 02:37

Hi Lucian,

Thanks for that tip. It worked. I am using void pointer because I want to use it to construct different types of packets in the same function, using packet type as handle. Each packet has different size which is not known in the begining. So I have to use void pointer for the packet and then cast it to appropriate packet type.

Thanks,

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