help on copying memory

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I am quite new to C++, could anyone help me to fix memory copy issue (invalid pointer of destination):

cut out code looks like this:

C++

int cWrapper::EncryptRun(const unsigned char *data, NewCamDData *cd, int len, bool UseMsgId, comm_type_t commType)
{
	memcpy(desKey , cd->desKey, 16);	
	int sz = sizeof(DES_key_schedule);
	memcpy(&ks1, &cd->ks1, sz);
	memcpy(&ks2, &cd->ks2, sz);

unsigned char netbuf[CWS_NETMSGSIZE];
memset(&netbuf[2],0,cdLen+2);
memcpy(&netbuf[cdLen+4],data,len);
netbuf[cdLen+4+1]=(data[1]&0xf0)|(((len-3)>>8)&0x0f);
netbuf[cdLen+4+2]=(len-3)&0xff;
//...assign values

//this is where the problem is for the data pointer
memcpy(&data,&netbuf,len);
}

data points to an array of 240 bytes, and netbuf is new byte array to be copied to data.

To complete calls, I am adding the method which calls this:

C++

DllExport int CALLBACK EncryptA(NewCamDData *cd, unsigned char *pBuffer, int len, bool UseMsgId, comm_type_t commType)
{
    //cw assigned in class initialisation:
    cw = new cWrapper(525);
    int ret = cw->EncryptRun(pBuffer, cd, len, UseMsgId, commType);
    return ret; //*pBuffer;
}

This method called from VB.NET.

debugging shows me this:

?netbuf
0x0038ebec ""
    [0]: 0 <-first byte of array
    [1]: 48 '0'
    [2]: 169 '©'
?data
0x0280aee4 "€	¤+ØÔ&§HœõRøî°¶l-áþ«…"
?sizeof(data)
4
?*data
128 '€' <-first byte of array

Posted 12-Jan-12 5:39am

Tomazas77

Updated 12-Jan-12 7:57am

v4

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2 solutions

Solution 3

C#

memcpy(data,netbuf,len);

i think, this is how you should call. To call like this the data parameter declaration should be "unsigned char *data" should not be a constant pointer!.

Also make sure -

C#

CWS_NETMSGSIZE>=len.

Posted 12-Jan-12 6:42am

Kethu Sasikanth

Updated 12-Jan-12 6:44am

v2

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Jochen Arndt · Accepted Answer · 2012-01-12T06:08:00

Solution 2

Use

memcpy(data, netbuf, len);

because data and netbuf are already pointers.
However, this will produce a compiler error, because the destination data is declared as const. Due to the fact that cd is not used, I assume that

memcpy(cd, netbuf, len)

is what you want.

To be sure that no buffer overflows occur, you may also add some checks:

C++

ASSERT(cdLen + 4 + len <= CWS_NETMSGSIZE);
ASSERT(cdLen + 7 <= CWS_NETMSGSIZE);
// This only when copying to cd
ASSERT(len <= sizeof(NewCamDData));

Posted 12-Jan-12 6:08am

Jochen Arndt

Comments

Stefan_Lang 12-Jan-12 12:13pm

Perfect, my 5.

Tomazas77 12-Jan-12 13:26pm

not, true, cd is used its just strip out.

using first option compilation is not possible with error:
"Error 1 error C2664: 'memcpy' : cannot convert parameter 1 from 'const unsigned char *' to 'void *' c:\PROJECTS\C++\NewCamdLibrary\NewCamdLibrary\CardServer\cWrapper.cpp 88"

Jochen Arndt 12-Jan-12 13:31pm

That's what I wrote ('this will produce a compiler error'). 'data' is declared as const. When you want to copy to data, you must remove the const specifier from the function declaration and implementation.

Tomazas77 12-Jan-12 13:42pm

netbuf data has to be copied to data value, not to cd.

CWS_NETMSGSIZE is constant and its value 240.

Jochen Arndt 12-Jan-12 13:50pm

Yes, I understand. Use my first memcpy example with data as destination, but you must remove the const specifier. When declaring a const pointer (here: 'const unsigned char *data'), you are not allowed to change the content at the memory location pointed to. The first argument to the memcpy function must not be const (see declaration / help for memcpy). Therefore, you must change the first parameter type of your EncryptRun function to 'unsigned char * data'.

Tomazas77 12-Jan-12 14:03pm

Thanks, removing const worked.