assign array to structure pointer - c++

Question

Getting segmentation fault while trying to print content member using pointer l_pContent in below program.
#include <iostream>
#include <string.h>
using namespace std;
struct responseStruct {
int Handle;
int headerLen;
int bodyLen;
unsigned char* content;
};
int main()
{
unsigned char l_httpResponse[] = {1,2,3,4,5,6,7,8,9,0,1,2,'a','b','c','d','e','f','g','h','i','j','k','l','a','b','c','d','e','f','g','h','i','j','k','l',0};
struct responseStruct *l_pContent = (struct responseStruct*)l_httpResponse;
cout << l_pContent->content << endl; // Error : Segmentation Fault
return 0;
}

Answer 1

The variable content is a pointer to an unsigned char and so is l_httpResponse. You can therefore create an instance of the responseStruct and then assign the instance's content pointer to l_httpResponse.
Here is an example:
#include <iostream>
#include <string.h>
using namespace std;
struct responseStruct {
int Handle;
int headerLen;
int bodyLen;
unsigned char* content;
};
int main()
{
unsigned char l_httpResponse[] = {1,2,3,4,5,6,7,8,9,0,1,2,'a','b','c','d','e','f','g','h','i','j','k','l','a','b','c','d','e','f','g','h','i','j','k','l',0};
// Create instance of an responseStruct struct
responseStruct rs;
// Make content point to the start of l_httpResponse
rs.content = l_httpResponse;
// Test for access without segfault
cout << static_cast<unsigned>(rs.content[1]) << endl;
return 0;
}
Or here is a live demo.

Answer 2

Omitting the fact that the idea of such code is mysterious to me, here is what causes the error:
If we assume that members of the responseStruct will match ideally the data from the l_httpResponse, that sizeof(int) and sizeof(unsigned char *) are 4, that your architecture uses little-endian notation, and that your compiler uses ASCII (which it probably does), you end with:
Handle == 0x04030201
headerLen == 0x08070605
bodyLen == 0x02010009
content == 0x64636261
Now, content is a pointer, so 0x64636261 is an address in your memory. It doesn't point to your "abcde..." string. It's made up of it's first four bytes. And points at some non existing region. That's why you end up with segmentation fault.