memcpy assistance please

Basically what I am trying to do is modify the contents of the place where the structures data pointer points to. I have to add 5 arb characters onto the packet's data.

When I run the program below, it crashes when performing the memcpy and I have no idea why. Please could somone assist me.

Thanks!

Code:

---

struct packet {
    int length; // length of the data packet
    unsigned char *data; // pointer to data area
};

void addHeader(packet);
void stripHeader(packet);

int main() {
    unsigned char *d = (unsigned char*) "45100035A0ED400";
    packet p = {15, d};
    cout << "Original data: " << d << endl;
    addHeader(p);
    cout << "headerAdd: " << p.data << endl;
    //stripHeader(p);
    //cout << "stripHeader: " << p.data << endl;
   
    cin.get(); // pauses program;
    return 0;
}
   
void addHeader(packet x) {
    unsigned char temp[20] = {'A','F','A','F','1'};
    int count = 0;
   
    for(int i = 5; i < (sizeof(temp)/sizeof(char)); i++) { // appends x.data onto temp
        temp[i] = x.data[count];
        count++;
    }
    memcpy(x.data, temp, 20); 
}

---

Prototype of memcpy is:

Code:

---

void *memcpy(
  void *dest,
  const void *src,
  size_t count
);

---

As you can see it needs addresses of variables!
Make sure you have enough room to place 20 characters!
Also your function addHeader receive copy of variable p, maybe you shpould use pointers!
According to C/C++ standard sizeof(char) is always 1 so you don't need to write (sizeof(temp)/sizeof(char) since it is 20.

Code:

---

    memcpy(&x.data, &temp, 20);

---

Right I see that. It does not crash now, but however it outputs the original data that was first outputted.

As for the sizeof, yeah I guess you're right.

I have to stick to the way I have defined the struct according to the problem's specs.

Quote:

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Originally Posted by cyberCLoWn

Basically what I am trying to do is modify the contents of the place where the structures data pointer points to. I have to add 5 arb characters onto the packet's data.

When I run the program below, it crashes when performing the memcpy and I have no idea why. Please could somone assist me.

---

Use writable arrays.

Code:

---

void addHeader(packet x) {
    unsigned char temp[20] = {'A','F','A','F','1'};
    int count = 0;
   
    for(int i = 5; i < 20; i++) {
        temp[i] = x.data[count];
        count++;
    }
    cout << temp;
    memcpy(&x.data, temp, 20); 
}

---

When using the above code, it outputs the temp array perfectly and memcpy doesn't crash the program. However later on in the main function when it outputs p.data nothing has been changed.

String literals are not writeable.

Aren't you going about this a little backwards anyways? Wouldn't you have a structure that contains the header and the packet? I'm thinking something along this line:

Code:

---

struct packet
{
  int length;
  unsigned char *data;
};

struct T
{
  unsigned char header[5];
  struct packet packet;
};

void addPacket(struct packet *packet)
{
  struct packet stuff = { 15, "45100035A0ED400" };
  *packet = stuff;
}

int main()
{
  struct T t = { { 'A','F','A','F','1' } };
  addPacket(&t.packet);
  /* ... */
  return 0;
}

---

(Poor and unfinished and all that -- merely a poke in a different direction.)

[edit=yet another]More work in progress:

Code:

---

struct packet
{
  int length;
  unsigned char *data;
};

struct T
{
  unsigned char header[5];
  struct packet packet;
};

void addHeader(struct T *t)
{
  unsigned char header[] = { 'A','F','A','F','1' };
  memcpy(t->header, header, sizeof t->header);
}

void addPacket(struct packet *packet)
{
  struct packet stuff = { 15, "45100035A0ED400" };
  *packet = stuff;
}

int main()
{
  struct T t;
  addHeader(&t);
  addPacket(&t.packet);
  /* ... */
  return 0;
}

---

Try something like this

Code:

---

#include <iostream>
using namespace std;

struct packet {
    int length;          // length of the data packet
    unsigned char *data; // pointer to data area
};

void addHeader(packet& );

const int hdrlen = 5;

int main() {
    char * d = "45100035A0ED400";                      // this is a string literal that cannot be modified   
    packet p;
   
    p.length = strlen(d);                              // don't like magic numbers
    p.data = new unsigned char[p.length + hdrlen + 1];  // added 1 to make it printable
    strcpy( reinterpret_cast<char*>(p.data), d );      // cast needed char and unsigned char are not related
   
    cout << "Original data: " << d << endl;
    addHeader(p);
    cout << "headerAdd: " << p.data << endl;
   
    delete [] p.data;
   
    return 0;
}
   
void addHeader(packet & x) {                          // needs to be a reference x is supposed to be modified
    unsigned char temp[21] = {'A','F','A','F','1'};
    int count = 0;
       
    for(int i = 0; i < x.length + 1; i++) {
        temp[hdrlen+i] = x.data[count];
        count++;
    }
   
    memcpy( x.data, temp, x.length + hdrlen + 1);
}

---

Kurt

EDIT: little late ( did I really need half an hour to do this ? )