How can I declare a structure of an unknow size?

Question

I have this structure :

  struct          __attribute__((packed)) BabelPacket
  {
    unsigned      senderId;
    unsigned      dataLength;
    unsigned char data[0];
  };

And to declare it I do :

  BabelPacket *packet = reinterpret_cast<BabelPacket *>(new char[sizeof(BabelPacket) + 5]);
  packet->senderId = 1;
  packet->data = "kappa";
  packet->dataLength = 5;

But when I compile I have this error :

error: incompatible types in assignment of ‘const char [6]’ to ‘unsigned char [0]’
   packet->data = "kappa";

            ^

Have you an idea how I can do that ? And I need to send this structure through a socket, to get the object back in my server, so I can use only C types.

PD: https://stackoverflow.com/questions/688471/variable-sized-struct-c Looking for a better one though. — Baum mit Augen, Oct 17 '16 at 09:58
This works in C ([like this](http://stackoverflow.com/a/19293671/335858)) but not in C++. — Sergey Kalinichenko, Oct 17 '16 at 09:59
You could use pointer there and allocate the string separately. — Kami Kaze, Oct 17 '16 at 10:02
C++ does not support array assignment. You cannot set an array equal to anything. So `packet->data = "kappa";` makes no sense under the rules of C or C++. — David Schwartz, Oct 17 '16 at 10:06
What the hell!!! what's the intention in writting `reinterpret_cast(new char [sizeof(BabelPacket)])` ???? — Luis Colorado, Oct 18 '16 at 09:22
@LuisColorado I need to allocate more memory than the size of the structure, because I use a little hack that allows to have an unknown size array at the end of my structure. — Dimitri Danilov, Oct 18 '16 at 21:11
@DimitriDanilov, then you lead to undefined behaviour, as new is going to call the `char[]` constructor (it doesn't exist but it should call the one for the object type you use in `new`) instead of the one you could define for your `struct`. And no way to call it after. The best approach is to define two structs, one with the hack and other without it (one can be a subclass of the other, so you don't need to reimplement things) and call `new` with the proper argument. Don't think like in C when you are programming C++. — Luis Colorado, Oct 19 '16 at 05:31

Some programmer dude · Answer 1 · 2016-10-17T13:05:49.243

3

If this was a C program, the error you get is because you try to assign to an array, which is not possible. You can only copy to an array:

memcpy(packet->data, "kappa", 5);

Also note that if you want the data to be a C string, you need to allocate an extra character for the string terminator '\0'. Then you can use strcpy instead of memcpy above. Or strncpy to copy at most a specific amount of characters, but then you might need to manually terminate the string.

However, this should not work in C++ at all, unless your compiler have it as an extension.

edited Oct 17 '16 at 13:05

answered Oct 17 '16 at 10:02

Some programmer dude

400,186
35
402
621

1

Better provide a safer alternative to `strcpy`. – Michael Foukarakis Oct 17 '16 at 10:11
1

I think you mean `memcpy(packet->data, "kappa", 5);` – juanchopanza Oct 17 '16 at 13:05

John Q. · Answer 2 · 2016-10-19T06:32:14.410

0

You can't assign a literal string that way. You'll need to allocate additional memory for the string, then copy to the data pointer.

struct A {
    size_t datasize;
    char data[0]; // flexible member must appear last.
};

A* create_A(const char* str)
{
    size_t datasize = strlen(str) + 1; // null terminated (?)
    A* p = reinterpret_cast<A*>(new char[sizeof(A) + datasize]);
    memcpy(p->data, str, datasize);
    p->datasize = datasize;
    return p;
}

A* p = create_A("data string");

This solution is only applicable in environments supporting zero-length or flexible arrays. In fact, a better solution may be to write the sockets code in C and export that interface for use in C++.

edited Oct 19 '16 at 06:32

answered Oct 17 '16 at 10:23

John Q.

177
4

`char data[0];` That isn't allowed in C++. – juanchopanza Oct 17 '16 at 10:49
@juanchopanza Considering he is using attributes, it's safe to assume he's using GCC which supports it as an extension. – John Q. Oct 17 '16 at 10:53
That would depend on compiler flags. – juanchopanza Oct 17 '16 at 10:56
Nobody is stopping you from writing a complete, precise answer. – juanchopanza Oct 17 '16 at 12:16
@juanchopanza That's funny, since this is the only correct answer. The accepted answer only allocates 5 bytes, then uses strcpy which copies 6 bytes. The other answer has the operands out of order for memcpy. Actually I'm missing a comma and a semicolon. So much for precision. – John Q. Oct 17 '16 at 12:59

Nikos Kazazakis · Accepted Answer · 2016-10-17T14:36:43.870

If you are willing/allowed to change the unsigned char to a regular char, you can use strcpy:

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

struct          __attribute__((packed)) BabelPacket
{
  unsigned      senderId;
  unsigned      dataLength;
  char data[0]; // I changed this to char in order to use strcpy
};

int main(){
  BabelPacket *packet = reinterpret_cast<BabelPacket *>(new char[sizeof(BabelPacket) + 5]);
  packet->senderId = 1;
  // Copy the string. Add NULL character at the end of 
  // the string to indicate its end
  strcpy(packet->data, "kappa\0"); 
  packet->dataLength = 5;

  // Verify that the string is copied properly
  for (int i=0;i<packet->dataLength;++i){
    std::cout<<packet->data[i];
  }
  std::cout<<std::endl;

  return 0;
}

Note that this will only work if data is at the end of the struct, otherwise there is no contiguous memory to allocate data. If I swap the order of the elements to:

struct          __attribute__((packed)) BabelPacket
{
  unsigned      senderId;
  char data[0]; // I changed this to char in order to use strcpy
  unsigned      dataLength;
};

the output of the code above (instead of "kappa"), would be "a".

A more reliable way if you are determined to use C-arrays would be to assume a maximum number of elements and preallocate the array, i.e.:

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

#define MAX_NUMBER_OF_CHARACTERS 5 // Many ways to do this, I defined the macro for the purposes of this example

struct          __attribute__((packed)) BabelPacket
{
  unsigned      senderId;
  // I changed this to char in order to use strcpy. Allocate the
  // max number + 1 element for the termination string
  char data[MAX_NUMBER_OF_CHARACTERS+1]; 
  unsigned      dataLength;
};

int main(){
  BabelPacket *packet = reinterpret_cast<BabelPacket *>(new char[sizeof(BabelPacket) + 5]);
  packet->senderId = 1;
  packet->dataLength = 5;
  if (dataLength>MAX_NUMBER_OF_CHARACTERS){
    std::cout<<"String greater than the maximum number of characters"<<std::endl;
  }
  // Copy the string. Add NULL character at the end of 
  // the string to indicate its end
  strcpy(packet->data, "kappa\0");  

  // Verify that the string is copied properly
  for (int i=0;i<packet->dataLength;++i){
    std::cout<<packet->data[i];
  }
  std::cout<<std::endl;

  return 0;
}

This code produces the correct output, and protects you against violations. As you can see, it can get messy pretty quickly, which is why I would recommend to use std::vector for this. The dataLength may then be retrieved automatically as the size of the vector, and you are always protected against overflows.

You should explain that allocation is hacked in a way to append free room at the end of the struct, and the trick will fail if the array isn't the last field. And you forgot to allocate an extra byte for the string terminator. — , Oct 17 '16 at 13:17
Unfortunatly I cannot use a std::vector because I send this structure through a socket so I must C types. — Dimitri Danilov, Oct 17 '16 at 22:39
You could use the pointer to the first element in the structure, and use the vector in the rest of the code to make sure memory is managed properly no? — Nikos Kazazakis, Oct 17 '16 at 22:42

How can I declare a structure of an unknow size?

3 Answers3