Resizing a char[x] to char[y] at runtime

Question

OK, I hope I explain this one correctly. I have a struct:

typedef struct _MyData
{
   char Data[256];
   int  Index;
} MyData;

Now, I run into

Answer 1

You cant reacolate manualy.

You can do some tricks wich i was uning when i was working aon simple data holding sistem. (very simple filesystem).

typedef struct
{
    int index ;     
    char x[250];
} data_ztorage_250_char;

typedef struct
{
    int index;      
    char x[1000];
} data_ztorage_1000_char;

int main(void)
{
      char just_raw_data[sizeof(data_ztorage_1000_char)];
      data_ztorage_1000_char* big_struct;
      data_ztorage_250_char* small_struct;
      big_struct = (data_ztorage_1000_char*)big_struct; //now you have bigg struct
      // notice that upper line is same as writing 
      // big_struct = (data_ztorage_1000_char*)(&just_raw_data[0]);

      small_struct = (data_ztorage_250_char*)just_raw_data;//now you have small struct

      //both structs starts at same locations and they share same memory
     //addresing data is 
      small_struct -> index = 250;
}

Answer 2

How about a really, really simple solution? Could you do:

typedef struct _MyData
{
   char Data[1024];
   int  Index;
} MyData;

I have a feeling I know your response will be "No, because the other program I don't have control over expects 256 bytes"... And if that is indeed your answer to my answer, then my answer becomes: this is impossible.

Answer 3

You don't state what the Index value is for.

As I understand it you are passing data to another program using the structure shown. Is there a reason why you can't break your data to send into chunks of 256bytes and then set the index value accordingly? e.g.

Data is 512 bytes so you send one struct with the first 256 bytes and index=0, then another with the next 256 bytes in your array and Index=1.

Answer 4

The problem is in the way you're putting the question. Don't think about C semantics: instead, think like a hacker. Explain exactly how you are currently getting your data into the other process at the right time, and also how the other program knows where the data begins and ends. Is the other program expecting a null-terminated string? If you declare your struct with a char[300] does the other program crash?

You see, when you say "passing data" to the other program, you might be [a] tricking the other process into copying what you put in front of it, [b] tricking the other program into letting you overwrite its normally 'private' memory, or [c] some other approach. No matter which is the case, if the other program can take your larger data, there is a way to get it to them.

Answer 5

You can use a flexible array member

typedef struct _MyData
{
   int  Index;
   char Data[];
} MyData;

So that you can then allocate the right amount of space

MyData *d = malloc(sizeof *d + sizeof(char[100]));
d->Data[0..99] = ...;

Later, you can free, and allocate another chunk of memory and make a pointer to MyData point to it, at which time you will have more / less elements in the flexible array member (realloc). Note that you will have to save the length somewhere, too.

In Pre-C99 times, there isn't a flexible array member: char Data[] is simply regarded as an array with incomplete type, and the compiler would moan about that. Here i recommend you two possible ways out there

• Using a pointer: char *Data and make it point to the allocated memory. This won't be as convenient as using the embedded array, because you will possibly need to have two allocations: One for the struct, and one for the memory pointed to by the pointer. You can also have the struct allocated on the stack instead, if the situation in your program allows this.
• Using a char Data[1] instead, but treat it as if it were bigger, so that it overlays the whole allocated object. This is formally undefined behavior, but is a common technique, so it's probably safe to use with your compiler.

Answer 6

You would re-arrange the structure like that

typedef struct _MyData
{
   int  Index;
   char Data[256];
} MyData;

And allocate instances with malloc/realloc like that:

my_data = (MyData*) malloc ( sizeof(MyData) + extra_space_needed );

This is an ugly approach and I would not recommend it (I would use pointers), but is an answer to your question how to do it without a pointer.

A limitation is that it allows for only one variable size member per struct, and has to be at the end.