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I have a array of somestruct_t I'm adding to but don't know in advance how many elements it will need. So I malloc 5 slots of space to start with but I need to encapsulate it in another struct that saves the array boundary and whats used so far so I don't overflow. I'll realloc if it looks like it will overflow.

typedef struct {
  somestruct_t[] *data;
  uint32_t max_size;
  uint32_t used;
} something_box_t;

Is this the idiomatic way to do this?

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5
  • Use std::vector, assuming you didn't tag the question with both C and C++, but only wanted C. Commented Oct 11, 2012 at 19:59
  • 2
    Are you using C or C++? Choose one as the solution will be different. Commented Oct 11, 2012 at 20:01
  • In C, i didn't think it'd be different for C++. Commented Oct 11, 2012 at 20:07
  • 2
    @BreezyChick89: Very different in C++. While any C solution will likely be legal and valid in C++, there are much better ways of doing this in C++ that you can't do in C. Commented Oct 11, 2012 at 20:08
  • 1
    Do you have a practical maximum number of somestruct_t objects that will be in a something_box_t? How large are these somestruct_t objects? Commented Oct 11, 2012 at 20:11

2 Answers 2

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One thing that's done in C sometimes is to use a zero length array at the end of a struct and to allocate extra space for a variable number of elements. I'm not sure if zero length arrays are legal in C (they're not in C++) but it's commonly supported.

struct something_box_t {
  uint32_t max_size;
  uint32_t used;
  somestruct_t data[];
};

something_box_t *box = malloc(sizeof(something_box_t) + N*sizeof(somestruct_t));
box.max_size = N;
box.used = 0;

box.data[0] = ...;
box.data[1] = ...;

Of course, if you can switch to C++ you won't have to hack together a solution on your own. You could just do std::vector<somestruct_t> box; and have a solution far easier to use, as performant if not more performant, and more reliable than what you're likely to write in C.

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1 Comment

This question has been re-tagged [C] only.
2

You can see this often done by linked list (in C, for example in kernel mode windows drivers), here's an example how to implement your own.

C++ offers you templates to make lists more flexible.

And C++ also offers you stl containers (like std::vector) but bames53 already suggested that one.

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5 Comments

A linked list is NOT the common practice. It's much more common to use dynamic arrays (such as std::vector). You only want linked lists when there will be many insertions/deletions in the middle of the list.
@EmileCormier I've spent a lot of time in Windows kernel drivers recently and the only thing you saw there was a linked list. Do you have any reference for good dynamic array implementation for C?
I was assuming C++ when I wrote my comment. I can see how linked lists would be easier to implement in C.
@EmileCormier I've added explict (in C), I meant linked lists only for C implementation. Anyway thank you for pointing out that my answer can be misunderstand.
Please revise your answer to say "a common practice," this is a perfectly valid way depending on application requirements but by no means is the most standard solution and the OP should not be led to think as such.

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