Create Dynamic Arrays of Pointers to Structures in C

Question

I have a code like the following, which works fine:

AVFrame *frame = NULL;
open_input(&frame);

where the input argument of open_input is something like: AVFrame **frame;

Now, I want to extend this code to work with an array of frames (say, N frames). I tried the following code but my code stops working after being compiled by gcc in MingW:

int i, N = 3;
AVFrame **frame;
frame = (AVFrame *) malloc(N * sizeof(AVFrame *));
for(i=0;i<N;i++){
   frame[i] = (AVFrame *) malloc(sizeof(AVFrame));
   open_input(&frame[i]);
}

Do you know, what is the problem?

First you don't need to cast malloc's return, then your first cast is false — Yabada, Dec 23 '13 at 15:10
@EoiFirst Did you also know that you don't have to not cast the malloc? — this, Dec 23 '13 at 15:13
@EoiFirst read this: http://stackoverflow.com/a/14879184/2327831 — this, Dec 23 '13 at 15:29
let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/43771/discussion-between-eoifirst-and-self) — Yabada, Dec 23 '13 at 15:31
@self.: Don't go off in discussions like this: This question is tagged _C_, not _C++_. Next, you might aswell argue to write a script that copies all your .c files to a `.cpp` file... in C, casting `malloc`'s return pointer is generally not done — Elias Van Ootegem, Dec 23 '13 at 15:33
@self.: Can't help myself, but none of the arguments in your linked answer make sense if you use `malloc` as you should use it: `someVar = malloc(sizeof(*someVar));` change the type of `someVar`, and you don't have to change your `malloc` call. You'd have to refactor all related code, changing your cast. casting actually makes for more error-prone development, IMO — Elias Van Ootegem, Dec 23 '13 at 15:40
@EliasVanOotegem Those aren't my arguments, I was merely sharing information with a curious member of SO. *Don't go off in discussions like this*; it's a good thing you don't own the site, it almost looks like you are trying to exert influence. Discussion ended. — this, Dec 23 '13 at 15:42
@self.: I'm not trying to exert influence, merely pointing out [that you're not using comments the way they were intended by the people who _do_ own the site](http://stackoverflow.com/help/privileges/comment). If I wanted to exert influence, I'd have flagged your comments for moderation attention. I'm not petty, so I didn't, and besides: the mods have enough work already — Elias Van Ootegem, Dec 23 '13 at 17:30

score 2 · Answer 1 · answered Dec 23 '13 at 15:06

2

If you want to allocate an array of frames, you could simply do

AVFrame *frame = malloc(N * sizeof(*frame));

accessing each element using frame[index]

A pointer to a pointer to AVFrame would only be required if you wanted an array of AVFrame arrays.

answered Dec 23 '13 at 15:06

simonc

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score 0 · Answer 2 · answered Dec 23 '13 at 15:06

0

You should probably be passing frame[i] to open_input rather than &frame[i]

I also think that this:

frame = (AVFrame *) malloc(N * sizeof(AVFrame *));

should be

frame = (AVFrame **) malloc(N * sizeof(AVFrame *));

I'd have thought your compiler might be warning about that too.

If you haven't already done so, then turn up the warning level on the compiler (probably /Wall on your compiler), and deal with the results.

answered Dec 23 '13 at 15:06

Will Dean

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You've given no description of the problem other than 'stops working', so it's not really possible to offer more help. Run under a debugger would be my next suggestion, as long as you've dealt with the compiler warnings (which you must be getting with /Wall, particularly if you've completely changed the type of the argument to open_input.) – Will Dean Dec 23 '13 at 15:15

score 0 · Answer 3 · edited May 23 '17 at 10:25

0

&frame[i] is of type AVFrame **. It seems that open_input is expecting the argument of type AVFrame *.
Change

open_input(&frame[i]);

to

open_input(frame[i]);

Here frame[i] is of type AVFrame *.

And also do not cast the return value of malloc.

edited May 23 '17 at 10:25

Community

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answered Dec 23 '13 at 15:07

haccks

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Create Dynamic Arrays of Pointers to Structures in C

3 Answers3