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
– Yabada, Commented Dec 23, 2013 at 15:10
@EoiFirst Did you also know that you don't have to not cast the malloc? — this
– this, Commented Dec 23, 2013 at 15:13
@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
– Elias Van Ootegem, Commented Dec 23, 2013 at 15:33

simonc · Accepted Answer · 2013-12-23 15:06:53Z

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, 2013 at 15:06

simonc

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Comments

Will Dean · Accepted Answer · 2013-12-23 15:06:29Z

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, 2013 at 15:06

Will Dean

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1 Comment

Will Dean Over a year ago

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.)

Community · Accepted Answer · 2017-05-23 10:25:30Z

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, 2017 at 10:25

CommunityBot

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answered Dec 23, 2013 at 15:07

haccks

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