Is malloc neccesary when creating array in function call?

Question

I have a function call

void copy_data(FILE *fin, FILE *fout, int size) {
     char buf[size];
     memset(buf, 0, size);
     fread(buf, sizeof buf, 1, fin);
     fwrite(buf, sizeof buf, 1, fout);

}

Is malloc here necessary, because I read that I need to use malloc when I don't know the size at compile time which I don't know here, but I feel like malloc is not neccesary here.

I am not using the malloc here because I think I can create array just like this? That's why I am asking if I should be using malloc here. — , Jan 08 '18 at 14:41

haccks · Answer 1 · 2018-01-08T15:12:02.233

4

C99 and later allow variable length arrays (VLA). buf is a variable length array and if the size is small you can use it. For large size (stack size limitations) arrays you need to allocate memory dynamically.

Note that since C11, VLA is made optional.

edited Jan 08 '18 at 15:12

answered Jan 08 '18 at 14:41

haccks

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It is not an extension, it is an optional feature and gcc supports it. – Jens Gustedt Jan 08 '18 at 14:58
@JensGustedt; The thing is GCC supports VLA in C90 as an extension and I thought it is also an extension for C99 an later. – haccks Jan 08 '18 at 15:11

gonczor · Accepted Answer · 2018-01-08T15:56:41.307

3

No it's not necesssary here provided that you are not going to use large amount of memory. Data related to this function will be allocated on a stack together with other variables (you are putting size elements of type char on the stack). You might, however take these points into consideration. If you want to use heap which is bigger, you would need to use malloc() and then manually free the memory.

EDIT As it has been pointed out, the C11 standard makes Variable Length Arrays optional, so problem of portability of your code also comes accross.

edited Jan 08 '18 at 15:56

answered Jan 08 '18 at 14:43

gonczor

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@SergeBallesta thanks for mentioning it. I've made an update. – gonczor Jan 08 '18 at 15:57

Is malloc neccesary when creating array in function call?

2 Answers2