Malloc or normal array definition?

Question

When shall i use malloc instead of normal array definition in C?

I can\'t understand the difference between:

int a[3]={1,2,3}
int array[sizeof(a)/siz         


        

Answer 1

Your code is very strange.

The answer to the question in the title is probably something like "use automatically allocated arrays when you need quite small amounts of data that is short-lived, heap allocations using malloc() for anything else". But it's hard to pin down an exact answer, it depends a lot on the situation.

Not sure why you are showing first an array, then another array that tries to compute its length from the first one, and finally a malloc() call which tries do to the same.

Normally you have an idea of the number of desired elements, rather than an existing array whose size you want to mimic.

The second line is better as:

int array[sizeof a / sizeof *a];

No need to repeat a dependency on the type of a, the above will define array as an array of int with the same number of elements as the array a. Note that this only works if a is indeed an array.

Also, the third line should probably be:

array = malloc(sizeof a);

No need to get too clever (especially since you got it wrong) about the sizeof argument, and no need to cast malloc()'s return value.