Array creation syntax in C

Question

Are the following two methods of creating arrays in C equivalent?

int main( void )
{
    int *array = malloc(sizeof(int) * 10);
    int array2[10];
}

My thought is that method two is syntactic sugar for method one, but I'm not sure.

Also, what do array and array2 contain after these declarations? I know array is a pointer to the start of an array, but what is the array initialized to? How about for array2?

No they are not the same. Do some reading on stack vs heap memory and what malloc actually does. — CollinD
– CollinD, Commented Jan 28, 2016 at 0:28
One declares an array. The other declares a pointer, calls malloc and makes the pointer to hold the value that malloc returned. — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Jan 28, 2016 at 2:19

Community · Accepted Answer · 2017-05-23 12:04:07Z

5

They are not remotely equivalent. This:

int *array = malloc(sizeof(int) * 10);

will allocate a block of memory of the heap, and leave you with a pointer to that memory.

This:

int array2[10];

will allocate some memory on the stack. Read this excellent answer about stack and heap memory: What and where are the stack and heap?

edited May 23, 2017 at 12:04

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answered Jan 28, 2016 at 0:28

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8 Comments

Adam Over a year ago

So array will persist until the program exits whereas array2 will go out of scope as soon as the function it is created in returns. For both, we have no way of knowing what the values are intially (i.e. they're not all initialized to 0 or something).

Adam Over a year ago

Or you could say that array is allocated at runtime (dynamically) whereas array2 is allocated statically at compile time. Assuming array2 is declared in the main() function.

Adam Over a year ago

@user3386109 Fair enough. Say this was all happening in main() then. Then the two methods would be essentially equivalent (except we might run into problems if we tried to create a really large array on the stack).

Adam Over a year ago

@user3386109 If I did int array[10] inside some function I call in main then array would go out of scope as soon as the function returns.

Stack Exchange Broke The Law Over a year ago

@Adam You seem to be saying that they are "basically equivalent, except for <list of all the ways in which they're not equivalent>". Along those lines, statues and oranges are basically equivalent, except that statues are art pieces and oranges are fruit, statues are bigger than oranges, oranges are edible while statues aren't, oranges are a commodity while statues are often unique, and so on.

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