C array malloc() VS {}

Question

I know that the result of

int *x = malloc(sizeof(int)*100);

and

int x[100];

is same,but the first one is allocating heap memory,and second one is allocating stack memory.

Now i need to create a huge array(about 10000 element,not in a pattern),I think malloc() is more suitable.

But when i ready to initialize the array, I face a problem. I cannot use any looping to init the array,how can I init an array that created by using malloc ,just like using

int x[100] = {1,2,3,4,......,6,7,5};

StackOverflow Rule of Thumb #3 is in effect: "If the OP says 'I know', they don't." — Kerrek SB
– Kerrek SB, Commented Nov 18, 2012 at 19:50
int *x = malloc(sizeof(int)*100); and int x[100]; are not the same, and not just because of stack and heap. — effeffe
– effeffe, Commented Nov 18, 2012 at 20:21

Kerrek SB · Accepted Answer · 2012-11-18 19:52:34Z

2

When you say int a[] = { 1, 2, 3 };, you are using an initializer to provide the initial data for (and infer the size of) the array a. This is part of the grammar of C.

When you say int * p = malloc(1000);, you are simply making a library call and storing a pointer. There is no mechanism in the language or library to provide initial values for the memory to which this pointer points, nor is it required that the pointer point to anything (it may be NULL).

You should notice that arrays are not pointers, and pointers are not arrays. a and p are entirely different animals, notwithstanding the fact that you can say p[1] = a[1];.

answered Nov 18, 2012 at 19:52

Kerrek SB

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2 Comments

Paul Over a year ago

Are there cases where a cannot be treated like b? This is with the exception of functions like free() and realloc(), or it being potentially unsafe to pass pointers to a.

Kerrek SB Over a year ago

@Paul: Sure: int n; p = &n; is valid, but a = &n; is not. Array decay produces "rvalues" (if that makes sense). Also, &p has type int**, while &a has type (int*)[3].

Aniket Inge · Accepted Answer · 2012-11-18 19:52:45Z

0

If you cannot initialize the array with looping, you can use memset() and get away with it.

answered Nov 18, 2012 at 19:52

Aniket Inge

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Comments

gnasher729 · Accepted Answer · 2015-06-05 15:29:59Z

0

If the data doesn't change, the best way is to write

static const int x [100] = { 23, 12, 5, 7, ... };

No code. No time needed for initialisation. I have actually seen code that initialised megabytes of data that way, with no problems.

answered Jun 5, 2015 at 15:29

gnasher729

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