Initialized a pointer array in C - Variable sized object may not be initialized

Question

I was trying to initialise a array made by pointer:

the code I used was:

int c = 15;
Struct *Pointer[c] = {NULL};

but C give me a error message which says:

"message": "variable-sized object may not be initialized",

but when I change my code to:

Struct *Pointer[15] = {NULL};

it worked!

Is there any way to fix it? I can't use 15 instead of variable "c"

Cheers!

Like it says, if the array size isn't constant you can't use an initializer. You'll have to write a loop: for (int i = 0; i < c; i++) Pointer[i]=NULL;. Or use memset if your platform has NULL pointers as all-bits-zero (most do). — Nate Eldredge
– Nate Eldredge, Commented Apr 25, 2021 at 15:20
@NateEldredge Thanks Nate - I tried memset, it worked!! Thank you - One thing I do not understand, even if I made C as Const int, it still doesn't work. Is that normal.? — leo
– leo, Commented Apr 25, 2021 at 15:24
Yes, that's normal. Just part of the somewhat peculiar way that C treats const. C++ is different. — Nate Eldredge
– Nate Eldredge, Commented Apr 25, 2021 at 15:33

qingy2019 · Accepted Answer · 2021-04-28 15:27:45Z

4

You need to make a loop to initialize the array:

for (int i = 0; i < c; i++)
    Pointer[i] = NULL; // set all of these values to NULL

Now, Pointer will be of size c.

Also, this link may help, too: Array[n] vs Array[10] - Initializing array with variable vs real number

edited Apr 28, 2021 at 15:27

answered Apr 25, 2021 at 15:23

qingy2019

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Vlad from Moscow · Accepted Answer · 2021-04-25 15:24:48Z

2

Variable length arrays may not be initialized in their declarations.

You can use the standard string function memset to initialize the memory occupied by a variable length array.

For example

#include <string.h>

//...

int c = 15;
Struct *Pointer[c];

memset( Pointer, 0, c * sizeof( *Pointer ) );

Pay attention to that variable length arrays shall have automatic storage duration that is they may be declared in a function and may not have the storage specifier static.

answered Apr 25, 2021 at 15:24

Vlad from Moscow

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2 Comments

leo Over a year ago

Hey Vlad - I think you are right - By using memset(), it somehow initialized another array....is there anyway to fix it?

Nate Eldredge Over a year ago

@leo: That won't happen if you use memset correctly, so you must have a bug somewhere. You could ask it as a new question, but be sure to include a minimal reproducible example.

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