zero-sized array in struct/union

_Flaviu

I have a legacy C code:

char* file_name[0];

which generate a warning: warning C4200: nonstandard extension used : zero-sized array in struct/union it is correct to write:

char* file_name[_MAX_PATH];

? I don't know the impact of this modify...

Victor Nijegorodov

_Flaviu wrote:

it is correct to write:

char* file_name[_MAX_PATH];

Yes, it is correct presuming you are going to define the array of pointers

Lost User

That is not a struct or a union. Is there some other code that you need to show us?

_Flaviu

Or

char* file_name[1];

? anyway, is more than 0 :)

_Flaviu

The original code is:

typedef struct {
....
....
		char\* file\_name\[0\];			/\* File name in Unicode. \*/
};  // warning C4094: untagged 'struct' declared no symbols

also, I get another warning here: warning C4094: untagged 'struct' declared no symbols I don't know how to get rid of this warnings ...

Victor Nijegorodov

Well, it all depends upon what you are going to achieve... :rolleyes:

Lost User

_Flaviu wrote:

Or

char* file_name[1];

Yes. The only reason someone would declare zero-length is to dynamic allocate the array. You should change the array length to [1]. If you change it to _MAX_PATH (260) then you will be wasting 1036 bytes on a 32 bit machine and wasting 2072 bytes on a 64 bit machine. Wasting bytes is punishable by death. Best Wishes, -David Delaune

CPallini

Read carefully the documentation[^] (see the sample code). Using _MAX_PATH (or whatever >0) is correct, the impact is in memory: each time the struct is allocated, _MAX_PATH character pointers are allocated too. You might instead choose to disable the warning, if it makes sense (e.g. there is an additional field in the struct specifying the actual size of the array).

CPallini

Quote:

You should change the array length to [1]

Quote:

Wasting bytes is punishable by death

Take your own conclusions. :-D

_Flaviu

:laugh: Thank you !

_Flaviu

I guess disabling this warning is best solution … how can I do that ? With pragma statement ? If yes, which version of pragma should I use ?

Lost User

Hmmm, The law states that wasting bytes less or equal to 1 * sizeof(pointer) is allowed but only in the month of August. I guess he could remove the array qualifier but then that would probably break his compile. :) Best Wishes, -David Delaune

Lost User

You are using a typedef but have not given it the name that you wish to use. It should be something like:

typedef struct {
....
....
char* file_name[0]; /* File name in Unicode. */
} myStruct;
// myStruct is now an alias for the above structure

Also the comment on the last line makes no sense; firstly it is declaring an aray of pointers rather than characters. And secondly, you should not store Unicode characters in a char type array. It will most likely cause problems at run time. The zero length array is possibly valid, but it depends on how the code uses the struct. It can be used as a placeholder name for space that will be allocated for a dynamic structure at run time. Something like:

struct foo
{
int i;
char text[0];
};

// ... other code

struct foo* myFoo = (struct foo*)malloc(sizeof(struct foo) + 20); // additional 20 bytes for the char data.

Lost User

Never disable warnings, they are there to help you.

_Flaviu

Sorry, I didn't saw the link first time ...

CPallini

Disabling the warning is an option.

CPallini

Quote:

_MAX_PATH (260) then you will be wasting 236 bytes on a 32 bit machine and wasting 472 bytes on a 64 bit machine

Hey David, the math there is not clear to me. Do I need more caffeine this morning?

Lost User

Yes, but that is punished by [tar and feathering](https://en.wikipedia.org/wiki/Tarring\_and\_feathering) :laugh: Best Wishes, -David Delaune

Lost User

CPallini wrote:

Do I need more caffeine this morning?

No, but I do... Best Wishes, -David Delaune

leon de boer

If you are on C11 ... C11 6.7.9/14 allows the option

char file_name[];

It was addedd for exactly that reason

In vino veritas