具有文件作用域（C）的已分配变量

Question

I'm trying to declare a variable that will be available to multiple .c files. It's a malloced array of pointers to structs.

From what I understand I'll have to declare it with extern in a header file, but since I can't malloc in the header file do I declare it in the header file and then malloc in the .c file? I can't see how this would be done.

Answer 1

You put the declaration in the header, the definition inside a .c file and the call to malloc in a function that gets executed before the first time you dereference the pointer (like the main function for example). So it will look something like this:

foo.h:

extern struct your_struct** pointer;
void foo_init();
void foo_cleanup();

foo.c:

#include <stdlib.h>
#include "foo.h"

struct your_struct** pointer;

void foo_init() {
    pointer = malloc(sizeof(your_struct*) * some_size);
    // initialize the pointer in the array
}

void foo_cleanup() {
    // free the pointers in the array if you used malloc to initialize them
    free(pointer);
}

main.c:

#include "foo.h"

int main() {
    foo_init();
    // do other stuff
    foo_cleanup();
    return 0;
}

Answer 2

Since you want to use a dynamic variable, you have to use a pointer. And beside declaration, you need to define it somewhere. In your case, you in addition should allocate the memory as early as possible in your code, ie in the first lines of main.

Answer 3

Yes, you declare it in the header and malloc it in the .c file. Malloced pointers don't have a scope as such and are only de-allocated when they are free d. So you can use the pointer throughout your entire program.

Answer 4

1. Declare as extern in header file
2. Define it as global in one of your .c files
3. Allocate the pointed array in the same .c file

You're good to go.

Note though, if you access it in different files, you need to ensure that at the point of each access the data has been already allocated (and not freed yet).

Answer 5

In the header file declare the pointer. Include the header in all .c modules that refer to the pointer.

extern struct yourstruct *ps;

Then define the variable in the most pertinent .c module

struct yourstruct *ps;

ps = malloc(....

Answer 6

FILE **file_array=NULL;

function(){
    file_array = malloc(size*sizeof(FILE*));
}

Answer 7

You will need to malloc it in c file and declare it in header file. You will also need to take care about its access and deallocation. It has to be de-allocated with free and only once.Probably at the end of you program execution. Also you need to put guard condition (check for null) before accessing it in multiple files.