在文件范围Struct C中修改的变量

Question

Hi I get compile error when I try to assign the struct with array size of global variable which user will input in command line, then pass value back to variable that was declared globally.

Here is just sample code

#include<stdlib.h>
#include<stdio.h>

int Type;

struct list_el {
    int val;
} list1[Type];

struct list_el item;

int main() {
    Type = 10; //just sample
}

error

variable modified at file scope Struct C

Is there anyway to fix the problem? I need it to work globally.

Thank you so much.

Answer 1

When declaring an array, you need a compile-time constant for the size, which Type isn't.

If you want to allocate during runtime, use pointers and malloc .

---

Well, actually you can have non-constant variables for array size, it's called variable-length arrays . The problem with using a global variable for the size is that global variables are initialized to zero, so what you are doing is actually creating an array with zero elements, but only if the variable Type is initialized before the array is created.

Answer 2

Your code is not valid: you can't first declare an array using a variable as the size, and then assign to the variable.

Or, you can, but the array certainly won't magically change.

Also, it's very strange to have the variable and array be global.

You should probably just do:

int main(void)
{
  int num;

  printf("enter size:\n");
  if(scanf("%d", &num) == 1 && num > 0)
  {
    struct list_el list1[num];

    /* here, work with the array. */
  }
}

Note that using a variable like this is a C99 feature.

Answer 3

If you want both the list and its magnitude Type to be global and dynamically allocated, you need to do just that: dynamically allocate .

#include<stdlib.h>
#include<stdio.h>

int Type;

struct list_el {
    int val;
} *list;

int main() 
{
    Type = 10;

    list = malloc(Type * sizeof(*list));

}

That said, seriously question whether you need these globally. If your compiler supports it you can use a local variable-length array (VLA). It is a C99 optional feature, testable by checking if __STDC_NO_VLA__ is not defined.

Answer 4

You could use alloca() in main to stack-allocate the memory.

struct list_el {
  int n;
} *list;
static int Type;

int main(int argc, char** argv) {
  Type = argc;
  list = alloca(sizeof(struct list_el) * Type);

  // use list...
}

The memory will stay in-scope until main() returns. I don't know what happens in an atexit() callback. That's an interesting question.