Allocating array size inside struct from user input in C - c

Question

I'm recently working on a project in C. I have a requirement to allocate an array inside a struct and its size need to be obtained from the user. But because of specific requirements, I cannot use pointers and then allocate memory using malloc.
My code is as follows:
#define arraySize size
typedef struct sample{
int keys[arraySize]
int pointers[arraySize + 1]
} sample;
int main(){
//size should be obtained from user input
size = 15;
}
If the struct is defined inside main, it works fine but the issue is then the struct won't be global. If I declare the struct as mentioned in the code it gives the error message stating that the array size needs to be constant. Can anyone help me with this issue?

Answer 1

(Just giving examples for the comments.)
I think there are a few typical ways of handling your scenario:
Method 1: Global pointer to resources allocated on main's stack (already suggested in other comments):
This variant uses the C99 feature / GCC extension (w.r.t C89) for variably-modified types / variably modified arrays, per Jonathan Leffler's comment.
struct sample *globalSample = NULL;
int main(){
//size should be obtained from user input
size = 15;
struct sample{
int keys[size];
int pointers[size + 1];
} mySample;
globalSample = &mySample;
}
This variant uses alloca and "flexible array member". Per Jonathan Leffler's comment below note the change of officially defined syntax for this between C89 and later revisions of the C spec.
struct inner {
int key;
int pointer;
};
struct sample {
unsigned int num_allocated;
struct inner members[1]; // struct hack
};
struct sample *globalSample = NULL;
int main(){
//size should be obtained from user input
size = 15;
globalSample = alloca((size+1) * sizeof(struct sample));
globalSample->num_allocated = size; /*just for book-keeping*/
globalSample->members[0].key = /*...*/;
globalSample->members[0].pointer = /*...*/;
globalSample->members[1].key = /*... reaching past declared size */
globalSample->members[1].pointer = /*...*/;
}
Method 2: Define a max size, and validate user input against it.
#define MAX_ENTRIES 4096
struct sample{
int keys[MAX_ENTRIES];
int pointers[MAX_ENTRIES + 1];
} globalSample;
int main(){
//size should be obtained from user input
size = 15;
if(MAX_ENTRIES < size) { /* error case */ }
}