我如何从一个函数返回一个结构，该结构包含一个带有正确数组的数组？

Question

I'm making a program that returns a struct containing an array, but the elements in the array are completely wrong. I keep searching for an answer on this site, Google, and even Bing and nothing. The best I can find are answers like this:

Functions can't return arrays in C.
However, they can return structs. And structs can contain arrays...

from How to make an array return type from C function?

Now, how do I fix this without the use of pointers?

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

struct Codes{
int as;
int a[];
};

struct Codes create(int as){
    int a[as];
    for(int j = 0;j<as;j++)
        a[j]=j+1;
    struct Codes c;
    c.as = as;
    c.a[c.as];
    for(int i=0; i<as; i++)
        c.a[i] = a[i];

    for(int i=0; i<as; i+=1)
        printf("%d \n", c.a[i]);

    return c;
}

int main(int argc, char **argv) {

    struct Codes cd;
    int as = 4;
    cd = create(as);

    for(int i=0; i<4; i+=1)
        printf("%d \n", cd.a[i]);

}

Actual output:

1 
2 
3 
4 
0 
0 
2 
-13120 

Expected output:

1 
2 
3 
4 
1
2
3
4

Answer 1

In your function, struct Codes create(int as) , the struct Codes c; is allocated on the stuck, so the memory is no longer valid once the function returns...

...It is true that the core struct is copied in the return value... but the variable array length ca isn't part of the struct (it's a memory "trailer" or "footer") and isn't copied along with the return value.

Either:

1. allocate the struct and pass it to a struct Codes create(struct Codes *dest, int as) function; OR

2. make the struct array fixed in size struct Codes{ int as; int a[4]; }; struct Codes{ int as; int a[4]; };

Good luck.

Answer 2

struct s with flexible value are not meant to be manipulated by value, only by pointer.

You cannot return a struct with a flexible member by value, because C does not know how many items it needs to allocate to the return value, and how many bytes it needs to copy.

Allocate your struct in dynamic memory using malloc of sufficient size, copy your data into it, and return a pointer to the caller:

struct Codes *c = malloc(sizeof(struct Codes)+as*sizeof(int));
c->as = as;
for (int i = 0 ; i != as ; i++) {
    c->a[i] = i+1;
}
return c;

Change your function to return a pointer; make sure the caller frees the result.