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Is it possible to create a struct containing two dynamically size arrays in c?

I have something like this:

#define MAX_DATA 512

    struct Address {
    int id;
    int set;
    char name[MAX_DATA];
    char email[MAX_DATA];
};

I'd like MAX_DATA to be defined at run time.

I have seen the struct hack:

How to include a dynamic array INSIDE a struct in C?

But I think this only works for one field per struct?

This code is from http://c.learncodethehardway.org/book/ex17.html

The extra credit section near the bottom of that page contains the bit about changing the sizes to be dynamic.

Community
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  • Sure, you just change the arrays to pointers, and then you need one call to `malloc` for each dynamically sized array... – Nicu Stiurca Apr 10 '14 at 10:26
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    I really need to read the tags first, I was almost suggesting "use a std::vector" – Marco A. Apr 10 '14 at 10:27
  • It's not a hack but clearly mentioned in the standard. It's called flexible array member or zero-length array. – ajay Apr 10 '14 at 10:46
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    IMHO cut your losses and use a normal malloc instead of a struct hack , you'll thank yourself for it a few years later when you maintain the code. – M.M Apr 10 '14 at 10:53
  • @MattMcNabb I don know that a "normal" malloc (i.e. three mallocs) is more maintainable. Any complexity from sharing one buffer can usually be encapsulated inside the function that creates `Address` instances. And using one buffer means that an Address can be freed using a normal `free()`. – Adrian Ratnapala Apr 11 '14 at 03:41

4 Answers4

3

I once did this:

struct Thing {
    int Number;
    char *MoreBytes;
    char Bytes[]
}


Thing *MakeThing(int nBytes, int nMoreBytes) 
{
    Thing *th = malloc(sizeof(Thing) + nBytes + nMoreBytes);
    // Error checking is for grrrlz.

    th->Number = 42;
    th->MoreBytes = th->Bytes + nBytes; 

    return th;
}

Thus the array th->Bytes actually holds both "arrays", and the pointer th->MoreBytes tells us where one array ends and another begins.

Adrian Ratnapala
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You can do this using the struct hack, now known as a flexible array. It just requires you to pack both arrays into the flexible part of the struct.

Suppose that you want the arrays to be of length N and M respectively. Then allocate a flexible array as if you were allocating a single array of length N+M. Then use indices 0..N-1 for the first array, and indices N..N+M-1 for the second array.

David Heffernan
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It works (at least for GCC 4.7.2) if you put your struct Address definition in a function, like this:

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

int
main(int argc, char *argv[])
{
    int len = atoi(argv[1]);

    struct Address {
        int id;
        char name[len];
        int set;
        char email[len];
    };

    printf("sizeof(struct Address) = %zu\n", sizeof(struct Address));
    exit(EXIT_SUCCESS);
}

Testing:

$ ./a.out 10
sizeof(struct Address) = 32
$ ./a.out 20
sizeof(struct Address) = 48
Lee Duhem
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  • Again, that's not the struct hack. Ask yourself what happens when `len` becomes very large. Or you want the struct to live beyond the scope in which it was declared. – David Heffernan Apr 10 '14 at 10:31
  • @DavidHeffernan Yeah, it is not struct hack, but something like variable length array (VLA). And it may not work or work well for those situations you just mentioned. – Lee Duhem Apr 10 '14 at 10:37
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struct Address 
{
int id;
int set;
char *name;
char *email;
};

Now in the main() function, Use some variable, lets say len to store length of the array, and dynamically allocate required memory using malloc().

int len;
struct Address Add;
printf("Enter the lenght of the array you want?");
scanf("%d",&len);
Add.name=(char *)malloc(len); 
Add.email=(char *)malloc(len);

otherwise you can add len as the member of the struct Address

struct Address 
{
int id;
int set;
int len;
char *name;
char *email;
};

Now in main()

struct Address Add;
printf("Enter the lenght of the array you want?");
scanf("%d",&Add.len);
Add.name=(char *)malloc(Add.len); 
Add.email=(char *)malloc(Add.len);
Vinay
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