Question

I want to hide a specified struct in my c module, so struct declaration is in the *.c file and the header contains a typedef. Something like this:

/* member.h */
typedef struct MEMBER_T *member_t;

/* member.c */
#include "member.h"
struct MEMBER_T {
    unsigned int member_id;
    char name;
};

and later in the c file I want to do a member table:

members = calloc(10, sizeof(member_t));

but it’s wrong, I know.
How can I use thie solution? I mean how can I create an object for member_t?

Thanks in advance!

Answer 1

You are allocating an array. you still need to allocate each of its elements.

If you want to hide the struct in .c file, than it would also be a good idea to encapsulate its creation. So the user of you struct will not ha to bother whether to user sizeof(member_t) or sizeof(*member_t) or sizeof(struct MEMBER_T).

define a function membet_t* create_member_array(int) in you header in addition to typedef struct MEMBER_T* member_t.

implement it in you .c file

member_t* create_member_array(int length) {
    member_t* arr = (member_t*) malloc(length * sizeof(member_t));
    if(arr) {
        int i = 0;
        for(; i < length; i++) {
            arr[i] = (member_t) malloc(sizeof(struct MEMBER_T));
            if(arr[i]) {
                arr[i]->member_id = 0;
                arr[i]->name = NULL;
            }
        }
    }
    return arr;
}

the declaration in you struct char name; combined with the statement members[0]->name = "test" is not correct. You declar a single char and try to a ssign a string to it, witch itself is not correct. it should be char* name and you shold allocate mememory before trying to copy a string to ist using strcpy()

member[0]->name = malloc((strlen("test") + 1)  * sizeof(char)); // 1 more the terminating '\0' !
strcpy(member[0]->name, "test");

alternativ:

member[0]->name = strdup("test");

This is a lot of memory allocation witch you need to make sure you release / free it. You will have to create another function to free all this memory

void free_member_array(member_t* members, int length) {
    for(int i = 0; i < length; i++) {
        if(members[i]->name) {
            free(members[i]->name);
        }
        free(members[i]);
    }
    free(members);
}