在结构内使用指针

Question

Suppose I have the following:

#include <stdio.h>

typedef struct
{
    int num;
} myObj;

typedef struct
{
    myObj *obj_ptr;
} myStruct;

void populate(myStruct *m1)
{
    m1->obj_ptr->num = 123;
}

int main()
{
    printf("Hello World");
    myStruct m1;
    populate(&m1);

    printf("%d", m1.obj_ptr->num);

    return 0;
}

The purpose of this code is I want to get an item eg m1 populated instead of being returned by the populate function, hence the way it is coded.

I'm concerned about the line m1.obj_ptr->num since it doesn't seem like a good design. Are there any issues with this code even as I got it to run? Is there a better approach to this?

Answer 1

It is OK to have a pointer in a struct, you must however allocate memory for it:

void populate(myStruct *m1)
{
    m1->obj_ptr->num = 123;
}

is wrong, because m1->obj_ptr is not initialized. You can do this:

void populate(myStruct *m1)
{
    // always check for validity of arguments
    if(m1 == NULL)
        return;

    m1->obj_ptr = malloc(sizeof *m1->obj_ptr);
    if(m1->obj_ptr == NULL)
        return;

    m1->obj_ptr->num = 123;
}

I personally would make populate return 1 on success and 0 on failure, thus the caller knows if the allocation was successful.

But you don't have to forget to free the memory:

int main()
{
    printf("Hello World");
    myStruct m1;
    populate(&m1);

    if(m1.obj_ptr)
        printf("%d", m1.obj_ptr->num);

    free(m1.obj_ptr);

    return 0;
}

If you say, you don't want to allocate memory in populate , then you have to initialize the pointer prior to the call of the populate , like this:

int main()
{
    printf("Hello World");
    myStruct m1;
    myObj obj;

    m1.obj_ptr = &obj;

    populate(&m1);

    printf("%d", m1.obj_ptr->num);

    return 0;
}

But this has the disadvantage that you cannot return this struct to another function. This would be incorrect:

myStruct get_me_a_struct(void)
{
    myStruct m1;
    myObj obj;

    m1.obj_ptr = &obj;

    populate(&m1);

    return m1;
}

void foo(void)
{
    myStruct m2 = get_me_a_struct();

    printf("%d\n", m2.obj_ptr->num); // <-- you cannot do that
}

because m2.obj_ptr would point to an invalid location. For that reason, using malloc for the allocation is better.

Answer 2

Two things; both within populate() :

First, nowhere are you allocating or deallocating the space for obj_ptr in your code. You simply set the value of num within in it and hope for the best. Before the line m1->obj_ptr->num = 123; allocate some memory for that 123 to live in with m1->obj_ptr = malloc(sizeof(myObj)) . Without this, when you set num it is actually pointeing to an unknown memory address which is probably not set aside for your program to work in.

Similarly, populate() does not check whether m1 is a valid address before assigning to its members. In this sample code, it is (only mostly) okay, because when you call populate() you are passing a reference( & ) rather than a pointer( * ), but, if you ever use it in the future, you may be calling with a pointer. So really, the first line in populate() needs to be if(m1 != NULL) with the rest of the function inside.