Does allocating using sizeof yield wrong size for structure pointers?

Question

Using valgrind to read this I get: Invalid write/read of size 4

 struct Person{
        char* name;
        int age;
    };

    struct Person* create_pers         


        

Answer 1

You are using the wrong size in the call to malloc.

struct Person* me = (struct Person*)malloc(sizeof(struct Person*));
                                                  ^^^^^^^^^^^^^^^

That is a size of a pointer, not the size of an object. You need to use:

struct Person* me = (struct Person*)malloc(sizeof(struct Person));

To avoid errors like this, use the following pattern and don't cast the return value of malloc (See Do I cast the result of malloc?):

struct Person* me = malloc(sizeof(*me));

It's a coincidence that malloc(sizeof(struct Person*)+4) works. Your struct has a pointer and an int. It appears sizeof(int) on your platform is 4. Hence, sizeof(struct Person*)+4 happen to match the size of struct Person.

Answer 2

Because you want to allocate enough space to hold an entire structure, not just a pointer to it.

That is, use sizeof(struct Person) and not sizeof(struct Person*).

sizeof(struct Person*)+4 is coincidentally big enough on your platform.