Consider the following code:

#include<stdio.h>

struct word {
    char* data;
};

struct sentence {
    struct word* data;
    int word_count;
};

struct paragraph {
    struct sentence* data  ;
    int sentence_count;
};

struct document {
    struct paragraph* data;
    int paragraph_count;
};

void main()
{
    int total_paragraph = 5;   //I'm myself assigning total number of paragraphs for simplicity
    
    struct document doc;
    
    doc.data = malloc(total_paragraph*(sizeof(struct paragraph)));  //Statement which I have a doubt!!!

    ....
    ....
    ....
}

Firstly, logically, Is the statement(malloc one, in which I have a doubt) valid?

If yes, how does the computer assigns 5 units of memory (each unit is of struct paragraph size) without knowing the size of struct paragraph (as we haven't malloced its content, the pointer data pointing to a struct sentence type)?

CodePudding user response：

It's impossible to allocate structures without its substructures being defined. After all, you can't know the size of a structure without knowing the size of its substructures.

However, struct paragraph doesn't contain any substructures. It contains a structure pointer, an int, and possibly padding. The size of all those things are known.

The code you posted is perfectly fine.

CodePudding user response：

without knowing the size of struct paragraph (as we haven't malloced its content, the pointer data pointing to a struct sentence type)? c pointers struct

Why do we need to malloc a structure to know it's size? What is the size of this structure?

struct document {
    struct paragraph* data;
    int paragraph_count;
};

It has a pointer and an integer. A pointer has a fixed size(usually 64-bit on 64-bit machines and 32-bit on 32-bit machines) and and integer has a fixed size(usually 32 bit). so, the size of the struct in bytes is sizeof(any_pointer) sizeof(int) padding. All of those three are known to the compiler.