define a function returning struct pointer

Question

Please bear with me, i m from other language and newbie to c and learning it from http://c.learncodethehardway.org/book/learn-c-the-hard-way.html

struct Person {         


        

Answer 1

The Person_create function returns a pointer to a struct Person so you have to define the return value to be a pointer (by adding the *). To understand the reason for returning a pointer to a struct and not the struct itself one must understand the way C handles memory.

When you call a function in C you add a record for it on the call stack. At the bottom of the call stack is the main function of the program you're running, at the top is the currently executing function. The records on the stack contain information such as the values of the parameters passed to the functions and all the local variables of the functions.

There is another type of memory your program has access to: heap memory. This is where you allocate space using malloc, and it is not connected to the call stack.

When you return from a function the call stack is popped and all the information associated with the function call are lost. If you want to return a struct you have two options: copy the data inside the struct before it is popped from the call stack, or keep the data in heap memory and return a pointer to it. It's more expensive to copy the data byte for byte than to simply return a pointer, and thus you would normally want to do that to save resources (both memory and CPU cycles). However, it doesn't come without cost; when you keep your data in heap memory you have to remember to free it when you stop using it, otherwise your program will leak memory.