What is the difference between the function "malloc()" and a created variable?

Question

I was reading some codes in C and I realized that to generate some specific object is used the function malloc(). It is not easier to create this object in a variable than to use the malloc()?

Here is an example:

typedef struct
{
    uint32_t idade;
} Pessoa;

void main()
{
    // Geralmente eu vejo isto:
    Pessoa *p = (Pessoa *)malloc(sizeof(Pessoa));
    // ... (usa a struct pessoa)


    // Mas não é mais fácil fazer isto?:
    Pessoa p;
    // ... (usa a struct pessoa)
}

Why the malloc() is usually used to allocate memory if I can create the object variable, as is the Pessoa p;? And are there differences between these two ways of allocating memory? If so, which ones?

Answer 1

To create an object it is not necessary to call malloc(), even because it is made to allocate dynamic memory, only that. It allocates memory and returns an address where there was allocation. Creating and placing an object there is another separate operation, usually will be done with the assignment operator (=) or a function that does the same.

The comment that says use the structure Pessoa is mistaken, because there is no object there, at least not one initialized, so there is only garbage, you can even access that place as if you had, but in practice you will be collecting garbage.

Inclusive it makes no sense to do the cast in function.

It is easier to make the second option, if it is what you want. If you want to allocate in automatic memory is much easier, even because then you will not have to release the allocation made with malloc(). But this allocation is only available for this function, does not allow the object to survive the end of the function, nor can it be used elsewhere. In many cases this is enough, in other cases not, dynamic allocation is necessary when you do not know how long it should survive or if you know it is beyond the limit from where it was created. In some cases size may be the determinant. But this can be seen below.

You can read more on:

• When to dynamically allocate memory?
• Variables declared within blocks or subprograms are dynamically allocated?
• Using static or dynamic variables in my codes? What would be more efficient or better seen in the job market?
• Dynamic Memory Allocation X vector
• Data access performance in heap and stack and object allocation
• Returning function string directly and with array

And can also research more on the subject here on the site, has a lot of useful information. Already have some links available there in the questions linked. Folks missed a little bit of learning from the answers here. Tough luck.

The question changed as I answered.