Casting Allocation In C

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If I allocate block of memory for the obj_t it’s look like this

obj_t *obj = (obj_t *)malloc(sizeof(obj_t));
obj_t *obj = (obj_t *)calloc(nmemb, sizeof(obj_t));

After reading  this post, I’m wondering why I use cast anyway?

My thought was because malloc and friends is returning void * so I need to cast to the appropriate type.

And now if I ask myself “Why do you need to cast anyway? Is there any effect?”. My answer was “I don’t know”. So I decide to doing some research about this.

I found this  StackOverflow answer which provide excellent information about the bad of using cast for malloc and friends.

This is a quick summary from that discusion

Do I cast the result of malloc?

No; you don’t cast the result, since:

It is unnecessary, as void * is automatically and safely promoted to any other pointer type in this case.
It can hide an error, if you forgot to include <stdlib.h>. This can cause crashes (or, worse, not cause a crash until way later in some totally different part of the code). Consider what happens if pointers and integers are differently sized; then you’re hiding a warning by casting and might lose bits of your returned address.
It adds clutter to the code, casts are not very easy to read (especially if the pointer type is long).
It makes you repeat yourself, which is generally bad.

It’s turnout that cast is a harmful, I should avoid them as much as possible.

Is there any effect using cast?

It’s a failure to include the cast, even if you got it right. There are simply no benefits to doing it, but a bunch of potential risks, and including the cast indicates that you don’t know about the risks.

Well, 100% Agree.

The last one

To add further, your code needlessly repeats the type information (int) which can cause errors. It’s better to dereference the pointer being used to store the return value, to “lock” the two together:

int *sieve = malloc(length * sizeof *sieve);

This also moves the length to the front for increased visibility, and drops the redundant parentheses with sizeof; they are only needed when the argument is a type name. Many people seem to not know (or ignore) this, which makes their code more verbose. Remember: sizeof is not a function! :)

“It’s better to dereference the pointer being used to store the return value”

I don’t cast allocation anymore and use dereference the pointer instead of type name as an argument of sizeof.

obj_t *obj = malloc(sizeof *obj);
obj_t *obj = calloc(nmemb, sizeof *obj);

The good news is it also improve my code to be more readable.

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