[masklinn]

And you really shouldn't use it with a type if you can avoid it anyway, it makes code brittle e.g.

    int *foo;
    // code
    foo = malloc(sizeof(int));

a few months later, change foo to be a double. Code still compiles, no warning, but you're allocating half the memory you need.

[xamuel]

This is a really great example why you SHOULDN'T think of sizeof as a function. If sizeof were a function, the code

  int *foo = NULL;
  foo = malloc(sizeof(*foo));

would be undefined behavior (dereferencing NULL)!

[Kranar]

Dereferencing a null pointer is legal in C. It's the conversion of a null pointer from from an r-value to an l-value that's illegal, which does not happen in that snippet of code.

That's why it's perfectly legal in C to do this (&*foo), even if foo is a null pointer.

[masklinn]

> Dereferencing a null pointer is legal in C. It's the conversion of a null pointer from from an r-value to an l-value that's illegal, which does not happen in that snippet of code.

And it does not happen in that snippet of code because sizeof is nothing like a function.

[imglorp]

Which is why it's nice to lift stuff out into typedefs. It centralizes them (DRY principle) and avoids this issue.

    typedef int thing_t;
    ...
    thing_t *foo;
    // code
    foo = malloc(sizeof(thing_t));

[viraptor]

Why is that better than just getting `sizeof(* foo)`? Even with typedef, I can see this happening in the future:

    typedef int thing_t;
    ...
    thing_t *foo_internal;
    thing_wrapper_t *foo;
    // code
    foo = malloc(sizeof(thing_t));

If you do:

    foo = malloc(sizeof(*foo));

That's at least always on the same line.

[userbinator]

That just makes it more verbose.

This, on the other hand, always allocates one object of foo's pointed-to-size, whatever its type:

    foo = malloc(sizeof(*foo));

[dllthomas]

As an aside, I think nearly any time you want a typedef, it's worth wrapping it in a struct.

    typedef struct { int value; } thing_t;

That way the compiler catches it when you try to pass the wrong thing (at least, more of the time).

[dllthomas]

DRY is good, but making the structure of your code reflect the actual semantics you want is better. What you want is to allocate space for foo. So write that.

[crazy2be]

Well, in this (simple) case you can just do

    int *foo;
    foo = malloc(sizeof(*foo));

And avoid the brittleness mentioned.