在这种情况下，malloc做了什么？

Question

What does this line of C code do?

      be_node *ret = malloc(sizeof(*ret));

The definition of be_node can be found in this file: http://funzix.git.sourceforge.net/git/gitweb.cgi?p=funzix/funzix;a=blob_plain;f=bencode/bencode.h;hb=HEAD

The line of code above was found in this file: http://funzix.git.sourceforge.net/git/gitweb.cgi?p=funzix/funzix;a=blob_plain;f=bencode/bencode.c;hb=HEAD

I don't understand what the sizeof(*ret) would return if it has only just been declared?

Answer 1

It's no different to any other use of sizeof ; it will evaluate the size of its operand. sizeof is based on compile-time information, ¹ so it doesn't matter that ret has only just been declared.

This idiom is the preferred way of using malloc . If you were to use be_node *ret = malloc(sizeof(be_node)) , then consider what would happen if you change the type of ret at a later date. If you forget to replace both uses of " be_node ", then you will have introduced a subtle bug.

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_{1. Except in the case of variable-length arrays.}

Answer 2

sizeof(*ret) is resolved by the compiler and only looks at the type of *ret , not its contents. In this case, it's the size of a be_node. It also gets resolved at compile time, not at runtime so it doesn't "return" per se, it just gets substituted with a constant.

The compiler is going to take out sizeof(*ret) and substitute in a constant number that is the size of the be_node in bytes.

Answer 3

It's a shortcut, or whatever you might want to call it.

You can either write

be_node *ret = malloc(sizeof(be_node));

or

be_node *ret = malloc(sizeof(*ret));

In the first case, you're basically saying "allocate a memory block large enough to hold a be_node". In the second case you're saying "allocate a memory block large enough to hold whatever ret points to". Which one you prefer is mostly a matter of preference.

Answer 4

sizeof works with both data types and actual variables. In your case, you invoke it with a variable as a parameter. By the time you invoke sizeof the variable is DECLARED (not initialized but declared) so it will know the var's type and will be able to compute the memory requirements in bytes which will be returned and used by the malloc.