List_head

[dayalsoap]

I'm new to C. In this project I'm working on, there's this line of code in bold.

typedef struct dimensions_
{
    char *name;
      LIST_HEAD(alts, altname_) altlist;
      LIST_ENTRY(dimensions_) entries;
} dimensions;

altname_ is a struct defined above, but there is no alts defined anywhere.

I've checked all of the associated header files, but I can't seem to find alts... what is this? How is this used?

[dayalsoap]

Am I just an idiot?

[smokeyangel]

man page LIST_HEAD section 3

A list is headed by a structure defined by the LIST_HEAD macro. This
structure contains a single pointer to the first element on the list.
The elements are doubly linked so that an arbitrary element can be
removed without traversing the list. New elements can be added to the
list after an existing element, before an existing element, or at the
head of the list. A LIST_HEAD structure is declared as follows:

LIST_HEAD(HEADNAME, TYPE) head;

So alts isn't defined before this because it is defined using this macro.

[dayalsoap]

man page LIST_HEAD section 3



So alts isn't defined before this because it is defined using this macro.

I see.. so I could pass in anything then, even xyzalts and it would work exactly the same?

[smokeyangel]

Yup - any legal variable name.

It's a macro -- it'll expand to something like

struct xyzalts = Some init value;

If it helps to know what it's expanded to, I think gcc -E will run the output through the preprocessor and output the result. Sometimes it's the easiest way to untangle complex poorly documented macros.

[dayalsoap]

Yup - any legal variable name.

It's a macro -- it'll expand to something like

struct xyzalts = Some init value;

If it helps to know what it's expanded to, I think gcc -E will run the output through the preprocessor and output the result. Sometimes it's the easiest way to untangle complex poorly documented macros.

thank you very much for your help. that explains everything.