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不要对struct使用typedef

我们经常看到这样的代码

typedef struct st{

char name[20];

int id;

} student, * pstudent;

student a1;

pstudent pa1;

这段代码中,用typedef给struct结构体和结构体指针起了别名。除了用的时候少打struct这几个字之外,这种做法没有任何好处。反而会给理解代码带来困扰。

Linus 在Linux内核编程规范第五章中,也规定了不要对 struct 使用 typedef。

Chapter 5: Typedefs

Please don’t use things like “vps_t”.

It’s a _mistake_ to use typedef for structures and pointers. When you see a

vps_t a;

in the source, what does it mean?

In contrast, if it says

struct virtual_container *a;

you can actually tell what “a” is.

Lots of people think that typedefs “help readability”. Not so. They are
useful only for:

(a) totally opaque objects (where the typedef is actively used to _hide_
what the object is).

Example: “pte_t” etc. opaque objects that you can only access using
the proper accessor functions.

NOTE! Opaqueness and “accessor functions” are not good in themselves.
The reason we have them for things like pte_t etc. is that there
really is absolutely _zero_ portably accessible information there.

(b) Clear integer types, where the abstraction _helps_ avoid confusion
whether it is “int” or “long”.

u8/u16/u32 are perfectly fine typedefs, although they fit into
category (d) better than here.

NOTE! Again – there needs to be a _reason_ for this. If something is
“unsigned long”, then there’s no reason to do

typedef unsigned long myflags_t;

but if there is a clear reason for why it under certain circumstances
might be an “unsigned int” and under other configurations might be
“unsigned long”, then by all means go ahead and use a typedef.

(c) when you use sparse to literally create a _new_ type for
type-checking.

(d) New types which are identical to standard C99 types, in certain
exceptional circumstances.

Although it would only take a short amount of time for the eyes and
brain to become accustomed to the standard types like ‘uint32_t’,
some people object to their use anyway.

Therefore, the Linux-specific ‘u8/u16/u32/u64’ types and their
signed equivalents which are identical to standard types are
permitted — although they are not mandatory in new code of your
own.

When editing existing code which already uses one or the other set
of types, you should conform to the existing choices in that code.

(e) Types safe for use in userspace.

In certain structures which are visible to userspace, we cannot
require C99 types and cannot use the ‘u32’ form above. Thus, we
use __u32 and similar types in all structures which are shared
with userspace.

Maybe there are other cases too, but the rule should basically be to NEVER
EVER use a typedef unless you can clearly match one of those rules.

In general, a pointer, or a struct that has elements that can reasonably
be directly accessed should _never_ be a typedef.

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