struct / typedef confusion

[Guillaume Dargaud]

I just saw the following two lines in /usr/include/usb.h and my head is
spinning:

struct usb_dev_handle;
typedef struct usb_dev_handle usb_dev_handle;

What is that supposed to mean ?
I assume the first line is a forward declaration.
I can't find a definition for it, so does that mean it's like a void ?
Apparently gdb is confused as well as I can't seem to display anything based
on this struct.

 

[christian.bau]

I just saw the following two lines in /usr/include/usb.h and my head is
spinning:

struct usb_dev_handle;
typedef struct usb_dev_handle usb_dev_handle;

It means that both "struct usb_dev_handle" and the typedef
"usb_dev_handle" now refer to a struct with a tag of usb_dev_handle,
but you don't know what is inside that struct.

But it is enough information to declare for example a function
returning a pointer to a usb_dev_handle or taking one as an argument.
Like

usb_dev_handle* create_usb_dev_handle (void);
void destroy_usb_dev_handle (usb_dev_handle* h);
size_t read_from_usb (usb_dev_handle* h, void* buffer, size_t size);

and now you can write code like

char buffer [10];
usb_dev_handle* p = create_usb_dev_handle ();
read_from_usb (p, buffer, sizeof (buffer));
destroy_usb_dev_handle (p);

even though you don't have any idea what's inside the struct.

 

[Walter Roberson]

I just saw the following two lines in /usr/include/usb.h and my head is
spinning:

struct usb_dev_handle;
typedef struct usb_dev_handle usb_dev_handle;

What is that supposed to mean ?

It means that struct usb_dev_handle is a structure of some kind,
whose contents are undefined to this section of code. "Incomplete"
declartions such as this are useful when writing library code in
which the internals of the defined object should be "opaque"
(known only to the implementation).

I assume the first line is a forward declaration.
I can't find a definition for it, so does that mean it's like a void ?

No, not like void, but a pointer to such a structure is akin to
void* -- a pointer to -something- that will be converted by the
library code to point to the real structure at need.

 

[Guillaume Dargaud]

It means that struct usb_dev_handle is a structure of some kind,

whose contents are undefined to this section of code. "Incomplete"
declartions such as this are useful when writing library code in
which the internals of the defined object should be "opaque"
(known only to the implementation).

Thanks both of you. I didn't know this was possible in C, although I've
known for a long time that it isn't strongly typed.

Still, isn't there a contradiction in having both a struct and a typedef
with the same name ?

 

[ais523]

I just saw the following two lines in /usr/include/usb.h and my head is
spinning:

struct usb_dev_handle;
typedef struct usb_dev_handle usb_dev_handle;

What is that supposed to mean ?
I assume the first line is a forward declaration.
I can't find a definition for it, so does that mean it's like a void ?
Apparently gdb is confused as well as I can't seem to display anything based
on this struct.

The line is declaring that a struct with a tag of usb_dev_handle
exists (which code would refer to as "struct usb_dev_handle"). The
second line allows a shorter form to be used for it's name (just
"usb_dev_handle"). Because no information about what's in the struct
has been given, it's a so-called "incomplete type"; you can pass
pointers to it around, but not create an object of that type (at least
not safely, if you don't know how big it is). Normally, there'll be
some library function around that does know the details of the type
and which can create objects of that type; however, you have no way to
access the details yourself without that info, and neither does your
debugger (if you loaded debug information into it from the library,
assuming you have that information, it'll be able to display
information as it would with a complete type).

void is linked to this, in that it's an incomplete type that cannot be
completed, and so you can perform the same operations with it, but no
more:

struct usb_dev_handle;
typedef struct usb_dev_handle usb_dev_handle;

void* a; /* fine */
usb_dev_handle* b; /* fine */
void c; /* error */
usb_dev_handle d; /* error */

void func(void) /* void has a special meaning here that has nothing
to do with incomplete types */
{
usb_dev_handle e; /* error */
usb_dev_handle *f; /* fine */
void g; /* error */
void *h; /* fine */
h=malloc(10); /* legal, but probably a bad idea to use a magic
number here */
h=malloc(10*sizeof(void)); /* would be safer if it made sense, but
it doesn't */
f=malloc(10); /* legal but very stupid, as you have no idea how big
a
usb_dev_handle is */
f=malloc(10*sizeof(*f)); /* error, because the compiler has no idea
how big a
usb_dev_handle is either */
}

 

[Ben Pfaff]

Still, isn't there a contradiction in having both a struct and a typedef
with the same name ?

It can be confusing to human readers to have a different
structure with the same struct tag and typedef, but it does not
confuse the compiler. Struct tags and typedefs are in different
namespaces: tags appear only immediately after the reserved word
"struct", and typedef names never do.

I don't think it's very confusing to give the same structure the
same struct tag and typedef name, but it's unnecessary. Usually,
in my code, I don't typedef my structs at all.

 

[Walter Roberson]

Still, isn't there a contradiction in having both a struct and a typedef
with the same name ?

No, C has a few different "name spaces", with a usage in one
name space not conflicting with the usage in another name space.
For example, statement labels don't conflict with ordinary
identifiers. In 30 seconds reading of C89 I'm left uncertain
exactly which name space is used for typedefs, but the
tags of structures, unions, and enumerations are specifically
noted as being disambiguated by the presence of the
'struct', 'union' or 'enum' tag.

 

[Keith Thompson]

It can be confusing to human readers to have a different
structure with the same struct tag and typedef, but it does not
confuse the compiler. Struct tags and typedefs are in different
namespaces: tags appear only immediately after the reserved word
"struct", and typedef names never do.

I don't think it's very confusing to give the same structure the
same struct tag and typedef name, but it's unnecessary. Usually,
in my code, I don't typedef my structs at all.

Neither do I in most cases, but this:

struct usb_dev_handle;
typedef struct usb_dev_handle usb_dev_handle;

is a case where using a typedef for a struct actually makes sense.
The type "usb_dev_handle" is an opaque type; the only thing client
code needs to know about it is that it's an object type, and you can
manipulate pointers to it (presumably using functions in the same
header that take arguments of type usb_dev_handle*). Presumably
client code will never refer to members of the type, so it doesn't
need to know that it's a struct.

There's no real reason to use different identifiers for the struct tag
and the typedef name; using the same identifier makes it clearer that
they're really the same thing.

It's very similar to the handling of type FILE in <stdio.h>. It could
have been declared as "struct FILE", but using the simple identifier
"FILE" *deliberately* hides the internals. (Well, it doesn't
completely hide them, but it expresses the intent that they shouldn't
be used.)

On the other hand, if client code is intended to refer to members of
the structure, then adding a typedef merely creates as second name for
something that already has a perfectly good name, and purports to hide
information (the fact that the type is a struct) that really isn't
hidden at all.

 

[David Thompson]

No, C has a few different "name spaces", with a usage in one
name space not conflicting with the usage in another name space.
Right.

For example, statement labels don't conflict with ordinary
identifiers. In 30 seconds reading of C89 I'm left uncertain
exactly which name space is used for typedefs, but the

The 'ordinary identifier' namespace, which also contains (named)
objects (aka variables), functions, and enum constants.

tags of structures, unions, and enumerations are specifically
noted as being disambiguated by the presence of the
'struct', 'union' or 'enum' tag.

- formerly david.thompson1 || achar(64) || worldnet.att.net