problem understanding c code

[Andreas]

hi

i'm going to develope an mp3 encoder and therefor i'm studiing the source
code of lame mp3 encoder. but i'm not able to anderstand the nixt three
lines of the sourde.

struct lame_global_str uct;
typedef struct lame_global_str uct lame_global_fla gs;
typedef lame_global_fla gs *lame_t;

maybe you can tell me how it works.

thank you
andi

[Spiros Bousbouras]

Re: problem understanding c code

On Nov 14, 11:56 am, Andreas <a.kas...@kabsi .atwrote:

hi
>
i'm going to develope an mp3 encoder and therefor i'm studiing the source
code of lame mp3 encoder. but i'm not able to anderstand the nixt three
lines of the sourde.
>
struct lame_global_str uct;
typedef struct lame_global_str uct lame_global_fla gs;
typedef lame_global_fla gs *lame_t;

The 2nd line defines "lame_global_fl ags" to be a name
for the type "struct lame_global_str uct" and the 3rd
line defines "lame_t" to be a name for the type "pointer
to lame_global_fla gs". In other words lame_t is a name
for the type "pointer to struct lame_global_str uct".

[Andreas]

Re: problem understanding c code

and the fist line? what means it? - no structure of the datatype is
defined?

struct lame_global_str uct;




Am Wed, 14 Nov 2007 12:03:54 +0000 schrieb Spiros Bousbouras:

On Nov 14, 11:56 am, Andreas <a.kas...@kabsi .atwrote:

>hi
>>
>i'm going to develope an mp3 encoder and therefor i'm studiing the
>source code of lame mp3 encoder. but i'm not able to anderstand the
>nixt three lines of the sourde.
>>
>struct lame_global_str uct;
>typedef struct lame_global_str uct lame_global_fla gs; typedef
>lame_global_fl ags *lame_t;

>
The 2nd line defines "lame_global_fl ags" to be a name for the type
"struct lame_global_str uct" and the 3rd line defines "lame_t" to be a
name for the type "pointer to lame_global_fla gs". In other words lame_t
is a name for the type "pointer to struct lame_global_str uct".

[Mark Bluemel]

Re: problem understanding c code

Spiros Bousbouras wrote:

On Nov 14, 11:56 am, Andreas <a.kas...@kabsi .atwrote:

>hi
>>
>i'm going to develope an mp3 encoder and therefor i'm studiing the source
>code of lame mp3 encoder. but i'm not able to anderstand the nixt three
>lines of the sourde.
>>
>struct lame_global_str uct;
>typedef struct lame_global_str uct lame_global_fla gs;
>typedef lame_global_fla gs *lame_t;

>
The 2nd line defines "lame_global_fl ags" to be a name
for the type "struct lame_global_str uct" and the 3rd
line defines "lame_t" to be a name for the type "pointer
to lame_global_fla gs". In other words lame_t is a name
for the type "pointer to struct lame_global_str uct".

Which the OP could have found out by opening his C reference text.

I suspect his question is really about the declaration of the "opaque"
structure...

This is a common technique in interface design, when a subsystem needs
to be passed a (pointer to a) structure but we don't want the users of
the subsystem to be aware of its composition.

I would expect users of lame to only really work with lame_t data -
pointers to structures, but we don't know (or care) what the structures
looks like.

[Ben Bacarisse]

Re: problem understanding c code

Andreas <a.kasper@kabsi .atwrites:

Am Wed, 14 Nov 2007 12:03:54 +0000 schrieb Spiros Bousbouras:

>On Nov 14, 11:56 am, Andreas <a.kas...@kabsi .atwrote:

>>i'm going to develope an mp3 encoder and therefor i'm studiing the
>>source code of lame mp3 encoder. but i'm not able to anderstand the
>>nixt three lines of the sourde.
>>>
>>struct lame_global_str uct;
>>typedef struct lame_global_str uct lame_global_fla gs;
>>typedef lame_global_fla gs *lame_t;

>>
>The 2nd line defines "lame_global_fl ags" to be a name for the type
>"struct lame_global_str uct" and the 3rd line defines "lame_t" to be a
>name for the type "pointer to lame_global_fla gs". In other words lame_t
>is a name for the type "pointer to struct lame_global_str uct".

>
and the fist line? what means it? - no structure of the datatype is
defined?

Please don't top post.

struct lame_global_str uct;

It says that there is a struct called lame_global_str uct and no more.
It is used when the programmer wants to talk about a structure without
saying anything more about it. In the example above, the programmer
wants to define a synonym for the structure type and for a pointer to
it. There is not much else you can do with it which is sometimes the
whole purpose (look up "opaque types").

There is no need to write it out like that.

typedef struct lame_global_str uct lame_global_fla gs;
typedef lame_global_fla gs *lame_t;

serves the same purpose (in C). The original may have been written
because, to a C++ compiler, a structure tag is a legal type name and
the code may have once have had things like:

some_function(l ame_global_stru ct *sp);

in it once, or simply the author may have adopted that style because
he/she is used to it from C++.

--
Ben.