Sometimes we use the 'struct' keyword in headers to help us
reduce dependencies between header files.  Document that
practice.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
Changes v1 -> v2:
* Use paragraphs written by Paolo Bonzini at
  https://www.mail-archive.com/qemu-devel@nongnu.org/msg586214.html
* Fix typos spotted by Thomas Huth
---
 HACKING | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/HACKING b/HACKING
index 0fc3e0fc04..035276e668 100644
--- a/HACKING
+++ b/HACKING
@@ -100,7 +100,19 @@ pointer, you're guaranteed that it is used to modify the storage
 it points to, or it is aliased to another pointer that is.
 
 2.3. Typedefs
-Typedefs are used to eliminate the redundant 'struct' keyword.
+
+Typedefs are used to eliminate the redundant 'struct' keyword, since type
+names have a different style than other identifiers ("CamelCase" versus
+"snake_case").  Each struct should have a CamelCase name and a
+corresponding typedef.
+
+Since certain C compilers choke on duplicated typedefs, you should avoid
+them and declare a typedef only in one header file.  For common types,
+you can use "include/qemu/typedefs.h" for example.  However, as a matter
+of convenience it is also perfectly fine to use forward struct
+definitions instead of typedefs in headers and function prototypes; this
+avoids problems with duplicated typedefs and reduces the need to include
+headers from other headers.
 
 2.4. Reserved namespaces in C and POSIX
 Underscore capital, double underscore, and underscore 't' suffixes should be
-- 
2.21.0

On 8/13/19 1:46 AM, Eduardo Habkost wrote:
> Sometimes we use the 'struct' keyword in headers to help us
> reduce dependencies between header files.  Document that
> practice.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
> Changes v1 -> v2:
> * Use paragraphs written by Paolo Bonzini at
>   https://www.mail-archive.com/qemu-devel@nongnu.org/msg586214.html
> * Fix typos spotted by Thomas Huth
> ---
>  HACKING | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/HACKING b/HACKING
> index 0fc3e0fc04..035276e668 100644
> --- a/HACKING
> +++ b/HACKING
> @@ -100,7 +100,19 @@ pointer, you're guaranteed that it is used to modify the storage
>  it points to, or it is aliased to another pointer that is.
>  
>  2.3. Typedefs
> -Typedefs are used to eliminate the redundant 'struct' keyword.
> +
> +Typedefs are used to eliminate the redundant 'struct' keyword, since type
> +names have a different style than other identifiers ("CamelCase" versus
> +"snake_case").  Each struct should have a CamelCase name and a

Maybe s/Each struct/Each global struct/ ? Or "non-local" or something
similar? Sometimes, you also define a struct just within a function, and
in that case we don't require the typedef, do we?

> +corresponding typedef.
> +
> +Since certain C compilers choke on duplicated typedefs, you should avoid
> +them and declare a typedef only in one header file.  For common types,
> +you can use "include/qemu/typedefs.h" for example.  However, as a matter
> +of convenience it is also perfectly fine to use forward struct
> +definitions instead of typedefs in headers and function prototypes; this
> +avoids problems with duplicated typedefs and reduces the need to include
> +headers from other headers.

Reviewed-by: Thomas Huth <thuth@redhat.com>

On 13/08/19 08:44, Thomas Huth wrote:
> Maybe s/Each struct/Each global struct/ ? Or "non-local" or something
> similar? Sometimes, you also define a struct just within a function, and
> in that case we don't require the typedef, do we?

I changed it to "each named struct" and queued.

Thanks,

Paolo