Thread: How to include the header files effectively

How to include the header files effectively

From

Andy Fan

Date:

12 April 2019, 05:08:52

I find the dependency is complex among header files in PG. At the same time, I find the existing code still can use the header file very cleanly/alphabetically. so I probably missed some knowledge here.

for example, when I want the LOCKTAG in .c file, which is defined in "storage/lock.h". then I wrote the code like this:

#include "storage/lock.h"

...

LOCKTAG tag;

compile and get errors.

In file included from

.../src/include/storage/lock.h:21:

/../../../src/include/storage/lockdefs.h:50:2: error: unknown type name

'TransactionId'

TransactionId xid; /* xid of holder of AccessExclusiveLock */

so I HAVE TO

1. include the header file which contains the TransactionId

2. add it before the lock.h.

normally I think we can add the dependency in lock.h directly to resolve this issue.

so how can I include header file effectively ?

Thanks

Re: How to include the header files effectively

From

Tom Lane

Date:

12 April 2019, 05:21:14

Andy Fan <zhihui.fan1213@gmail.com> writes:
> for example,  when I want the LOCKTAG in .c file,   which is defined in
> "storage/lock.h".  then I wrote the code like this:

> #include "storage/lock.h"
> ...
> LOCKTAG tag;

> compile and get errors.

> In file included from
> .../src/include/storage/lock.h:21:
> /../../../src/include/storage/lockdefs.h:50:2: error: unknown type name
>       'TransactionId'
>         TransactionId xid;                      /* xid of holder of
> AccessExclusiveLock */

The reason that's failing is that you didn't include postgres.h first.

The general expectation --- and we do mechanically verify this,
periodically --- is that any Postgres header should have enough #include's
that you can include it without further work, so long as you included
postgres.h (or postgres_fe.h, or c.h, depending on context) beforehand.
One of those three headers must be the first inclusion in every Postgres
.c file.  There are portability reasons behind that rule, which you
don't really want to know about ;-) ... just do it like that.

            regards, tom lane

Re: How to include the header files effectively

From

Alvaro Herrera

Date:

12 April 2019, 05:22:56

On 2019-Apr-12, Andy Fan wrote:

> for example,  when I want the LOCKTAG in .c file,   which is defined in
> "storage/lock.h".  then I wrote the code like this:
> 
> #include "storage/lock.h"
> ...
> 
> /../../../src/include/storage/lockdefs.h:50:2: error: unknown type name
>       'TransactionId'
>         TransactionId xid;                      /* xid of holder of AccessExclusiveLock */

What are you trying to do?  Your .c file must include "postgres.h"
before any other header file.  There should be no other dependencies.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

Re: How to include the header files effectively

From

Michael Paquier

Date:

12 April 2019, 07:31:10

On Thu, Apr 11, 2019 at 10:22:56PM -0400, Alvaro Herrera wrote:
> What are you trying to do?  Your .c file must include "postgres.h"
> before any other header file.  There should be no other dependencies.

The usual rule when it comes to develop extensions or a patch is to
include headers in the following order:
1) postgres.h for backend code and postgres_fe.h for frontend (use
ifdef FRONTEND if a file is used in both context, see src/common/*.c).
2) System-related headers, like <unistd.h> or such.
3) Other PostgreSQL internal headers, in any patch posted to the lists
these in alphabetical order is warmly welcome.
--
Michael

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