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Re: [PATCH] libpq: Return of NULL from PQexec

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Mark Hills <Mark(dot)Hills(at)framestore(dot)com>
Cc: pgsql-docs(at)postgresql(dot)org
Subject: Re: [PATCH] libpq: Return of NULL from PQexec
Date: 2011-11-10 18:00:55
Message-ID: 201111101800.pAAI0t926533@momjian.us (view raw or flat)
Thread:
Lists: pgsql-docs
Thanks. Applied and back-patched to 9.1.

---------------------------------------------------------------------------

Mark Hills wrote:
> I'm sending the patch below for inclusion.
> 
> Originally I ran this by pgsql-hackers, just to confirm the docs (and not 
> the code) were in need of the fix, which seems to be the case:
> 
> http://archives.postgresql.org/pgsql-hackers/2011-09/msg00614.php
> 
> Thanks
> 
> -- 
> Mark
> 
> ---------- Forwarded message ----------
> Date: Mon, 12 Sep 2011 16:40:28 +0100 (BST)
> From: Mark Hills <mark(dot)hills(at)framestore(dot)com>
> To: pgsql-hackers(at)postgresql(dot)org
> Subject: libpq: Return of NULL from PQexec
> 
> The libpq documentation for PQexec states:
> 
>   "If a null pointer is returned, it should be treated like a 
>    PGRES_FATAL_ERROR result"
> 
> But this contradicts the example programs; eg. from Example Program 1:
> 
>    /* Start a transaction block */
>    res = PQexec(conn, "BEGIN");
>    if (PQresultStatus(res) != PGRES_COMMAND_OK)
>    {
>        fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn));
>        PQclear(res);
>        exit_nicely(conn);
>    }
> 
> which does not check if (res != NULL).
> 
> The example is not buggy -- PQresultStatus, PQerrorMessage and, 
> importantly, PQclear deal with the NULL value; eg. src/libpq/fq-exec.c:
> 
>   ExecStatusType
>   PQresultStatus(const PGresult *res)
>   {
>       if (!res)
>           return PGRES_FATAL_ERROR;
>       return res->resultStatus;
>   }
> 
> So I took the example to be correct, and attempted to fix the 
> documentation in the patch below. I hope this is useful.
> 
> In a straw-poll of code I could find using libpq, I found most follows the 
> example and is reliant on libpq for its NULL check.
> 
> The same also applies to PQconnect functions -- and PQstatus, PQfinish, 
> which I also tried to clarify in the documentation.
> 
> Thanks
> 
> -- 
> Mark
> 
> 
> diff --git a/doc/src/sgml/libpq.sgml b/doc/src/sgml/libpq.sgml
> index 163a893..57be7e1 100644
> --- a/doc/src/sgml/libpq.sgml
> +++ b/doc/src/sgml/libpq.sgml
> @@ -62,7 +62,7 @@
>     return a non-null object pointer, unless perhaps there is too
>     little memory even to allocate the <structname>PGconn</> object.
>     The <function>PQstatus</> function should be called to check
> -   whether a connection was successfully made before queries are sent
> +   the return value for a successful connection before queries are sent
>     via the connection object.
>  
>     <warning>
> @@ -1748,8 +1748,10 @@ PGresult *PQexec(PGconn *conn, const char *command);
>          Returns a <structname>PGresult</structname> pointer or possibly a null
>          pointer.  A non-null pointer will generally be returned except in
>          out-of-memory conditions or serious errors such as inability to send
> -        the command to the server.  If a null pointer is returned, it should
> -        be treated like a <symbol>PGRES_FATAL_ERROR</symbol> result.  Use
> +        the command to the server.  The <function>PQresultStatus</> function
> +        should be called to check the return value for any errors (including
> +        the value of a null pointer, in which case it will return
> +        <symbol>PGRES_FATAL_ERROR</symbol>).  Use
>          <function>PQerrorMessage</function> to get more information about such
>          errors.
>         </para>
> 
> -- 
> Sent via pgsql-docs mailing list (pgsql-docs(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-docs

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

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