Обсуждение: libpd - PQfinish() - crashes client application in some cases

I shall try to write english well enough :)

I'm writing a GUI client to access my PostrgeSQL DB, with FLTK libraries. Sometimes, the client crashes.

I have written un source code, as little as possible, without FLTK libraries, to test the behavior of PQfinish(), and I noticed :

If no connexion has ever been made, [ PQstatus() retuns 'CONNECTION_BAD' ] , a call to PQfinish(...) does not do something wrong.
But il I connect with  PQsetdblogin(..), [ PQstatus returns 'CONNECTION_OK ], a first call to PQfinish(...) disconnects the client, [ PQstatus returns 'CONNECTION_BAD' ], and A SECOND call to PQfinish(..) make the client to crash with a system message 'segmentation fault'.

The source code I made to test interactively connections, disconnections and status is attached to this mail.

I could be told that one does not call PQfinish() when PQstatus is 'CONNECTION_BAD, but it is done in the PostgreSQL documentation example (Programmer's guide, chapter 1.11)

I hope this will be useful for you.

Here is my computer configuration :
    CPU    :    Pentium II 350 MHz
    OS        :    Linux  SuSE 7.2 upgraded with kernel 2.4.8
    Compiler :  GNU gcc version2.95.3
    Linker  :  GNU ld version 2.10.91 (with BFD 2.10.91.0.4)
PostgreSQL : Version 7.1.3 (downloaded on ftp.fr.postgresql.org )
 

-- 
Jean-Michel Chabanne
77450 MONTRY (FRANCE)
48" 54' N - 2" 49' E
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"Jean-Michel.chabanne" <jeanmichel.chabanne@free.fr> writes:
> ... a first call to PQfinish(...) disconnects the client,
> [ PQstatus returns 'CONNECTION_BAD' ], and A SECOND call to PQfinish(..)
> make the client to crash with a system message 'segmentation fault'.

Freeing the same chunk of memory twice leads to a crash in most
implementations of malloc/free :-(.  I don't think that libpq can be
expected to protect you from this.  Don't PQfinish() the same connection
object more than once, because after the first one it's not a connection
object anymore --- only a dangling pointer to freed memory.  For that
matter, you shouldn't call PQstatus() or any other libpq function using
an already-PQfinished connection pointer.

            regards, tom lane