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Re: Segmentation fault when overwriting a k/v with MDB_RESERVE ?

Oh ok it makes sense now. Thank you for the heads up

On 18 August 2015 at 15:36, Howard Chu <hyc@symas.com> wrote:
Simon Majou wrote:
This is version 0.9.16

I used sample-bdb.txt for instance, with the DUP_SORT option, and the mdb_put
with the MDB_RESERVED flags, and execute it 2 times. The first run (creation)
is ok, the second (update) gets the seg fault.

DUPSORT and RESERVED are incompatible. RESERVED means you're not providing a data value, only a length. DUPSORT sorts *values* - since you didn't provide a valid value, it probably chased a pointer off into space.

I ran it in gdb and got the stack :
#0  mdb_cursor_set (mc=mc@entry=0x7fffffffdb40, key=key@entry=0x7fffffffdda0,
     data="" op=op@entry=MDB_SET,
     exactp=exactp@entry=0x7fffffffd7f0) at mdb.c:5867
#1  0x00007ffff7bd024c in mdb_cursor_put (mc=0x7fffffffdb40,
     key=0x7fffffffdda0, data="" flags=32768) at mdb.c:6391
#2  0x00007ffff7bd095f in mdb_cursor_put (mc=0x7fffffffd9b0,
     key=0x7fffffffdd90, data="" flags=<optimized out>)
     at mdb.c:6780
#3  0x00007ffff7bd29b2 in mdb_put (txn=<optimized out>, dbi=<optimized out>,
     key=0x7fffffffdd90, data="" flags=65536) at mdb.c:8611
#4  0x0000000000400ccf in main (argc=1, argv=0x7fffffffded8) at sample-mdb.c:43

It seems the mp_lower value of the page is set to 0 instead of a value inside
the page.

On 18 August 2015 at 13:58, Howard Chu <hyc@symas.com <mailto:hyc@symas.com>>

    Simon Majou wrote:


        I wonder, why do I get a Segmentation fault when I use mdb_put with
        MDB_RESERVE on a key that already exists ?

    Have no idea. Provide some more details and maybe we can guess.

    LMDB version? stack trace from SEGV? What's the size of the existing
    record, what's the size of the new record? Show sample code that
    demonstrates the problem?

       -- Howard Chu
       CTO, Symas Corp. http://www.symas.com
       Director, Highland Sun http://highlandsun.com/hyc/
       Chief Architect, OpenLDAP http://www.openldap.org/project/


Best regards,


  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/


Best regards,