gdbm programming with C

Question

I am trying to learn gnu gdbm programming with C but can't proceed due to the paucity of the gdbm tutorial, books etc. so the only thing I have to follow is couple of simple gdbm c api codes available on w3. I wrote and compiled the following code by the help of two separate .c files but it can't fetch data from the database "testdb" so please tell me where it goes wrong. First it stores a string and, in the second part, it fetches the data. Output is; key not found.

#include <stdio.h>
#include <gdbm.h>
#include <stdlib.h>
#include <string.h>

int
main(void)
{
  GDBM_FILE dbf;
  datum key = { "testkey", 7 };     /* key, length */
  datum value = { "testvalue", 9 }; /* value, length */

  printf ("Storing key-value pair... \n");

  dbf = gdbm_open("testdb", 0, GDBM_NEWDB,0666, 0);
  gdbm_store (dbf, key, value, GDBM_INSERT);
  printf ("key: %s size: %d\n", key.dptr, key.dsize);
  gdbm_close (dbf);
  printf ("done.\n\n");

   dbf = gdbm_open("testdb", 0, GDBM_READER, 0666, 0);
   if (!dbf)
   {
      fprintf (stderr, "File %s either doesn't exist or is not a gdbm file.\n", "testdb");
      exit (1);
   }

   key.dsize = strlen("testkey") + 1;

   value = gdbm_fetch(dbf, key);

   if (value.dsize > 0) {
      printf ("%s\n", value.dptr);
      free (value.dptr);
   } 
    else {
      printf ("Key %s not found.\n", key.dptr);
   }
   gdbm_close (dbf);

   return 0;
}

Morpfh · Answer 1 · 2012-09-12T19:27:00.980

2

Include trailing '\0' in length.

  datum key = { "testkey", 8 };     /* key, length */
  datum value = { "testvalue", 10 }; /* value, length */

-- Edit:

Regarding the link you comment with: http://www-rohan.sdsu.edu/doc/gdbm/example.html

Read first bullet point carefully: "I'm assuming that the process which wrote the key and data included the terminating null character. …"

So; either:

datum key = { "testkey", 8 }; /* Include \0 in length */
datum value = { "testvalue", 10 };

And:

key.dsize = strlen("testkey") + 1; /* +1 for the trailing \0 */

or

datum key = { "testkey", 7 }; /* Skip \0 in length */
datum value = { "testvalue", 9 };

And:

key.dsize = strlen("testkey"); /* Do not +1 */

First version is often preferred as c-strings that's not null terminated can be a pain to work with.

Hope it helps.

-- Edit 2 (sorry, keep thinking of new things):

Also note that if you say i.e.:

datum value = { "testvalue", 5 }; /* value, length */

The value stored would be "testv".

edited Sep 12 '12 at 19:27

answered Sep 12 '12 at 18:12

Morpfh

4,033
18
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**Thank you very much for your kind help.** Now it's ok and gives the desired output. I took the content from the following two .c files: http://www.network-theory.co.uk/docs/gccintro/gccintro_22.html used to store data and the fetch part I wrote based on this file - http://www-rohan.sdsu.edu/doc/gdbm/example.html I wonder why they didn't add the NULL char at the end of two strings in their code (network-theory.co.uk) where it seems impossible to store data without '\0'. – Bishnu Sep 12 '12 at 19:08
**Thanks again.** I got the point. I really failed to notice it due to exhaustion as I had been trying to solve the problem for the last 24 hrs.(sometimes with gdb - gdb showed key size was <0 and I couldn't detect the reason). But without your help and favour, I wouldn't be able to solve it because I haven't got any experience either in gdbm or in c. Thanks. – Bishnu Sep 12 '12 at 19:35
@Bishnu: Your're welcome. You might find this page useful: http://stackoverflow.com/tags/c/info. I'd recommend you read "The C Programming Language (Second edition)". (Keep in mind it is written for c89 standard; but it is still _the_ book.) Also "Expert C Programming: Deep C Secrets" has, among other things, some interesting facts about the language and its history: gives you a understanding of "why is it like this or that" – which can be very useful when learning. – Morpfh Sep 13 '12 at 04:04
Yes. I have bought the book "C Programming Language, 2nd edition" with "C answer book" by Tondo (answer book book is very good) and I have just got a copy of "Deep C Secretes" so surely it can be of great help. Thank you. :) – Bishnu Sep 13 '12 at 08:50

gdbm programming with C

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