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From Tom Donovan <>
Subject Re: mod_dbd and prepared statements (httpd-2.2.9)
Date Sat, 18 Oct 2008 16:27:32 GMT
Andrej van der Zee wrote:
> Hi,
> I did not find a solution, I just stopped using prepared statements
> altogether. But I tried to isolate the problem just now, and found somehow
> that I cannot use FLOAT in prepared statement somehow (when I tried INT
> columns it even segfaults). Below the source code of a mini-module to
> illustrate this. This is the table I created in MySQL:
> And this is what appears in the MySQL log:
> Prepare    INSERT INTO simple_table (duration) VALUES (?)
> Execute    INSERT INTO simple_table (duration) VALUES ('')
> If you want to reproduce it, dont forget to put this in httpd.conf:
> LoadModule prep_stmt_module   modules/
> PrepStmt on

You missed a couple of details:

* apr_dbd_pvquery is only for string values.

   You must use apr_dbd_pvbquery (with a "b") for binary values.


* You don't pass a float value directly - %f takes a *pointer* to a float.

* It is best not to call ap_dbd_prepare and ap_dbd_acquire directly.

   You should populate your own function pointers at config time using APR_RETRIEVE_OPTIONAL_FN.

   If APR_RETRIEVE_OPTIONAL_FN gives you NULL pointers, that means that mod_dbd is not loaded.

   When you need to prepare a stmt or acquire a connection, call these function pointers.

Here is your code back with these changes (and with all definitions moved to the top of functions
I could compile it on Windows).  It works for me with httpd-2.2.10.


#include <httpd.h>
#include <http_config.h>
#include <http_core.h>
#include <http_log.h>
#include <http_protocol.h>
#include <http_connection.h>
#include <apr_file_info.h>
#include <apr_file_io.h>
#include <apr_dbd.h>
#include <mod_dbd.h>

module AP_MODULE_DECLARE_DATA prep_stmt_module;
static int prep_stmt_write(request_rec *r);
static void (*dbd_prepare_fn)(server_rec*, const char*, const char*) = NULL;
static ap_dbd_t *(*dbd_acquire_fn)(request_rec*) = NULL;

typedef struct prep_stmt_config
   int prep_stmt_on;
} prep_stmt_config;

static const char * prep_stmt_config_set_prep_stmt_on(cmd_parms *cmd,
                                                         void *dummy, int flag)
   prep_stmt_config *cfg =
   dbd_prepare_fn (cmd->server,
     "INSERT INTO simple_table (duration) VALUES(%f)", "insert_row");
   cfg->prep_stmt_on = flag;
   return NULL;

static int prep_stmt_write(request_rec *r)
     int rv, nrows;
     ap_dbd_t * dbd ;
     apr_dbd_prepared_t *prepared ;
     float val = 10.2;

     prep_stmt_config *cfg =
     if (!cfg->prep_stmt_on)
         return DECLINED;
     dbd = dbd_acquire_fn(r);
     prepared = apr_hash_get(dbd->prepared, "insert_row",APR_HASH_KEY_STRING);
     if (!prepared) {
         ap_log_error(APLOG_MARK, APLOG_ERR, 0, r->server,
                 "DBD Log: Failed to get prepared statement: update_request");
         return DECLINED;

     if (rv = apr_dbd_pvbquery(dbd->driver, r->pool, dbd->handle, &nrows,
                                 prepared, &val, NULL)) {
         const char *errmsg = apr_dbd_error(dbd->driver, dbd->handle, rv);
         ap_log_error(APLOG_MARK, APLOG_ERR, 0, r->server,
                 "DBD Log: Failed to execute prepared statement: insert_row");
         return DECLINED;

static const command_rec prep_stmt_cmds[] = {
     AP_INIT_FLAG("PrepStmt", prep_stmt_config_set_prep_stmt_on,
             NULL, RSRC_CONF, "Enable DBD Log"),
     { NULL }

static void prep_stmt_register_hooks(apr_pool_t *p)
     ap_hook_log_transaction(prep_stmt_write, NULL, NULL, APR_HOOK_MIDDLE);

static void * prep_stmt_create_config(apr_pool_t *pool, server_rec *s)
   prep_stmt_config *cfg = apr_pcalloc(pool, sizeof(prep_stmt_config));
   dbd_prepare_fn = APR_RETRIEVE_OPTIONAL_FN(ap_dbd_prepare);
   dbd_acquire_fn = APR_RETRIEVE_OPTIONAL_FN(ap_dbd_acquire);
   return cfg;

module AP_MODULE_DECLARE_DATA prep_stmt_module =
     STANDARD20_MODULE_STUFF,    /* stuff in every 2.0 mod                */
     NULL,                       /* create per-directory config structure */
     NULL,                       /* merge per-directory config structures */
     prep_stmt_create_config,    /* create per-server config structure    */
     NULL,                       /* merge per-server config structures    */
     prep_stmt_cmds,             /* command apr_table_t                   */
     prep_stmt_register_hooks    /* register hooks                        */

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