Re: [sqlite] Odd issue when SELECT querying
Thanks all !
I will receive a C book tomorrow.
I need it.
David.
James Dennett wrote:
David Hautbois wrote:
Hi
I have an odd issue.
My function :
char * get_config_value (sqlite3 * db, char * config_name) {
[...]
configvalue = (char *) sqlite3_column_text(stmt, 0);
[...]
return configvalue;
}
Why the variable content changes ??
Why the variable configvalue has not the same content ??
The variable has the same content (a pointer), but the pointer is
invalid by the time your function returns it. You need to copy the
*string*, not just a pointer to it.
This is essentially the same issue that Igor described when he wrote:
Strings passed to the callback are valid only within the callback. As
soon as the callback returns, the memory may be deallocated or reused
for other purposes. If the callback wants to keep some strings around
beyond a single call, it should allocate its own memory and copy the
value over.
It's vitally important when using C libraries that you read the
documentation and avoid making any assumptions about the lifetimes of
objects referenced by pointers.
C++ wrappers can return std::string objects and avoid this issue (though
even in C++ it's important to consider validity/lifetime issues for both
pointers and iterators).
-- James
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Re: [sqlite] Odd issue when SELECT querying
I found the solution :
I replaced this line :
configvalue = (char *) sqlite3_column_text(stmt, 0);
by
configvalue = g_strdup((gchar *) sqlite3_column_text(stmt, 0));
and the configvalue type : gchar
Now it works !!
A newbie error...
David.
David Hautbois wrote:
Hi
I have an odd issue.
My database schema :
sqlite> .schema
CREATE TABLE config (id INTEGER PRIMARY KEY, config_name TEXT ,
config_value TEXT);
CREATE TABLE waypoints (id INTEGER PRIMARY KEY, wp_date INTEGER,
wp_longitude REAL, wp_latitude REAL, wp_speed REAL);
The config table content :
sqlite> SELECT * from config;
1|version|1
2|ftpserver|A
3|ftp_remotedir|
4|ftp_login|
5|ftp_password|
My function :
char * get_config_value (sqlite3 * db, char * config_name) {
int ncols;
sqlite3_stmt *stmt;
char *sql;
const char *tail;
char * configvalue = 0;
sql = g_strdup_printf("SELECT config_value FROM config WHERE
config_name='%s'\n", config_name);
printf (sql);
sqlite3_prepare(db , sql, (int)strlen(sql), , );
ncols = sqlite3_column_count(stmt);
while(sqlite3_step(stmt) == SQLITE_ROW) {
printf ("gpstracer-cfg.c - get_config_value : Getting column
content\n");
configvalue = (char *) sqlite3_column_text(stmt, 0);
printf ("gpstracer-cfg.c - get_config_value : content=%s\n",
configvalue); > note this line
}
printf ("gpstracer-cfg.c - get_config_value : %s=%s\n",
config_name, configvalue); >and this one
sqlite3_finalize(stmt);
return configvalue;
}
***
When I call this function : get_config_value (db, "version")
I get :
SELECT config_value FROM config WHERE config_name='version'
gpstracer-cfg.c - get_config_value : Getting column content
gpstracer-cfg.c - get_config_value :
content=1 > ok
gpstracer-cfg.c - get_config_value : version=ftp_password
> Why the variable content changes ??
Why the variable configvalue has not the same content ??
It's the same issue than my previous post (Callback issue - using
pointer as argument), when I used a callback.
Thanks.
David.
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[sqlite] Odd issue when SELECT querying
Hi
I have an odd issue.
My database schema :
sqlite> .schema
CREATE TABLE config (id INTEGER PRIMARY KEY, config_name TEXT ,
config_value TEXT);
CREATE TABLE waypoints (id INTEGER PRIMARY KEY, wp_date INTEGER,
wp_longitude REAL, wp_latitude REAL, wp_speed REAL);
The config table content :
sqlite> SELECT * from config;
1|version|1
2|ftpserver|A
3|ftp_remotedir|
4|ftp_login|
5|ftp_password|
My function :
char * get_config_value (sqlite3 * db, char * config_name) {
int ncols;
sqlite3_stmt *stmt;
char *sql;
const char *tail;
char * configvalue = 0;
sql = g_strdup_printf("SELECT config_value FROM config WHERE
config_name='%s'\n", config_name);
printf (sql);
sqlite3_prepare(db , sql, (int)strlen(sql), , );
ncols = sqlite3_column_count(stmt);
while(sqlite3_step(stmt) == SQLITE_ROW) {
printf ("gpstracer-cfg.c - get_config_value : Getting column
content\n");
configvalue = (char *) sqlite3_column_text(stmt, 0);
printf ("gpstracer-cfg.c - get_config_value : content=%s\n",
configvalue); > note this line
}
printf ("gpstracer-cfg.c - get_config_value : %s=%s\n", config_name,
configvalue); >and this one
sqlite3_finalize(stmt);
return configvalue;
}
***
When I call this function : get_config_value (db, "version")
I get :
SELECT config_value FROM config WHERE config_name='version'
gpstracer-cfg.c - get_config_value : Getting column content
gpstracer-cfg.c - get_config_value :
content=1 > ok
gpstracer-cfg.c - get_config_value : version=ftp_password >
Why the variable content changes ??
Why the variable configvalue has not the same content ??
It's the same issue than my previous post (Callback issue - using
pointer as argument), when I used a callback.
Thanks.
David.
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Re: [sqlite] Re: Callback issue - using pointer as argument
It works better.
Thanks.
David.
Igor Tandetnik wrote:
David Hautbois <[EMAIL PROTECTED]> wrote:
char * get_config_value (sqlite3 * db, char * config_name) {
TabResult res;
rc= sqlite3_exec( db, query, exec_get_config_value_cb , ,
);
Strings passed to the callback are valid only within the callback. As
soon as the callback returns, the memory may be deallocated or reused
for other purposes. If the callback wants to keep some strings around
beyond a single call, it should allocate its own memory and copy the
value over.
Better still, stop using sqlite3_exec and switch over to
sqlite3_prepare / sqlite3_step / sqlite3_finalize interface.
Igor Tandetnik
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[sqlite] Callback issue - using pointer as argument
Hi
I spent 6 hours on this issue and I don't understand why I get a wrong
value.
My database content :
sqlite> select * from config;
1|version|1
2|ftpserver|A
3|ftp_remotedir|
4|ftp_login|
5|ftp_password|
A simple query :
sqlite> SELECT config_value FROM config WHERE config_name="version";
1
Ok, all right.
Now I try to execute the same query with this code :
*
typedef struct TabResult {
char *value;
} TabResult;
/***/
/* Get a config
value
*/
/***/
char * get_config_value (sqlite3 * db, char * config_name) {
gchar * query;
int rc;
char *zErrMsg = 0;
char *config_value=0;
TabResult res;
query = g_strdup_printf("SELECT config_value FROM config WHERE
config_name='version'\n");
printf (query);
rc= sqlite3_exec( db, query, exec_get_config_value_cb , , );
if (rc!=SQLITE_OK) {
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
return NULL;
}
printf ("gpstracer-cfg.c - get_config_value : %s : %s\n",
config_name, (char *)res.value);
return config_value;
}
/***/
/* Callback of the get_config_value
query */
/***/
static int exec_get_config_value_cb(void *result, int argc, char **argv,
char **azColName) {
int i;
TabResult *p = (TabResult*)result;
for(i=0; ivalue = argv[0];
printf ("gpstracer-cfg.c - exec_get_config_value_cb : returned value
: %s\n", argv[0]);
printf ("%s,%s\n", p->value, argv[0]);
return 0;
}
*
When I call the function :
get_config_value (sqlite3 * db, "version")
I get :
1: SELECT config_value FROM config WHERE config_name="version"
2: 0 : config_value = 1
3:
4: gpstracer-cfg.c - exec_get_config_value_cb : returned value : 1
5:
1,1
-> ok
6: gpstracer-cfg.c - get_config_value : version :
ftp_password ->nok
Line #4 : all right : version = 1
Line #5 : The returned value change to "ftp-password" !
I don't know why !
I use libsqlite3-0 3.4.1
Can some help me to understand this ?
Thanks.
David.
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