I'm not able to pass a callback function with a new argument i added in c - c

Question

I don't know how to pass a variable into a callback to do some stuff (print json output) inside this function.
In sample code there is no other functions i need to pass but with sqlite3 I think I have to do this in the callback, tell me if i'm doing wrong.
The callback is called in sendhosts() function in my code.
I get this error :
In function ‘sendhosts’:
sample.c:85:48: error: too few arguments to function ‘callback’
rc = sqlite3_exec(db, "SELECT * FROM hosts;", callback(req), 0, &zErrMsg);
Original code is :
#include <stdio.h>
#include <sqlite3.h>
static int callback(void *NotUsed, int argc, char **argv, char **azColName){
int i;
for(i=0; i<argc; i++){
printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
}
printf("\n");
return 0;
}
int main(int argc, char **argv){
sqlite3 *db;
char *zErrMsg = 0;
int rc;
if( argc!=3 ){
fprintf(stderr, "Usage: %s DATABASE SQL-STATEMENT\n", argv[0]);
return(1);
}
rc = sqlite3_open(argv[1], &db);
if( rc ){
fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db));
sqlite3_close(db);
return(1);
}
rc = sqlite3_exec(db, argv[2], callback, 0, &zErrMsg);
if( rc!=SQLITE_OK ){
fprintf(stderr, "SQL error: %s\n", zErrMsg);
sqlite3_free(zErrMsg);
}
sqlite3_close(db);
return 0;
}
My code is :
#include <sys/types.h> /* size_t, ssize_t */
#include <stdarg.h> /* va_list */
#include <stddef.h> /* NULL */
#include <stdint.h> /* int64_t */
#include <stdlib.h>
#include <string.h> /* memset */
#include <stdio.h>
#include <kcgi.h>
#include <kcgihtml.h>
#include <sqlite3.h>
enum page {
PAGE_HOSTS,
PAGE__MAX
};
enum key {
KEY_INTEGER,
KEY_FILE,
KEY_PAGECOUNT,
KEY_PAGESIZE,
KEY__MAX
};
struct tstrct {
struct khtmlreq req;
struct kreq *r;
};
typedef void (*disp)(struct kreq *);
static void sendhosts(struct kreq *);
static const disp disps[PAGE__MAX] = {
sendhosts, /* PAGE_SENDDATA */
};
static const struct kvalid keys[KEY__MAX] = {
{ kvalid_int, "integer" }, /* KEY_INTEGER */
{ NULL, "file" }, /* KEY_FILE */
{ kvalid_uint, "count" }, /* KEY_PAGECOUNT */
{ kvalid_uint, "size" }, /* KEY_PAGESIZE */
};
static const char *const pages[PAGE__MAX] = {
"sendhosts" /* PAGE_SENDDATA */
};
static void
resp_open(struct kreq *req, enum khttp http)
{
enum kmime mime;
if (KMIME__MAX == (mime = req->mime))
mime = KMIME_APP_OCTET_STREAM;
khttp_head(req, kresps[KRESP_STATUS],
"%s", khttps[http]);
khttp_head(req, kresps[KRESP_CONTENT_TYPE],
"%s", kmimetypes[mime]);
khttp_body(req);
}
static int callback(struct kreq *req, int argc, char **argv, char **azColName){
char *page;
struct khtmlreq r;
resp_open(req, KHTTP_200);
khtml_open(&r, req, 0);
int i;
for(i=0; i<argc; i++){
printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
struct khtmlreq r;
khtml_puts(&r, "OK");
}
khtml_close(&r);
free(page);
}
static void
sendhosts(struct kreq *req)
{
sqlite3 *db;
char *zErrMsg = 0;
int rc;
rc = sqlite3_open("/var/www/MaSSH/databases/massh.db", &db);
if(rc){
sqlite3_close(db);
}
rc = sqlite3_exec(db, "SELECT * FROM hosts;", callback(req), 0, &zErrMsg);
if(rc!= SQLITE_OK){
sqlite3_free(zErrMsg);
}
sqlite3_close(db);
}
int
main(void)
{
struct kreq r;
enum kcgi_err er;
/* Set up our main HTTP context. */
er = khttp_parse(&r, keys, KEY__MAX,
pages, PAGE__MAX, PAGE_HOSTS);
if (KCGI_OK != er)
return(EXIT_FAILURE);
/*
* Accept only GET, POST, and OPTIONS.
* Restrict to text/html and a valid page.
* If all of our parameters are valid, use a dispatch array to
* send us to the page handlers.
*/
if (KMETHOD_OPTIONS == r.method) {
khttp_head(&r, kresps[KRESP_ALLOW],
"OPTIONS GET POST");
resp_open(&r, KHTTP_200);
} else if (KMETHOD_GET != r.method &&
KMETHOD_POST != r.method) {
resp_open(&r, KHTTP_405);
} else if (PAGE__MAX == r.page ||
KMIME_TEXT_HTML != r.mime) {
resp_open(&r, KHTTP_404);
} else
(*disps[r.page])(&r);
khttp_free(&r);
return(EXIT_SUCCESS);
}

Answer 1

Your callback's signature
static int callback(struct kreq *req, int argc, char **argv, char **azColName)
is wrong. It should be
static int callback(void *ptr, int argc, char **argv, char **azColName) {
struct kreq *req = (struct kreq *)ptr;
The callback's signature should exactly match the function pointer's format, which, for sqlite3_exec, is int (*callback)(void *, int, char **, char **).
And this
rc = sqlite3_exec(db, "SELECT * FROM hosts;", callback(req), 0, &zErrMsg);
should be
rc = sqlite3_exec(db, "SELECT * FROM hosts;", callback, req, &zErrMsg);
callback(req) calls callback, which is not what you want; you're trying to pass a function pointer to a function, which is done with just the function's name.