Fabrice NA a écrit : Hi,
In sqlite3_exec, you pass a function pointer (callback). Sqlite call that function with each row of data Best wishes Noël > Hi all, > > > > I am trying to understand Sqlite and to make thing worse I am also learning > C++. You will not be surprised that I find it really hard to understand the > C++ example on the web. Can someone guide me to digest this? If you can > explain please do so by extrapolating since I am a total newbie. > > > > I have managed to compile the code example into a file called testdb.exe and > have created a database named Cars.db containing 7 rows. (notice that I have > removed some part of that code that I don't need help for) > > > > #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]); > > exit(1); > > } > > rc = sqlite3_open(argv[1], &db); > > > > rc = sqlite3_exec(db, argv[2], callback, 0, &zErrMsg); > > > > sqlite3_close(db); > > > > return 0; > > } > > > > Now in command line when I execute testdb.exe cars.db "select * from cars" > (the table is called cars too) everything works fine and I get an output > with all my seven rows. But how come this work? Is this some kind of magic? > > > > Even after reading the doc again and again I don't understand (probably > because I am still learning some basic C++ concepts). Here my questions; > > > > 1) What is the purpose of doing "sqlite3 *db" are we just creating a > pointer of type sqlite3 named db here? > > > > 2) At first I though that for the main function the first > parameter "cars.db" > was represented by the variable argc and the second "select * from cars" by > argv. Well at the end, it looks like that "cars.db" is argv[1] and that the > select statement is argv[2]. What is argc then? Seems like it's the number > of rows returned by the query (when looking at function callback) but how > the program find this out? How come we have a line " if( argc!=3 )" and see > this same argc variable in the callback function? > > > > 3) I don't understand the third argument from the query " rc = > sqlite3_exec(db, argv[2], callback, 0, &zErrMsg);" my problem is > understanding the callback function inside the sqlite3_exec function. > Reading this link http://www.sqlite.org/c3ref/exec.html didn't help too much > even though it has been written in plain English. > > > > 4) I am using VC++ Express from Microsoft on WinXP and would like to > know if it's possible to pass parameter when debugging i.e. tell the > debugger to use cars.db for file and use "select * from cars" as a statement > (this would allow me to see what's hapening witout replacing variables by > their real values). > > > > 5) It's really hard to find some simple example on internet about C++ > working with sqlite. Can any of you provide with simple sample codes that > shows how you can do and what you can do with sqlite and C++? > > > > I hope I didn't offended anyone with my lack of knowledge and I thank in > advance the courageous ones who managed to read this email until the end and > probably got answers to my questions. > > > > Fabou > _______________________________________________ > sqlite-users mailing list > sqlite-users@sqlite.org > http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users > > _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users