SQLite in C. sqlite3_exec：回调函数中的参数集在main（）中调用时打印不完整的数据， [英] SQLite in C. sqlite3_exec: parameter set in callback function prints incomplete data when called in main()

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问题描述

我想实现一些非常基本的东西。但不幸的是，我无法得到预期的结果。

我有一个名为 emp_info 的sqlite表，在以下字段中有9个员工记录：

SR_NO [0]，NAME [1]，AGE [2]，SEX [3]，ADDRESS [4]，EMPID [5]，CARDID [6]和SALARY [7] / p>

要访问表并根据sql查询获取员工记录，我在C中有以下程序：

  #include< stdio.h> 
 #include< stdlib.h> 
 #include< sqlite3.h> 
 #include< string.h> 
 
 static int callback（void * param，int argc，char * argv []，char ** azColName）{
 int i = 0; 
 
 if（argc == 0）return 0; 
 char ** res =（char **）param; 
 * res =（char *）realloc（* res，sizeof（* res））; 
 
 // for（i = 0; i  // printf（％s =％s\\\
，azColName [i]，argv [i ]？argv [i]：NULL）; 
 
 strcpy（* res，argv [1]，sizeof（* res））; 
 printf（％s\\\
，* res）; 
 //} 
 return 0; 
} 
 
 int main（int argc，char * argv []）
 {
 sqlite3 * db; 
 char * zErrMsg = 0; 
 int rc; 
 char * sql; 
 char * param; 
 
 rc = sqlite3_open（/ DBsqlite3 / foreign_key.db，& db）; 
 
 if（rc）
 {
 fprintf（stderr，无法打开数据库：％s\\\
，sqlite3_errmsg（db））; 
 exit（0）; 
} 
 else 
 {
 fprintf（stderr，\\\
Open数据库成功\\\
 \\\
）; 
} 
 
 sql =select * from emp_info; 
 
 rc = sqlite3_exec（db，sql，callback，& param，& zErrMsg）; 
 
 if（rc！= SQLITE_OK）
 {
 fprintf（stderr，SQL错误：％s\\\
，zErrMsg）; 
 sqlite3_free（zErrMsg）; 
} 
 else 
 {
 fprintf（stdout，Operation done successfully\\\
\\\
）; 
 printf（数据的值是％s \\\
\\\
，param）; 
} 
 
 
 free（param）; 
 sqlite3_close（db）; 
 return 0; 
}

输出如下：
`
数据库成功

  AAA 
 BBB 
 CCC 
 DDD 
 EEE $ b b FFF 
 GGG 
 HHH 
 III 
操作成功完成
 
数据值为III

我预计所有的AAA，BBB，CCC，...将存储在 param 但这不是发生。 param 只能使用最后一条记录。

为什么会发生这种情况？我怀疑strcpy（）正在创建问题。

请帮助。

是获取所有有效的雇员记录（按照sql查询）存储在 param 中，它应该显示所有字段（SR_NO [0]，NAME [1]，AGE [2]，SEX [3]，ADDRESS [4]，EMPID [5]，CARDID [6]和SALARY [7]）。例如如果我向 sql 提供以下查询
sql =SELECT * FROM emp_info WHERE AGE> ; 40;
那么 param 应显示结果如下
7 GGG 41 MALE G1G2G3G4 307 7777GGG 77777 8 HHH 42 FEMALE H1H2H3H4 308 8888HHH 88888 9 III 43 FEMALE I1I2I3I4 309 9999IIII 99999
不仅仅是一个列。 / p>

提前感谢。
解决方案
CL。我开始编写基于光标的API的代码。我得到了所需的输出。
#include< iostream> #include< sqlite3.h> #include< stdlib.h> #include< list> #include< iterator> #include< algorithm> using namespace std; class sqliteDB { private： int rc; sqlite3 * db; char * zErrMsg const char * sql; sqlite3_stmt * stmt; list< string>回答; public： // ======================== ====== XX ================================= // bool connectDB（） { rc = sqlite3_open（/ DBsqlite3 / foreign_key.db，& db）; if（rc） { cerr<< 无法打开数据库：< sqlite3_errmsg（db）< endl; sqlite3_close（db）; exit（1）; } else { std :: cout<<\\\ \\\ Database opened successfully\\\ \\\ ; } } // ============================== = XX ================================= // allEmp（） { int AGE; //准备语句 rc = sqlite3_prepare（db，SELECT * FROM emp_info WHERE AGE> 40;，-1，& stmt，NULL） if（rc！= SQLITE_OK）{ throw string（sqlite3_errmsg（db））; } else { cout<<\\\ \\\ 语句准备成功\\\ \\\ ; } //创建列表容器// cout<< \\\ \\\ List answer was successfully successfully\ n \\\ ; while（sqlite3_step（stmt）== SQLITE_ROW）{ cout<< Rows<< endl; int column = sqlite3_column_count（stmt）; for（int i = 0; i { cout< 列<< endl; answer.push_back（string（（const char *）sqlite3_column_text（stmt，i）））; } } sqlite3_finalize（stmt）; cout<< \\\ \\\ Database已成功关闭\\\ \\\ ; } // ========================== XX ================================= // void printList（）{ int s = answer.size（）; for（list< std :: string> :: const_iterator it = answer.begin（）; it！= answer.end（）; it ++） cout< it-> c_str（）<< endl; } // =============================== ==== XX =========================== // }; int main（） { sqliteDB object1; object1.connectDB（）; object1.allEmp（）; object1.printList（）; cout<< \\\ \\\ All语句已正确执行\\\ \\\ ; return 0; } // ======================代码结束======== =================== //
输出如下
数据库已成功打开语句已准备成功已成功创建列表答案行列列列列列列列列列列列列列 $ b b 行列列列列列列列列列列列列行列列列列列列列列列列列列数据库已关闭成功 7 GGG 41 MALE G1G2G3G4 307 7777GGG 77777 9833446677 abi@gmail.com 07：08：1947 NONE 8 HHH 42 FEMALE H1H2H3H4 308 8888HHH 88888 9833446688 abj@gmail.com 08：09：1947 NONE 9 III 43 FEMALE I1I2I3I4 309 9999IIII 99999 9833446699 abk@gmail.com 09 ：10：1947 NONE 所有语句正确执行
我遇到的主要问题是重载< <运算符。

谢谢CL。

I am trying to achieve something very basic. But unfortunately I am unable to get the expected result.

I have a sqlite table named emp_infohaving 9 employee records in following fields:

SR_NO[0], NAME[1], AGE[2], SEX[3], ADDRESS[4], EMPID[5], CARDID[6] and SALARY[7].

To access the table and fetch the employee record as per sql query, I have following program in C:
#include <stdio.h> #include <stdlib.h> #include <sqlite3.h> #include <string.h> static int callback(void *param, int argc, char *argv[], char **azColName){ int i=0; if(argc == 0) return 0; char **res = (char **)param; *res = (char *)realloc(*res, sizeof(*res)); //for(i=0; i<argc; i++){ //printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL"); strcpy(*res, argv[1], sizeof(*res)); printf("%s\n", *res); // } return 0; } int main(int argc, char* argv[]) { sqlite3 *db; char *zErrMsg = 0; int rc; char *sql; char *param; rc = sqlite3_open("/DBsqlite3/foreign_key.db", &db); if( rc ) { fprintf(stderr, "Can't open database: %s\n", sqlite3_errmsg(db)); exit(0); } else { fprintf(stderr, "\nOpened database successfully\n\n"); } sql="select * from emp_info;" rc = sqlite3_exec(db, sql, callback, &param, &zErrMsg); if( rc != SQLITE_OK ) { fprintf(stderr, "SQL error: %s\n", zErrMsg); sqlite3_free(zErrMsg); } else { fprintf(stdout, "Operation done successfully\n\n"); printf("the value of data is %s \n\n", param); } free(param); sqlite3_close(db); return 0; }
The output is as follows: ` Opened database successfully
AAA BBB CCC DDD EEE FFF GGG HHH III Operation done successfully the value of data is III
I expected that all the AAA,BBB,CCC,.... will get stored in param and will get printed. But that's not happening. param takes only the last record only.

Why is this happening ? I suspect strcpy() is creating the problem.

Kindly help.

Apart from this, my real aim is to get all the valid employee record (as per sql query) to get stored in param and it should display all the fields(SR_NO[0], NAME[1], AGE[2], SEX[3], ADDRESS[4], EMPID[5], CARDID[6] and SALARY[7]). say for example if i provide following query to sql
sql="SELECT * FROM emp_info WHERE AGE>40";
then the param should display result as follow
7 GGG 41 MALE G1G2G3G4 307 7777GGG 77777 8 HHH 42 FEMALE H1H2H3H4 308 8888HHH 88888 9 III 43 FEMALE I1I2I3I4 309 9999IIII 99999
And not just a column.How can I achieve this ?

Thank you in advance.
解决方案
working on the suggestions suggested by CL. I started working on writing the code on cursor-based API. And I got the desired output.
#include <iostream> #include <sqlite3.h> #include <stdlib.h> #include <list> #include <iterator> #include <algorithm> using namespace std; class sqliteDB { private: int rc; sqlite3 *db; char *zErrMsg; const char *sql; sqlite3_stmt * stmt; list<string> answer; public: //================================XX=====================================// bool connectDB() { rc = sqlite3_open("/DBsqlite3/foreign_key.db", &db); if( rc ) { cerr << "Can't open database: " << sqlite3_errmsg(db) << endl; sqlite3_close(db); exit(1); } else { std::cout<<"\n\nDatabase opened successfully\n\n"; } } //===============================XX=====================================// void allEmp() { int AGE; //Preparing the statement rc=sqlite3_prepare(db,"SELECT * FROM emp_info WHERE AGE>40;", -1, &stmt, NULL ); if (rc != SQLITE_OK){ throw string(sqlite3_errmsg(db)); } else{ cout<<"\n\nThe statement was prepared successfully\n\n"; } // Creating List Container // cout << "\n\nList answer was created successfully\n\n"; while(sqlite3_step(stmt) == SQLITE_ROW) { cout << "Rows" << endl; int column = sqlite3_column_count(stmt); for(int i = 0; i < column; i++) { cout << "Columns" << endl; answer.push_back(string((const char *) sqlite3_column_text(stmt, i))); } } sqlite3_finalize(stmt); cout << "\n\nDatabase was closed successfully\n\n"; } //==============================XX=====================================// void printList(){ int s = answer.size(); for( list<std::string>::const_iterator it = answer.begin(); it != answer.end(); it++) cout << it->c_str() << endl; } //===================================XX===============================// }; int main() { sqliteDB object1; object1.connectDB(); object1.allEmp(); object1.printList(); cout << "\n\nAll the statement were executed properly\n\n"; return 0; } //========================END OF CODE===========================//
The Output is as follows
Database opened successfully The statement was prepared successfully List answer was created successfully Rows Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Rows Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Rows Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Columns Database was closed successfully 7 GGG 41 MALE G1G2G3G4 307 7777GGG 77777 9833446677 abi@gmail.com 07:08:1947 NONE 8 HHH 42 FEMALE H1H2H3H4 308 8888HHH 88888 9833446688 abj@gmail.com 08:09:1947 NONE 9 III 43 FEMALE I1I2I3I4 309 9999IIII 99999 9833446699 abk@gmail.com 09:10:1947 NONE All the statement were executed properly
The major problem I faced was with the overloading of "<<" operator.

Thank you CL.

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SQLite in C. sqlite3_exec：回调函数中的参数集在main（）中调用时打印不完整的数据， [英] SQLite in C. sqlite3_exec: parameter set in callback function prints incomplete data when called in main()

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SQLite in C. sqlite3_exec：回调函数中的参数集在main（）中调用时打印不完整的数据， [英] SQLite in C. sqlite3_exec: parameter set in callback function prints incomplete data when called in main()

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