Slog2_如何使用C与Mysql进行数据交互

2019年10月1日 70次阅读来源: ArthurSlog

ArthurSlog
SLog-2
Year·1
Guangzhou·China
July 8th 2018

《Slog2_如何使用C与Mysql进行数据交互》

想让别人拉你一把，你的手在哪？别想着拉别人一把，否则你也会被拖垮。

开发环境MacOS

Mac自带的编译器，可以直接编译C语言的代码。所以现在，打开你的手机，计时5分钟，编译器了解一下

有点常识最原始的信息一般都是最有效的信息

如果面向百度或者QQ编程，那趁早转行

在Mysql的官网获取C与Mysql的交互手册Mysql官方手册

下面这段话是智商的分水岭

The MySQL C API is a C-based API that client applications written in C can use to communicate with MySQL Server.Client programs refer to C API header files at compile time and link to a C API library file, libmysqlclient, at link time.

这段话就是实现C与Mysql数据交互的关键如果你看不懂说明你想的太多做的太少

提醒：当你想进步的时候，第一时间打开官方手册

官方文档一般都不会考虑菜鸟们的阅读感受所以作为一只🙃菜鸟要有点灵性

下面是在MySQL官网找到的一个具体的调用流程：

step1 初始化MySQL客户端库，mysql_library_init()。
step2 初始化MySQL对象，mysql_init()；并连接到服务器， mysql_real_connect()。
step3 发出SQL语句并处理其结果，mysql_eqery()。
step4 关闭与MySQL服务器的连接，mysql_close()。
step5 结束使用MySQL客户端库，mysql_library_end()。

下面main.c是完整的C代码（1分钟浏览一下），关键是上面提到的五个步骤，函数的用法参考Mysql官方手册

main.c

#include <stdio.h>
#include <mysql.h>
#include <stdlib.h>

MYSQL mysql;
MYSQL_RES *result;
MYSQL_ROW row;
unsigned int num_fields;
unsigned int i;
const char host_name[] = "localhost";
const char user_name[] = "root";
const char password[] = "888888";
const char db_name[] = "my_db";
unsigned int port_num = 0;
unsigned long CLIENT_MULTI_STATMENTS = 0;

int main(void)
{
    /* step1 int mysql_library_init(int argc, char **argv, char **groups) */
    if (mysql_library_init(0, NULL, NULL))
    {
        fprintf(stderr, "could not initialize MySQL client library\n");
        exit(1);
    }

    /* step2 MYSQL *mysql_init(MYSQL *mysql) */
    mysql_init(&mysql);

    /* step2 MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd, const char *db, unsigned int port, const char *unix_socket, unsigned long client_flag) */
    if (mysql_real_connect(&mysql, host_name, user_name, password,
                           db_name, port_num, NULL, CLIENT_MULTI_STATEMENTS) == NULL)
    {
        printf("mysql_real_connect() failed\n");
        fprintf(stderr, "Failed to connect to database: Error: %s\n", mysql_error(&mysql));
        mysql_close(&mysql);
        exit(1);
    }

    /* step3 int mysql_query(MYSQL *mysql, const char *stmt_str) */
    int status = mysql_query(&mysql, "SELECT * FROM Account"); /* int mysql_query(MYSQL *mysql, const char *stmt_str) */

    if (status)
    {
        printf("Could not execute statement(s)");
        mysql_close(&mysql);
        exit(0);
    }

    /* process each statement result */
    do
    {
        /* did current statement return data? */
        /* MYSQL_RES *mysql_store_result(MYSQL *mysql) */
        result = mysql_store_result(&mysql);
        if (result)
        {
            /* unsigned int mysql_num_fields(MYSQL_RES *result) */
            num_fields = mysql_num_fields(result);
            /* MYSQL_ROW mysql_fetch_row(MYSQL_RES *result) */
            while ((row = mysql_fetch_row(result)))
            {
                unsigned long *lengths;
                lengths = mysql_fetch_lengths(result);
                for (i = 0; i < num_fields; i++)
                {
                    printf("[%.*s] ", (int)lengths[i],
                           row[i] ? row[i] : "NULL");
                }
                printf("\n");
            }
            /* yes; process rows and free the result set */
            /* process_result_set(mysql, result); */
            mysql_free_result(result) /* void mysql_free_result(MYSQL_RES *result) */;
        }
        else /* no result set or error*/
        {
            /* unsigned int mysql_field_count(MYSQL *mysql) */
            if (mysql_field_count(&mysql) == 0)
            {
                printf("lld rows affected\n");
                break;
            }
        }
        /* more results? -1 = no, >0 = error, 0 = yes (keep looping) */
        if ((status = mysql_next_result(&mysql)) > 0) /* int mysql_next_result(MYSQL *mysql) */
        {
            printf("Counld not execute statement\n");
        }
    } while (status == 0);

    /* step4 void mysql_close(MYSQL *mysql) */
    mysql_close(&mysql);

    /* step5 void mysql_library_end(void) */
    mysql_library_end();

    return EXIT_SUCCESS;
}

英文看不懂?
google翻译， chrome ，字典了解一下
流程代码分5个step

main.c

    /* step1 int mysql_library_init(int argc, char **argv, char **groups) */
    if (mysql_library_init(0, NULL, NULL))
    {
        fprintf(stderr, "could not initialize MySQL client library\n");
        exit(1);
    }

    /* step2 MYSQL *mysql_init(MYSQL *mysql) */
    mysql_init(&mysql);
    /* step2 MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd, const char *db, unsigned int port, const char *unix_socket, unsigned long client_flag) */
    if (mysql_real_connect(&mysql, host_name, user_name, password,
                           db_name, port_num, NULL, CLIENT_MULTI_STATEMENTS) == NULL)
    {
        printf("mysql_real_connect() failed\n");
        fprintf(stderr, "Failed to connect to database: Error: %s\n", mysql_error(&mysql));
        mysql_close(&mysql);
        exit(1);
    }

    /* step3 int mysql_query(MYSQL *mysql, const char *stmt_str) */
    int status = mysql_query(&mysql, "SELECT * FROM Account"); /* int mysql_query(MYSQL *mysql, const char *stmt_str) */
    if (status)
    {
        printf("Could not execute statement(s)");
        mysql_close(&mysql);
        exit(0);
    }

    /* step4 void mysql_close(MYSQL *mysql) */
    mysql_close(&mysql);

    /* step5 void mysql_library_end(void) */
    mysql_library_end();

这里面涉及到的C语法，google了解一下
上面的代码走完一个流程就算完了，但实际情况是需要‘增删改查’
So

main.c

    /* process each statement result */
    do
    {
        /* did current statement return data? */
        /* MYSQL_RES *mysql_store_result(MYSQL *mysql) */
        result = mysql_store_result(&mysql);
        if (result)
        {
            /* unsigned int mysql_num_fields(MYSQL_RES *result) */
            num_fields = mysql_num_fields(result);
            /* MYSQL_ROW mysql_fetch_row(MYSQL_RES *result) */
            while ((row = mysql_fetch_row(result)))
            {
                unsigned long *lengths;
                lengths = mysql_fetch_lengths(result);
                for (i = 0; i < num_fields; i++)
                {
                    printf("[%.*s] ", (int)lengths[i],
                           row[i] ? row[i] : "NULL");
                }
                printf("\n");
            }
            /* yes; process rows and free the result set */
            /* process_result_set(mysql, result); */
            mysql_free_result(result) /* void mysql_free_result(MYSQL_RES *result) */;
        }
        else /* no result set or error*/
        {
            /* unsigned int mysql_field_count(MYSQL *mysql) */
            if (mysql_field_count(&mysql) == 0)
            {
                printf("lld rows affected\n");
                break;
            }
        }
        /* more results? -1 = no, >0 = error, 0 = yes (keep looping) */
        if ((status = mysql_next_result(&mysql)) > 0) /* int mysql_next_result(MYSQL *mysql) */
        {
            printf("Counld not execute statement\n");
        }
    } while (status == 0);

根据Mysql官方手册
在代码间，当一个函数执行后会返回一个结果，这个结果的数据结构各不相同，参考Mysql官方手册
举个例子栗子：

main.c

mysql_library_init()

完整的写法

main.c

int mysql_library_init(int argc, char **argv, char **groups)

第一个 int，代表的就是返回值的数据结构
如果要储存这个函数执行完返回的值，需要声明一个相同的数据结构的对象来保存这个值
举个栗子：

main.c

int return_mli = null; //声明一个数据结构为int的对象，并让reture_mli的值为空

然后你就可以

main.c

int return_mli = null;
return_mli = mysql_library_init(0, NULL, NULL);

此时你就可以判断函数的返回值，以确定接下来要执行的操作
C 语言把任何非零和非空的值假定为 true，把零或 null 假定为 false。

main.c

int return_mli = 0;
return_mli = mysql_library_init(0, NULL, NULL);

if(return_mli)
{
    do something
}

明白了这些，再次看一下完整的代码

main.c

#include <stdio.h>
#include <mysql.h>
#include <stdlib.h>

MYSQL mysql;
MYSQL_RES *result;
MYSQL_ROW row;
unsigned int num_fields;
unsigned int i;
const char host_name[] = "localhost";
const char user_name[] = "root";
const char password[] = "888888";
const char db_name[] = "my_db";
unsigned int port_num = 0;
unsigned long CLIENT_MULTI_STATMENTS = 0;

int main(void)
{
    /* step1 int mysql_library_init(int argc, char **argv, char **groups) */
    if (mysql_library_init(0, NULL, NULL))
    {
        fprintf(stderr, "could not initialize MySQL client library\n");
        exit(1);
    }

    /* step2 MYSQL *mysql_init(MYSQL *mysql) */
    mysql_init(&mysql);

    /* step2 MYSQL *mysql_real_connect(MYSQL *mysql, const char *host, const char *user, const char *passwd, const char *db, unsigned int port, const char *unix_socket, unsigned long client_flag) */
    if (mysql_real_connect(&mysql, host_name, user_name, password,
                           db_name, port_num, NULL, CLIENT_MULTI_STATEMENTS) == NULL)
    {
        printf("mysql_real_connect() failed\n");
        fprintf(stderr, "Failed to connect to database: Error: %s\n", mysql_error(&mysql));
        mysql_close(&mysql);
        exit(1);
    }

    /* step3 int mysql_query(MYSQL *mysql, const char *stmt_str) */
    int status = mysql_query(&mysql, "SELECT * FROM Account"); /* int mysql_query(MYSQL *mysql, const char *stmt_str) */

    if (status)
    {
        printf("Could not execute statement(s)");
        mysql_close(&mysql);
        exit(0);
    }

    /* process each statement result */
    do
    {
        /* did current statement return data? */
        /* MYSQL_RES *mysql_store_result(MYSQL *mysql) */
        result = mysql_store_result(&mysql);
        if (result)
        {
            /* unsigned int mysql_num_fields(MYSQL_RES *result) */
            num_fields = mysql_num_fields(result);
            /* MYSQL_ROW mysql_fetch_row(MYSQL_RES *result) */
            while ((row = mysql_fetch_row(result)))
            {
                unsigned long *lengths;
                lengths = mysql_fetch_lengths(result);
                for (i = 0; i < num_fields; i++)
                {
                    printf("[%.*s] ", (int)lengths[i],
                           row[i] ? row[i] : "NULL");
                }
                printf("\n");
            }
            /* yes; process rows and free the result set */
            /* process_result_set(mysql, result); */
            mysql_free_result(result) /* void mysql_free_result(MYSQL_RES *result) */;
        }
        else /* no result set or error*/
        {
            /* unsigned int mysql_field_count(MYSQL *mysql) */
            if (mysql_field_count(&mysql) == 0)
            {
                printf("lld rows affected\n");
                break;
            }
        }
        /* more results? -1 = no, >0 = error, 0 = yes (keep looping) */
        if ((status = mysql_next_result(&mysql)) > 0) /* int mysql_next_result(MYSQL *mysql) */
        {
            printf("Counld not execute statement\n");
        }
    } while (status == 0);

    /* step4 void mysql_close(MYSQL *mysql) */
    mysql_close(&mysql);

    /* step5 void mysql_library_end(void) */
    mysql_library_end();

    return EXIT_SUCCESS;
}

接下来切换至代码路径下
执行编译指令

gcc -c main.c -I /usr/local/mysql/include

执行链接指令，得到执行文件main

gcc main.o -o main -L /usr/local/mysql/lib -lmysqlclient

此时当前路径下有这三个文件

main.c main.o main

启动可执行文件main

./main

此时会提示出错，解决办法第一时间应该想到的是Mysql官方手册
根据官方提供的信息，可以推断是系统链接的问题。google可得，由于MacOS系统的保护机制，解决方式如下：

ln -s /usr/local/mysql/lib/libmysqlclient.21.dylib /usr/local/lib/libmysqlclient.21.dylib

最后，启动可执行文件main

./main

前提是你配置好了数据库参数，下面是我自己配置的数据库信息

const char host_name[] = "localhost";
const char user_name[] = "root";
const char password[] = "888888";
const char db_name[] = "my_db";

int status = mysql_query(&mysql, "SELECT * FROM Account"); /* int mysql_query(MYSQL *mysql, const char *stmt_str) */

SQL语法不懂？SQL手册了解一下

SELECT * FROM Account

至此，我们使用C语言与MySQL数据库成功实现了数据交互。

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《Slog2_如何使用C与Mysql进行数据交互》

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谢谢

    原文作者：ArthurSlog
    原文地址: https://segmentfault.com/a/1190000016093160
    本文转自网络文章，转载此文章仅为分享知识，如有侵权，请联系博主进行删除。

开发环境MacOS

提醒：当你想进步的时候，第一时间打开官方手册

欢迎关注我的微信公众号 ArthurSlog

如果你喜欢我的文章 欢迎点赞 留言

谢谢

如果你喜欢我的文章欢迎点赞留言