Access MySQL from C （用C訪問MySQL數據庫）

This blog is mainly a collection of study notes and some simple tryout examples. For more details, refer to "Beginning Linux Programming", Chapter 8.

 

Most Commonly Used APIs for Accessing MySQL:

MYSQL *mysql_init(MYSQL *);

MYSQL *mysql_real_connect(MYSQL *connection,

const char *server_host,

const char *sql_user_name,

const char *sql_password,

const char *db_name,

unsigned int port_number,

const char *unix_socket_name,

unsigned int flags);

void mysql_close(MYSQL *connection);

int mysql_options(MYSQL *connection, enum option_to_set,

const char *argument);

int mysql_query(MYSQL *connection, const char *query);

my_ulonglong mysql_affected_rows(MYSQL *connection);

unsigned int mysql_errno(MYSQL *connection);

char *mysql_error(MYSQL *connection);

MYSQL_RES *mysql_store_result(MYSQL *connection);

my_ulonglong mysql_num_rows(MYSQL_RES *result);

MYSQL_ROW mysql_fetch_row(MYSQL_RES *result);	

void mysql_data_seek(MYSQL_RES *result, my_ulonglong offset);

MYSQL_ROW_OFFSET mysql_row_tell(MYSQL_RES *result);

MYSQL_ROW_OFFSET mysql_row_seek(MYSQL_RES *result, MYSQL_ROW_OFFSET offset);

void mysql_free_result(MYSQL_RES *result);

unsigned int mysql_field_count(MYSQL *connection);

MYSQL_FIELD *mysql_fetch_field(MYSQL_RES *result);

char *mysql_get_client_info(void);

char *mysql_get_host_info(MYSQL *connection);

char *mysql_get_server_info(MYSQL *connection);

char *mysql_info(MYSQL *connection);

int mysql_select_db(MYSQL *connection, const char *dbname);

int mysql_shutdown(MYSQL *connection, enum mysql_enum_shutdown_level);

 

Example1: how to connect to a mysql server

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

int main(int argc, char *argv[])
{
        MYSQL *conn_ptr;

        conn_ptr = mysql_init(NULL);
	if (!conn_ptr)
	{
                fprintf(stderr, "mysql_init failed\n");
                exit(EXIT_FAILURE);
        }

        conn_ptr = mysql_real_connect(conn_ptr, "localhost", "chenqi", "helloworld",
                                      "test", 0, NULL, 0);

	if (conn_ptr)
	{
         	printf("Connection Success \n");
	}
        else
	{
         	printf("Connection failed \n");
	}

        mysql_close(conn_ptr);

	exit(EXIT_SUCCESS);
}

gcc -I/usr/include/mysql connect1.c -L/usr/lib/mysql -lmysqlclient -o connect1

Example2: how to handle errors

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

int main(int argc, char *argv[])
{
        MYSQL conn;
        mysql_init(&conn);
        if (mysql_real_connect(&conn, "localhost", "chenqi",
	                       "i do not know", "test", 0, NULL, 0))
	{
                printf("Connection Success \n");
                mysql_close(&conn);
        }
        else
        {
                fprintf(stderr, "Connection Failed \n");
                if (mysql_errno(&conn))
                {
                        fprintf(stderr, "Connection error %d: %s \n",
                                mysql_errno(&conn), mysql_error(&conn));
                }
        }
        exit(EXIT_SUCCESS);
}

result:

chenqi@chenqi-laptop ~/MyPro/Database/access_with_c $ ./connect2
Connection Failed
Connection error 1045: Access denied for user 'chenqi'@'localhost' (using password: YES)

Example3: how to insert and update

/* insert a row into the table children */
                ret = mysql_query(&conn, "insert into children(fname, age) values('James', 23)");
                if (!ret)
                {
                        printf("Inserted %lu rows \n",
                               (unsigned long)mysql_affected_rows(&conn));
                }
                else
                {
                        fprintf(stderr, "Insert error %d: %s \n",
                                mysql_errno(&conn), mysql_error(&conn));
                }
                /* update a row in the table children */
                ret = mysql_query(&conn, "update children set age = 24 where fname = 'James'");
                if (!ret)
                {
                        printf("Update %lu rows \n",
                               (unsigned long)mysql_affected_rows(&conn));
                }
                else
                {
                        fprintf(stderr, "Update error %d: %s \n",
                                mysql_errno(&conn), mysql_error(&conn));
                }

result:

chenqi@chenqi-laptop ~/MyPro/Database/access_with_c $ ./insert-update
Connection Success
Inserted 1 rows
Update 2 rows

Example4: how to retrieve data into C application

Step1: Issue the query (mysql_query)
Step2: Retrieve the data (mysql_store_result, mysql_use_result)
Step3: Process the data (mysql_fetch_row)
Step4: Tidy up if necessary (mysql_free_result)

For a large data set, mysql_use_result should be considered, because it uses less storage.

MySQL, like other SQL databases, gives back two sorts of data:
1. The retrieved information from the table, namely the column data
2. Data about the data, so-called metadata, such as column types and names

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

#include "mysql.h"

MYSQL my_connection;
MYSQL_RES *res_ptr;
MYSQL_ROW sqlrow;

void display_row()
{
	unsigned int field_count = 0;
	while(field_count < mysql_field_count(&my_connection))
	{
		if (sqlrow[field_count])
			printf("%12s", sqlrow[field_count]);
		else
			printf("%12s", "NULL");
		field_count++;
	}
	printf("\n");
}

/* show metadata of each column */
void display_header()
{
	MYSQL_FIELD *field_ptr;

	printf("Column Details:\n");
	while((field_ptr = mysql_fetch_field(res_ptr)) != NULL)
	{
		printf("\t Name: %s\n", field_ptr->name);
		printf("\t Type: ");
		if (IS_NUM(field_ptr->type))
		{
			printf("Numeric Field\n");
		}
		else
		{
			switch(field_ptr->type)
			{
			case FIELD_TYPE_VAR_STRING:
				printf("VARCHAR\n");
				break;
			case FIELD_TYPE_LONG:
				printf("LONG\n");
				break;
			default:
				printf("Type is %d, check in mysql_com.h\n", field_ptr->type);
			}
		} /* else */
		printf("\t Max_width %ld\n", field_ptr->max_length);
		if (field_ptr->flags & AUTO_INCREMENT_FLAG)
		{
			printf("\t Auto increments\n");
		}
		printf("\n");
	}	  /* while */
}

int main(int argc, char *argv[])
{
	int ret;
	mysql_init(&my_connection);
	if (mysql_real_connect(&my_connection, "localhost", "chenqi",
			       "helloworld", "test", 0, NULL, 0))
	{
		printf("Conncetion Success\n");
		ret = mysql_query(&my_connection, "select * from children");
		if (ret)	/* error */
		{
			printf("select error: %s\n", mysql_error(&my_connection));
		}
		else		/* ok */
		{
			res_ptr = mysql_store_result(&my_connection); /* mysql_use_result for an alternative */
			if (res_ptr)
			{
				printf("Retrieved %lu rows \n", (unsigned long)mysql_num_rows(res_ptr));
				display_header();
				while (sqlrow = mysql_fetch_row(res_ptr))
				{
///					printf("Fetching data ... \n");
					display_row();
				}
				if (mysql_errno(&my_connection))
				{
					fprintf(stderr, "Retrieve error: %s\n", mysql_error(&my_connection));
				}
				mysql_free_result(res_ptr); /* res_ptr != NULL */
			}
		}
		mysql_close(&my_connection);
	}
	else
	{
		fprintf(stderr, "Connection Failed\n");
		if (mysql_errno(&my_connection))
		{
			fprintf(stderr, "Connection error %d: %s\n",
				mysql_errno(&my_connection), mysql_error(&my_connection));
		}
		exit(EXIT_FAILURE);
	}
	exit(EXIT_SUCCESS);
}

Result:

chenqi@chenqi-laptop ~/MyPro/Database/access_with_c $ ./select1 Conncetion Success Retrieved 12 rows Column Details:          Name: childno          Type: Numeric Field          Max_width 2          Auto increments          Name: fname          Type: VARCHAR          Max_width 8          Name: age          Type: Numeric Field          Max_width 2            1       Jenny          21            2        Feby          25            3    Chandler          12            4      Monica          23            5      Rachel          21            6        Ross           2            7         Joy          11            8        Emma          11            9       Gavin          14           10      Andrew          21           12       James          24           13         Tom          13