C Mysql API连接Mysql
最近都在查看MYsql C API文档,也遇到了很多问题,下面来简单的做一个总结。
mysql多线程问题
- mysql多线程处理不好,经常会发生coredump,见使用Mysql出core一文。
单线程
- 一般情况下,单线程连接mysql代码如下:
1 /* 2 single_thread_mysql_client.cpp 3 */ 4 #include <stdio.h> 5 #include <string.h> 6 #include <stdlib.h> 7 #include <mysql/mysql.h> 8 #include <pthread.h> 9 #include <unistd.h> 10 11 #define DBHOST "localhost" 12 #define DBUSER "pca" 13 #define DBPASS "pca" 14 #define DBPORT 3306 15 #define DBNAME "dxponline" 16 #define DBSOCK NULL //"/var/lib/mysql/mysql.sock" 17 #define DBPCNT 0 18 19 int main() 20 { 21 MYSQL_RES *result; 22 MYSQL_ROW row; 23 MYSQL_FIELD *field; 24 unsigned int num_fields; 25 unsigned int i; 26 const char *pStatement = "SHOW TABLES"; 27 mysql_library_init(0, NULL, NULL); 28 MYSQL *mysql = mysql_init(NULL); 29 unsigned int timeout = 3000; 30 mysql_options(mysql, MYSQL_OPT_CONNECT_TIMEOUT, &timeout); 31 32 if (mysql_real_connect(mysql, DBHOST, DBUSER, DBPASS, DBNAME, DBPORT, DBSOCK, DBPCNT) == NULL) 33 { 34 printf("connect failed: %s\n", mysql_error(mysql)); 35 mysql_close(mysql); 36 mysql_library_end(); 37 return 0; 38 } 39 40 printf("connect succssfully\n"); 41 42 if (0 != mysql_real_query(mysql, pStatement, strlen(pStatement))) 43 { 44 printf("query failed: %s\n", mysql_error(mysql)); 45 mysql_close(mysql); 46 mysql_library_end(); 47 return 0; 48 } 49 50 result = mysql_store_result(mysql); 51 52 if (result == NULL) 53 { 54 printf("fetch result failed: %s\n", mysql_error(mysql)); 55 mysql_close(mysql); 56 mysql_library_end(); 57 return 0; 58 } 59 60 num_fields = mysql_num_fields(result); 61 printf("numbers of result: %d\n", num_fields); 62 63 while (NULL != (field = mysql_fetch_field(result))) 64 { 65 printf("field name: %s\n", field->name); 66 } 67 68 while (NULL != (row = mysql_fetch_row(result))) 69 { 70 unsigned long *lengths; 71 lengths = mysql_fetch_lengths(result); 72 73 for (i = 0; i < num_fields; i++) 74 { 75 printf("{%.*s} ", (int) lengths[i], row[i] ? row[i] : "NULL"); 76 } 77 78 printf("\n"); 79 } 80 81 mysql_free_result(result); 82 mysql_close(mysql); 83 mysql_library_end(); 84 return 0; 85 }
-
执行
make single_thread_mysql_client LDFLAGS="-g -O2 -L/usr/lib64/mysql -lmysqlclient -lpthread -lz -lm -lssl -lcrypto -ldl"
即可获得对应单线程二进制。
多线程
- 多线程主要需要注意以下几点
mysql_library_init
和mysql_library_end
需要放入主线程;- 连接句柄需要多个才能加快并发,而连接句柄由
mysql_init
生成,而mysql_init
跟随机函数rand
有点相似,第一次需要初始化后才能线程安全,所以需要使用mysql_thread_init
和mysql_thread_end
两个函数来保证线程安全;
- 一般多线程连接mysql代码如下
1 /* 2 muti_thread_mysql_client.cpp 3 */ 4 #include <stdio.h> 5 #include <string.h> 6 #include <stdlib.h> 7 #include <mysql/mysql.h> 8 #include <pthread.h> 9 #include <unistd.h> 10 11 #define THREAD_NUM 4 12 #define DBHOST "localhost" 13 #define DBUSER "pca" 14 #define DBPASS "pca" 15 #define DBPORT 3306 16 #define DBNAME "dxponline" 17 #define DBSOCK NULL //"/var/lib/mysql/mysql.sock" 18 #define DBPCNT 0 19 20 typedef struct ThreadArgsST 21 { 22 int id; 23 pthread_t *thread_id; 24 } ThreadArgs; 25 26 void *func(void *arg) 27 { 28 ThreadArgs *args = (ThreadArgs *)arg; 29 MYSQL_RES *result; 30 MYSQL_ROW row; 31 MYSQL_FIELD *field; 32 unsigned int num_fields; 33 unsigned int i; 34 unsigned int timeout = 3000; 35 const char *pStatement = "SHOW TABLES"; 36 mysql_thread_init(); 37 MYSQL *mysql = mysql_init(NULL); 38 39 if (mysql == NULL) 40 { 41 printf("[%ld][%d]mysql init failed: %s\n", *args->thread_id, args->id, mysql_error(mysql)); 42 return (void *)0; 43 } 44 45 mysql_options(mysql, MYSQL_OPT_CONNECT_TIMEOUT, &timeout); 46 47 if (mysql_real_connect(mysql, DBHOST, DBUSER, DBPASS, DBNAME, DBPORT, DBSOCK, DBPCNT) == NULL) 48 { 49 printf("[%ld][%d]connect failed: %s\n", *args->thread_id, args->id, mysql_error(mysql)); 50 mysql_close(mysql); 51 mysql_thread_end(); 52 return (void *)0; 53 } 54 55 printf("[%ld][%d]connect succssfully\n", *args->thread_id, args->id); 56 57 if (0 != mysql_real_query(mysql, pStatement, strlen(pStatement))) 58 { 59 printf("[%ld][%d]query failed: %s\n", *args->thread_id, args->id, mysql_error(mysql)); 60 mysql_close(mysql); 61 mysql_thread_end(); 62 return (void *)0; 63 } 64 65 result = mysql_store_result(mysql); 66 67 if (result == NULL) 68 { 69 printf("[%ld][%d]fetch result failed: %s\n", *args->thread_id, args->id, mysql_error(mysql)); 70 mysql_close(mysql); 71 mysql_thread_end(); 72 return (void *)0; 73 } 74 75 num_fields = mysql_num_fields(result); 76 printf("[%ld][%d]numbers of result: %d\n", *args->thread_id, args->id, num_fields); 77 78 while (NULL != (field = mysql_fetch_field(result))) 79 { 80 printf("[%ld][%d]field name: %s\n", *args->thread_id, args->id, field->name); 81 } 82 83 while (NULL != (row = mysql_fetch_row(result))) 84 { 85 unsigned long *lengths; 86 lengths = mysql_fetch_lengths(result); 87 88 for (i = 0; i < num_fields; i++) 89 { 90 printf("[%ld][%d]{%.*s} ", *args->thread_id, args->id, (int) lengths[i], row[i] ? row[i] : "NULL"); 91 } 92 93 printf("\n"); 94 } 95 96 mysql_free_result(result); 97 mysql_close(mysql); 98 mysql_thread_end(); 99 return (void *)0; 100 } 101 102 int main(int argc, char *argv[]) 103 { 104 int thread_num; 105 106 if (argc == 2) 107 { 108 thread_num = atoi(argv[1]); 109 } 110 else 111 { 112 thread_num = THREAD_NUM; 113 } 114 115 mysql_library_init(0, NULL, NULL); 116 printf("argc: %d and thread_num: %d\n", argc, thread_num); 117 118 do 119 { 120 pthread_t *pTh = new pthread_t[thread_num]; 121 ThreadArgs *pArgs = new ThreadArgs[thread_num]; 122 int i; 123 124 for (i = 0; i < thread_num; i ++) 125 { 126 pArgs[i].id = i; 127 pArgs[i].thread_id = &pTh[i]; 128 129 if (0 != pthread_create(&pTh[i], NULL, func, &pArgs[i])) 130 { 131 printf("pthread_create failed\n"); 132 continue; 133 } 134 } 135 136 for (i = 0; i < thread_num; i ++) 137 { 138 pthread_join(pTh[i], NULL); 139 } 140 141 delete[] pTh; 142 delete[] pArgs; 143 } 144 while (0); 145 146 mysql_library_end(); 147 return 0; 148 }
-
执行
make muti_thread_mysql_client LDFLAGS="-g -O2 -L/usr/lib64/mysql -lmysqlclient -lpthread -lz -lm -lssl -lcrypto -ldl"
即可获得对应单线程二进制。
-
连接数与连接句柄是一一对应关系,故一般使用长连接,所以需要连接池,所以上面的代码可以有优化的空间,代码见:
1 /* 2 muti_thread_mysql_client_pool.cpp 3 */ 4 #include <stdio.h> 5 #include <string.h> 6 #include <stdlib.h> 7 #include <mysql/mysql.h> 8 #include <pthread.h> 9 #include <unistd.h> 10 #include <queue> 11 #include <string> 12 13 #define THREAD_NUM 4 14 #define DBHOST "localhost" 15 #define DBUSER "pca" 16 #define DBPASS "pca" 17 #define DBPORT 3306 18 #define DBNAME "dxponline" 19 #define DBSOCK NULL //"/var/lib/mysql/mysql.sock" 20 #define DBPCNT 0 21 22 using namespace std; 23 24 class CBlockQueue; 25 typedef struct ThreadArgsST 26 { 27 int id; 28 pthread_t *thread_id; 29 CBlockQueue *pQueue; 30 } ThreadArgs; 31 32 class CMutex 33 { 34 public: 35 CMutex() 36 { 37 pthread_mutex_init(&_mutex, NULL); 38 } 39 ~CMutex() 40 { 41 pthread_mutex_destroy(&_mutex); 42 } 43 44 int32_t lock() 45 { 46 return pthread_mutex_lock(&_mutex); 47 } 48 49 int32_t unlock() 50 { 51 return pthread_mutex_unlock(&_mutex); 52 } 53 54 int32_t trylock() 55 { 56 return pthread_mutex_trylock(&_mutex); 57 } 58 59 private: 60 pthread_mutex_t _mutex; 61 }; 62 63 class CGlobalFunction 64 { 65 public: 66 static MYSQL *connect() 67 { 68 unsigned int timeout = 3000; 69 mysql_thread_init(); 70 MYSQL *mysql = mysql_init(NULL); 71 72 if (mysql == NULL) 73 { 74 printf("mysql init failed: %s\n", mysql_error(mysql)); 75 return NULL; 76 } 77 78 mysql_options(mysql, MYSQL_OPT_CONNECT_TIMEOUT, &timeout); 79 80 if (mysql_real_connect(mysql, DBHOST, DBUSER, DBPASS, DBNAME, DBPORT, DBSOCK, DBPCNT) == NULL) 81 { 82 printf("connect failed: %s\n", mysql_error(mysql)); 83 mysql_close(mysql); 84 mysql_thread_end(); 85 return NULL; 86 } 87 88 printf("connect succssfully\n"); 89 return mysql; 90 } 91 }; 92 93 class CBlockQueue : public CMutex 94 { 95 public: 96 CBlockQueue() : _size(512) 97 { 98 } 99 ~CBlockQueue() 100 { 101 } 102 void set_size(int size) 103 { 104 _size = size; 105 } 106 int size() 107 { 108 this->lock(); 109 int size = q.size(); 110 this->unlock(); 111 return size; 112 } 113 bool push(void *m) 114 { 115 this->lock(); 116 // TODO 117 /* 118 if (q.size() > _size) 119 { 120 this->unlock(); 121 fprintf(stderr, "[QUEUE_IS_FULL]queue size over limit from push: %d\n", _size); 122 return false; 123 } 124 */ 125 q.push(m); 126 this->unlock(); 127 return true; 128 } 129 130 void *pop() 131 { 132 this->lock(); 133 134 if (q.empty()) 135 { 136 this->unlock(); 137 fprintf(stderr, "[QUEUE_IS_EMPTY]queue is no item from pop"); 138 return NULL; 139 } 140 141 void *m = q.front(); 142 q.pop(); 143 this->unlock(); 144 return m; 145 } 146 147 private: 148 queue q; 149 int _size; 150 }; 151 152 void *func(void *arg) 153 { 154 ThreadArgs *args = (ThreadArgs *)arg; 155 MYSQL_RES *result; 156 MYSQL_ROW row; 157 MYSQL_FIELD *field; 158 bool pushed = true; 159 unsigned int num_fields; 160 unsigned int i; 161 const char *pStatement = "SHOW TABLES"; 162 MYSQL *db = (MYSQL *)args->pQueue->pop(); 163 164 if (db == NULL) 165 { 166 db = CGlobalFunction::connect(); 167 168 if (db == NULL) 169 { 170 printf("[%ld][%d]mysql connect failed\n", *args->thread_id, args->id); 171 return (void *)0; 172 } 173 } 174 175 if (0 != mysql_real_query(db, pStatement, strlen(pStatement))) 176 { 177 printf("[%ld][%d]query failed: %s\n", *args->thread_id, args->id, mysql_error(db)); 178 args->pQueue->push(db); 179 return (void *)0; 180 } 181 182 result = mysql_store_result(db); 183 184 if (result == NULL) 185 { 186 printf("[%ld][%d]fetch result failed: %s\n", *args->thread_id, args->id, mysql_error(db)); 187 args->pQueue->push(db); 188 return (void *)0; 189 } 190 191 num_fields = mysql_num_fields(result); 192 printf("[%ld][%d]numbers of result: %d\n", *args->thread_id, args->id, num_fields); 193 194 while (NULL != (field = mysql_fetch_field(result))) 195 { 196 printf("[%ld][%d]field name: %s\n", *args->thread_id, args->id, field->name); 197 } 198 199 while (NULL != (row = mysql_fetch_row(result))) 200 { 201 unsigned long *lengths; 202 lengths = mysql_fetch_lengths(result); 203 204 for (i = 0; i < num_fields; i++) 205 { 206 printf("[%ld][%d]{%.*s} ", *args->thread_id, args->id, (int) lengths[i], row[i] ? row[i] : "NULL"); 207 } 208 209 printf("\n"); 210 } 211 212 mysql_free_result(result); 213 args->pQueue->push(db); 214 return (void *)0; 215 } 216 217 int main(int argc, char *argv[]) 218 { 219 CBlockQueue queue; 220 int thread_num; 221 222 if (argc == 2) 223 { 224 thread_num = atoi(argv[1]); 225 } 226 else 227 { 228 thread_num = THREAD_NUM; 229 } 230 231 mysql_library_init(0, NULL, NULL); 232 printf("argc: %d and thread_num: %d\n", argc, thread_num); 233 234 do 235 { 236 int i; 237 pthread_t *pTh = new pthread_t[thread_num]; 238 ThreadArgs *pArgs = new ThreadArgs[thread_num]; 239 240 for (i = 0; i < thread_num; i ++) 241 { 242 pArgs[i].id = i; 243 pArgs[i].thread_id = &pTh[i]; 244 pArgs[i].pQueue = &queue; 245 246 if (0 != pthread_create(&pTh[i], NULL, func, &pArgs[i])) 247 { 248 printf("pthread_create failed\n"); 249 continue; 250 } 251 } 252 253 for (i = 0; i < thread_num; i ++) 254 { 255 pthread_join(pTh[i], NULL); 256 } 257 258 delete[] pTh; 259 delete[] pArgs; 260 int qsize = queue.size(); 261 262 for (i = 0; i < qsize; i ++) 263 { 264 MYSQL *db = (MYSQL *)queue.pop(); 265 266 if (NULL != db) 267 { 268 mysql_close(db); 269 mysql_thread_end(); 270 } 271 } 272 } 273 while (0); 274 275 mysql_library_end(); 276 return 0; 277 }
-
执行
make muti_thread_mysql_client_pool LDFLAGS="-g -O2 -L/usr/lib64/mysql -lmysqlclient -lpthread -lz -lm -lssl -lcrypto -ldl"
即可获得对应单线程二进制。
-
上述代码就是利用队列来保持mysql连接,达到优化连接数。
总结
-
mysql连接与多线程处理不好,可能会造成很多问题,如
*MySQL Connection failed (#2058): This handle is already connected. Use a separate handle for each connection.*
Error in my_thread_global_end(): 1 threads didn't exit
- 甚至出现coredump
-
关于多线程连接mysql优化的思想,其实可以扩展到其他连接,如HTTP、Socket等连接中;
测试多线程连接Mysql:
1 int main() 2 { 3 mysql_library_init(0,NULL,NULL); 4 pthread_t producer; 5 pthread_t consumer_1; 6 pthread_t consumer_2; 7 //produce_fun不涉及数据库链接,不贴出来 8 pthread_create(&producer,NULL,produce_fun,NULL); 9 pthread_create(&consumer_1,NULL,consume_fun,NULL); 10 pthread_create(&consumer_2,NULL,consume_fun,NULL); 11 //下面的三句非常重要,main线程等待其他三个线程的结束,避免main线程执行到mysql_library_end时退出,而 12 //其他三个线程仍然在运行并报错的情形 13 pthread_join(producer,NULL); 14 pthread_join(consumer_1,NULL); 15 pthread_join(consumer_2,NULL); 16 mysql_library_end(); 17 return 0; 18 } 19 20 void addRecord_d(MYSQL *ptr_db,const char *t_name,int item){ 21 char query_buffer[100]; 22 sprintf(query_buffer,"insert into %s values(0,%d)",t_name,item); 23 int ret=mysql_query(ptr_db,query_buffer); 24 if(ret){ 25 fprintf(stderr,"%s%s\n","cannot add record to ",t_name); 26 return; 27 } 28 29 unsigned long long update_id=mysql_insert_id(ptr_db); 30 printf("add record (%llu,%d) ok.",update_id,item); 31 } 32 33 void * consume_fun(void *arg){ 34 MYSQL db; 35 MYSQL *ptr_db=mysql_init(&db); 36 mysql_real_connect(); 37 //蓝色部分可以改为其他任何带操作数据库语句的代码 38 //procedure 39 while(1){ 40 printf("consumer..."); 41 int item=consume(&p); 42 addRecord_d(ptr_db,"test",item); 43 } 44 mysql_thread_end(); 45 pthread_exit(NULL); 46 }
C/C++中调用api设置mysql连接的编码方式
mysql在C/C++中调用api设置连接mysql的编码方式有以下几种方法:
1. mysqli_set_charset
调用示例:
- ret = mysql_set_character_set(mysql, "utf8");
说明:
推荐使用的设置方法,与mysql的连接断开自动重连后仍能保持设置的编码格式,并且影响mysql_real_escape_string函数功能,使mysql_real_escape_string函数使用设置的编码格式转义字符串。
但该函数在mysql5.0.5版本以后才支持,故版本太低者...。
2. 执行sql语句:SET NAMES
调用示例:
- ret = mysql_real_query(mysql, "SET NAMES UTF8;",
- (unsigned long) strlen ("SET NAMES UTF8;"));
说明:
使用sql语句执行,只能影响当前与数据库的连接,断开自动重连后编码格式会重置为默认的配置。
3. 设置MYSQL_SET_CHARSET_NAME属性
调用示例:
- ret = mysql_options(mysql, MYSQL_SET_CHARSET_NAME, "utf8");
说明:
跟mysql_set_character_set类似,断开自动重连后仍能保持设置的编码格式,只是不会影响到mysql_real_escape_string函数。
并且在mysql5.0.5版本都可以使用,低版本可以使用该方法代替。
需要特别说明的是只有在调用mysql_real_connect连接数据库之前修改该属性才会生效。
mysql控制自动重连行为
当向mysql server发送statement时,mysql客户端发现连接失效后会自动尝试重新连接server。如果"自动重连"状态是enabled,客户端尝试连接server,并在连接成功后重新发送statement.
"自动重连“默认是disabled的。
如果应用程序需要知道连接是否可用(可以退出程序或显示相应的提示信息),确认“自动重连”是disabled。可以通过调用包含MYSQL_OPT_RECONNECT参数的mysql_options()函数以确认"自动重连"是否disabled:
my_bool reconnect = 0; mysql_options(&mysql, MYSQL_OPT_RECONNECT, &reconnect);
如果连接已经断开,mysql_ping()依赖“自动重连”的状态。如果“自动重连”是enabled,
mysql_ping()会重新获取连接。否则,会返回错误。
有些客户端程序可能提供了控制自动重连的功能。比如,mysql默认是允许重连的,但是使用 --skip-reconnect 选项将关闭这个行为。
如果发生了自动重连(比如,调用了mysql_ping()
),自动重连是透明的。为了检查是否发生了重连,在调用mysql_ping()函数前,先调用
mysql_thread_id()函数获取原始的连接id,然后调用
mysql_ping(),在调用
mysql_thread_id()函数,对比两次
mysql_thread_id()的结果是否变化了。
“自动重连”很方便,因为你不需要再实现自己重连代码,但是一旦发生了自动重连,connection的一些状态在server端就会被重置,而你的应用程序得不到这些变化的通知。
下面列出连接相关的状态变化:
-
任何未提交的事务将会回滚并且连接autocommit属性将会重置
-
事务中的锁将会被释放。
-
所有的临时表将会关闭(并释放)。
-
Session中的变量会重新初始化为系统默认值,包括在statements中显式声明的变量,比如
SET NAMES。
-
用户自定义的变量将会丢失。
-
Prepared statements将释放。
-
HANDLER
变量将会关闭。 -
LAST_INSERT_ID()的值将会重置为0.
-
通过
GET_LOCK()获取的锁将会释放。
如果连接失效,可能connection的session在server端依然在运行,当server没有检测到客户端失去连接。在这种情况下,原来connection的锁依然属于那个session,你可以调用mysql_kill()函数kill掉它。
Linux下C语言连接MySQL找不到mysql.h解决办法
昨天是把MySQL装上去了,今天尝试用C语言访问连接MySQL,发现使用编译指令:gcc -I/usr/include/mysql connect.c -lmysqlclient -o connect之后,提示找不到“mysql.h”文件,T_T|||。
原因其实很简单了,昨天我只装了server和client两个包,对于平常的使用,这已经足够了,但对于要在Linux下进行数据库的开发,则还需要安装devel开发包,并且该开发包的版本应该和我之前所安装的server和client包版本一致,否则会发生冲突而导致无法安装。
server和client包的安装见上一篇日志:Linux(Fedora 9)卸载自带的MySQL并安装MySQL的详细步骤。
好了,我们来安装devel包,注意版本要保持一致:
- [root@localhost install]# rpm -ivh MySQL-devel-community-5.1.46-1.rhel5.i386.rpm
- Preparing... ########################################### [100%]
- 1:MySQL-devel-community ########################################### [100%]
这次为了避免再出现问题,我把共享组件包shared也装上去了,哈哈:
- [root@localhost install]# rpm -ivh MySQL-shared-community-5.1.46-1.rhel5.i386.rpm
- Preparing... ########################################### [100%]
- 1:MySQL-shared-community ########################################### [100%]
装完之后,再编译就通过了,此时的“mysql.h”已经出现在了/usr/include/mysql目录下面。Happy~
undefined reference to `mysql_init'解决办法
写了一个很简单的测试数据库连接的程序conn.c如下:
1 #include <stdio.h> 2 #include <stdlib.h> 3 #include "/usr/local/mysql/include/mysql/mysql.h" 4 #include <string.h> 5 int main(int argc, char *argv[]) 6 { 7 MYSQL my_connection; 8 mysql_init(&my_connection); 9 if (mysql_real_connect(&my_connection, "localhost", "root", "","mysql",0,NULL,CLIENT_FOUND_ROWS)) 10 { 11 printf("Connection success\n"); 12 mysql_close(&my_connection); 13 } 14 else 15 { 16 fprintf(stderr, "Connection failed\n"); 17 if (mysql_errno(&my_connection)) 18 { 19 fprintf(stderr, "Connection error %d: %s\n",mysql_errno(&my_connection),mysql_error(&my_connection)); 20 } 21 } 22 return EXIT_SUCCESS; 23 }
gcc编译:gcc -o conn conn.c出现如下错误:
/tmp/ccY0JTdh.o(.text+0x1e): In function `main':
: undefined reference to `mysql_init'
/tmp/ccY0JTdh.o(.text+0x47): In function `main':
: undefined reference to `mysql_real_connect'
/tmp/ccY0JTdh.o(.text+0x6d): In function `main':
: undefined reference to `mysql_close'
/tmp/ccY0JTdh.o(.text+0x97): In function `main':
: undefined reference to `mysql_errno'
/tmp/ccY0JTdh.o(.text+0xad): In function `main':
: undefined reference to `mysql_error'
/tmp/ccY0JTdh.o(.text+0xc0): In function `main':
: undefined reference to `mysql_errno'
collect2: ld returned 1 exit status
出现该错误的原因是因为编译器找不到mysql_init,mysql_close等的具体实现.虽然我们包括了正确的头文件,但是 我们在编译的时候还是要连接确定的库.对于一些常用的函数的实现,gcc编译器会自动去连接一些常用库,这样我们就没有必要自己去指定了,如:printf函数.在本程序中要通过-L选项包含库文件的路径:
gcc -o conn conn.c -L /usr/local/mysql/lib/*.a -lz
通过,用*.a将库全部包含进来拉,其实只要包含mysqlclient.a就可以. -lz什么意思我也不清楚了
undefined reference to 'dlopen';undefined reference to 'dlclose';undefined reference to 'dlerror'等问题
在linux下,编译链接的时候,经常会遇到这样一个问题,undefined reference to.....,引起这个问题的原因在于在链接的时候缺少选项。下面举几个例子,并给出解决办法。
1、undefined reference to `dlerror'
undefined reference to `dlopen'
undefined reference to `dlerror';;解决方法:在makefile的 ldflags 后面把选项 -ldl添加上即可
2、undefined reference to `main';;解决方法:在makefile的 ldflags 后面添加 -nostartfiles 选项
3、undefined reference to `pthread_setspecific'
undefined reference to `pthread_key_delete'
undefined reference to `pthread_key_create'::解决办法:在makefile的LDFLAGS后面添加 -lpthread 选项。
最后::如果undefined reference to后面的内容是在自己的文件中声明或定义的东西,就不能用这种方法来解决了。这时就需要检查一下自己的makefile涉及到源文件、头文件的地方是否出错了,也有可能是其他的原因