这是官方网站找到的解释:http://www.sqlite.org/cvstrac/wiki?p=DatabaseIsLocked
下面我做了几个实验:
1、多个线程(pthread),使用同一个句柄(一次sqlite3_open,同一个数据库文件),在多个线程中同时使用此句柄,这些线程中有的是进行select操作的,有的是进行update操作的,使用的都是sqlite3_exec函数;结果是没有发现database is locked错误;
程序源码(mutitrhead.c ):
#include <sqlite3.h> #include <stdio.h> #include <pthread.h> #include <string.h> #include "main.h" #ifdef _WIN32 #include <windows.h> #define sleep(x) Sleep((x)*1000) #else #define sleep(x) sleep(x) #endif #define CREATE_SQL "CREATE TABLE IF NOT EXISTS \"mutiprocess\" (" "\"id\" INTEGER NOT NULL PRIMARY KEY," "\"src\" TEXT(1024)," "\"videotype\" INTEGER NOT NULL," "\"postfiledir\" TEXT(1024)" ");" #define REPLACE_SQL "REPLACE INTO mutiprocess (id,src,videotype,postfiledir) VALUES(NULL,%d,%d,%d)" #define UPDATE_SQL "UPDATE mutiprocess SET videotype = ‘2‘ WHERE id = 1" #define SELECT_SQL "SELECT * FROM mutiprocess" static void* select_cf(void *param) { int rc = -1; sqlite3 *db = (sqlite3*)param; char *szErrMsg; pthread_t tid; tid = pthread_self(); while(1){ rc = sqlite3_exec(db ,SELECT_SQL, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("tid %u -- select error.[%s] -- rc=[%d]\n", tid, szErrMsg, rc); if(strstr(szErrMsg, "database is locked")){ printf("<database is locked> when executing [%s], retry after 1 second.\n", SELECT_SQL); } }else{ printf("tid %u -- successfully execute [%s].\n", tid, SELECT_SQL); } sleep(1); } } static void* update_cf(void *param) { int rc = -1; sqlite3 *db = (sqlite3*)param; char *szErrMsg; pthread_t tid; tid = pthread_self(); while(1){ rc = sqlite3_exec(db ,UPDATE_SQL, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("tid %u -- update error.[%s] -- rc=[%d]\n", tid, szErrMsg, rc); if(strstr(szErrMsg, "database is locked")){ printf("<database is locked> when executing [%s], retry after 1 second.\n", UPDATE_SQL); } }else{ printf("tid %u -- successfully execute [%s].\n", tid, UPDATE_SQL); } sleep(1); } } int main() { pthread_t pid[20]; int rc; sqlite3 *db = 0; char *szErrMsg; int i = 0; char sql[1024] = {0}; rc = sqlite3_open("mutiprocess.db", &db); if (rc != SQLITE_OK) { printf("open sqlite3 error.\n"); } rc = sqlite3_exec(db ,CREATE_SQL, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("create db error-[%s]\n", szErrMsg); } for(i = 0; i < 4; i++){ sprintf(sql, REPLACE_SQL, i, i, i); rc = sqlite3_exec(db ,sql, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("replace db error-[%s]\n", szErrMsg); } } for(i = 0; i < 10; i++){ pthread_create(&(pid[i]), 0, select_cf, db); pthread_detach(pid[i]); } for(; i < 20; i++){ pthread_create(&(pid[i]), 0, update_cf, db); pthread_detach(pid[i]); } while(1){ sleep(-1); } return 0; }
编译:gcc -o mutitrhead mutitrhead.c -lsqlite3 -lpthread
运行:./mutithread
注意:只运行这一个进程没有发现任何database is locked的错误提示;如果在运行一个./mutithread进程,那么两个进程都会出现database is locked错误;如果kill掉其中一个进程,那么另外一个不再出现database is locked;
注意两个进程和一个进程的区别,一个是进程数不同,一个是使用的数据库句柄连接不同;上述程序虽然在一个进程中使用了多线程,但是多个线程都是使用了同一个数据库连接(使用一个sqlite3_open返回的句柄),不好区分是什么问题(其实官方网站有一句话很重要『SQLITE_BUSY means that another database connection (probably in another process) is using the database in a way that prevents you from using it』);这其实已经说明了产生database is locked的原因了,就是出在多个连接上,那么我们在做下面的实验;
2、同一个进程,启动多个线程,每个线程中都打开一个连接(connection,使用一个sqlite3_open返回的句柄),并且在其中做select或者update的操作;结果会出现database is locked错误;
程序源码(muticonnection.c):
#include <sqlite3.h> #include <stdio.h> #include <pthread.h> #include <string.h> #include "main.h" #ifdef _WIN32 #include <windows.h> #define sleep(x) Sleep((x)*1000) #else #define sleep(x) sleep(x) #endif #define CREATE_SQL "CREATE TABLE IF NOT EXISTS \"mutiprocess\" (" "\"id\" INTEGER NOT NULL PRIMARY KEY," "\"src\" TEXT(1024)," "\"videotype\" INTEGER NOT NULL," "\"postfiledir\" TEXT(1024)" ");" #define REPLACE_SQL "REPLACE INTO mutiprocess (id,src,videotype,postfiledir) VALUES(NULL,%d,%d,%d)" #define UPDATE_SQL "UPDATE mutiprocess SET videotype = ‘2‘ WHERE id = 1" #define SELECT_SQL "SELECT * FROM mutiprocess" static void* select_cf(void *param) { int rc; sqlite3 *db = 0; char *szErrMsg; int i = 0; char sql[1024] = {0}; pthread_t tid; rc = sqlite3_open("mutiprocess.db", &db); if (rc != SQLITE_OK) { printf("open sqlite3 error.\n"); } rc = sqlite3_exec(db ,CREATE_SQL, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("create db error-[%s]\n", szErrMsg); } for(i = 0; i < 4; i++){ sprintf(sql, REPLACE_SQL, i, i, i); rc = sqlite3_exec(db ,sql, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("replace db error-[%s]\n", szErrMsg); } } tid = pthread_self(); while(1){ rc = sqlite3_exec(db ,SELECT_SQL, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("tid %u -- select error.[%s] -- rc=[%d]\n", tid, szErrMsg, rc); if(strstr(szErrMsg, "database is locked")){ printf("<database is locked> when executing [%s], retry after 1 second.\n", SELECT_SQL); } }else{ printf("tid %u -- successfully execute [%s].\n", tid, SELECT_SQL); } sleep(1); } } static void* update_cf(void *param) { int rc; sqlite3 *db = 0; char *szErrMsg; int i = 0; char sql[1024] = {0}; pthread_t tid; rc = sqlite3_open("mutiprocess.db", &db); if (rc != SQLITE_OK) { printf("open sqlite3 error.\n"); } rc = sqlite3_exec(db ,CREATE_SQL, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("create db error-[%s]\n", szErrMsg); } for(i = 0; i < 4; i++){ sprintf(sql, REPLACE_SQL, i, i, i); rc = sqlite3_exec(db ,sql, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("replace db error-[%s]\n", szErrMsg); } } tid = pthread_self(); while(1){ rc = sqlite3_exec(db ,UPDATE_SQL, 0, 0, &szErrMsg); if (rc != SQLITE_OK) { printf("tid %u -- update error.[%s] -- rc=[%d]\n", tid, szErrMsg, rc); if(strstr(szErrMsg, "database is locked")){ printf("<database is locked> when executing [%s], retry after 1 second.\n", UPDATE_SQL); } }else{ printf("tid %u -- successfully execute [%s].\n", tid, UPDATE_SQL); } sleep(1); } } int main() { pthread_t pid[20]; int i = 0; for(i = 0; i < 10; i++){ pthread_create(&(pid[i]), 0, select_cf, 0); pthread_detach(pid[i]); } for(; i < 20; i++){ pthread_create(&(pid[i]), 0, update_cf, 0); pthread_detach(pid[i]); } while(1){ sleep(-1); } return 0; }
编译:gcc -o muticonnection muticonnection.c -lsqlite3 -lpthread
运行:./muticonnection
总结:看来出现这个错误是因为多数是因为使用了多个连接(多个sqlite3_open返回的句柄)导致的,这里我的说法是“多数”,因为还有别的原因,请见官方网站的解释:http://www.sqlite.org/cvstrac/wiki?p=DatabaseIsLocked
还有一点要注意:
database is locked是SQLITE_BUSY的错误,不要和SQLITE_LOCKED混淆;
SQLITE_LOCKED(6) means the source of contention is
internal and comes from the same database connection that received the SQLITE_LOCKED error.
SQLITE_BUSY(5) means that another database connection (probably in another process) is using the database in a way that prevents you from using it.
附:SQLITE_LOCKED(6)错误见:http://sqlite.1065341.n5.nabble.com/Begin-immediate-transaction-gt-SQLITE-BUSY-database-is-locked-td64878.html