1.前言
默认读者对SQL操作有一定的了解。
对于数值、字符串等数据的操作，用以下三个常用的接口就可以搞定。
#include "sqlite3.h"
// 打开数据库
SQLITE_API int sqlite3_open(
const char *filename, /* Database filename (UTF-8) */
sqlite3 **ppDb /* OUT: SQLite db handle */
);
// 执行SQL语句
SQLITE_API int sqlite3_exec(
sqlite3*, /* An open database */
const char *sql, /* SQL to be evaluated */
int (*callback)(void*,int,char**,char**), /* Callback function */
void *, /* 1st argument to callback */
char **errmsg /* Error msg written here */
);
// 关闭数据库
SQLITE_API int sqlite3_close(sqlite3*);
如果想操作图片等二进制数据呢？上面这三个接口肯定可能就不够用了，当然也可以考虑使用base64编码。
本文要介绍的是如何使用sqlite3操作二进制文件。
2.接口介绍
#include "sqlite3.h"
// SQL语句准备
SQLITE_API int sqlite3_prepare_v2(
sqlite3 *db, /* Database handle */
const char *zSql, /* SQL statement, UTF-8 encoded */
int nByte, /* Maximum length of zSql in bytes. */
sqlite3_stmt **ppStmt, /* OUT: Statement handle */
const char **pzTail /* OUT: Pointer to unused portion of zSql */
);
// 绑定二进制数据
SQLITE_API int sqlite3_bind_blob(sqlite3_stmt*, int, const void*, int n, void(*)(void*));
// 执行
SQLITE_API int sqlite3_step(sqlite3_stmt*);
// 释放
SQLITE_API int sqlite3_finalize(sqlite3_stmt *pStmt);
3.demo
将一张图片保存到数据库中，再读取数据库中的图片，存到另一个文件中。
git地址: https://gitee.com/yu-jiahang/sqlite3
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stddef.h>
#include <stdarg.h>
#include "sqlite3.h"
// 带文件名与行号log输出
#define LOG(...) printf("%s-%d: ",__FILE__, __LINE__);printf(__VA_ARGS__)
// 查询回调函数
int callback(void *data, int argc, char **argv, char **azColName);
int main(int argc, char* argv[])
{
sqlite3_stmt * stmt;
sqlite3 *db;
char *zErrMsg = 0; //记录返回的错误信息
char *sql_str; //存sql语句
char SQL[100] = {
0};
int sql_data = 0; //其作为回调函数的第一个参数，可在回调函数中改变其值。
int res;
int fd;
off_t fd_len;
void *data_p;
int data_len;
void *column_p;
int column_len;
// 打开文件
fd = open("test.jpg", O_RDONLY);
if(fd == -1)
{
LOG("Fail to open file.\r\n");
exit(EXIT_FAILURE);
}
// 获取文件大小
fd_len = lseek(fd, 0, SEEK_END);
LOG("The file size is %ld byte.\r\n", fd_len);
// 读取文件到内存 pread为指定光标位置读取 不改变当前光标位置
data_p = malloc(fd_len);
data_len = pread(fd, data_p, fd_len, 0);
if (data_len != fd_len)
{
LOG("Fail to read file.\r\n");
exit(EXIT_FAILURE);
}
close(fd);
// 打开数据库
res = sqlite3_open("test.db", &db);
if( res != SQLITE_OK )
{
LOG("Fail to open database.\r\n");
exit(EXIT_FAILURE);
}
// 查询表是否存在
sql_str = "SELECT COUNT(*) FROM sqlite_master WHERE name=='my_table';";
res = sqlite3_exec(db, sql_str, callback, &sql_data, &zErrMsg);
if( res != SQLITE_OK )
{
LOG("Select failed: %s.\r\n", zErrMsg);
sqlite3_free(zErrMsg);
}
LOG("The number of tables is %d.\r\n", sql_data);
// 表中无数据则创建
if(sql_data == 0)
{
sql_str = "create table my_table( \
key int primary key not null, \
data blob not null);";
res = sqlite3_exec(db, sql_str, callback, &sql_data, &zErrMsg);
if( res != SQLITE_OK )
{
LOG("Fail to create table: %s.\r\n", zErrMsg);
sqlite3_free(zErrMsg);
}
/****************************************写二进制值****************************************/
memset(SQL, 0, 100);
sprintf(SQL, "insert into my_table(key, data) values(1,?)");
// 把sql语句解析到stmt中去
res = sqlite3_prepare_v2( db, SQL, -1, &stmt, NULL);
if( res != SQLITE_OK )
{
LOG("Stmt preparation failed.\r\n");
}
// 绑定二进制数据
res = sqlite3_bind_blob( stmt, 1, data_p, data_len, NULL);
if( res != SQLITE_OK )
{
LOG("Stmt binding failed.\r\n");
}
// 写入数据库
res = sqlite3_step(stmt);
if( res != SQLITE_DONE )
{
LOG("Stmt executed failed.\r\n");
}
// 释放资源
free(data_p);
res = sqlite3_finalize(stmt);
if( res != SQLITE_OK )
{
LOG("Stmt released failed.\r\n");
}
}
/****************************************读二进制值****************************************/
res = sqlite3_prepare_v2( db, "select * from my_table", -1, &stmt, NULL);
if( res != SQLITE_OK )
{
LOG("Stmt preparation failed.\r\n");
}
// 读数据 每循环一遍就是获取一行数据
while(1)
{
res = sqlite3_step(stmt);
if( res == SQLITE_DONE )
{
LOG("Stmt executed successfully.\r\n");
break;
}
else if( res != SQLITE_ROW )
{
LOG("Stmt executed failed.\r\n");
break;
}
else
{
// 获取第1列的文件大小
column_len = sqlite3_column_bytes(stmt, 1);
// 获取第1列内容的首地址 不需要释放内存
column_p = (char *)sqlite3_column_blob(stmt, 1);
// 创建并写入文件
fd = open("test2.jpg", O_RDWR|O_CREAT|O_TRUNC, S_IRWXU);
if(fd == -1)
{
LOG("Fail to open file.\r\n");
exit(EXIT_FAILURE);
}
fd_len = write(fd, column_p, column_len);
if (fd_len != column_len)
{
LOG("Fail to write file.\r\n");
exit(EXIT_FAILURE);
}
close(fd);
}
}
res = sqlite3_finalize(stmt);
if( res != SQLITE_OK )
{
LOG("Stmt released failed.\r\n");
}
sqlite3_close(db);
return 0;
}
// 回调函数 打印查询的信息
int callback(void *data, int argc, char **argv, char **azColName)
{
int i;
*(int *)data = atoi(argv[0]);
for(i = 0; i < argc; i++)
{
printf("%s = %s\n", azColName[i], argv[i] ? argv[i] : "NULL");
}
printf("\n");
return 0;
}