Linux下std::ifstream成员函数对应系统调用验证

news/2024/11/30 0:31:31/

最近在分析离线数据使用时的bug，发现代码中对std::ifstream成员函数使用存在疑问，所以就写了个简单测试程序来分析std::ifstream成员函数对应那些系统调用。

目录

1.gcount

2.seekg和tellg

3.read

代码如下：

test.cpp

#include <iostream>
#include <fstream>
#include <atomic>
#include <string>using namespace std;/*
1.fseek函数：C语言标准库函数
作用：设置文件读写位置
函数原型：int fseek(FILE *fp, LONG offset, int origin);
参数含义：fp 文件指针 offset 相对于origin规定的偏移位置量 origin 指针移动的起始位置，取值如下： SEEK_SET 文件开始位置 SEEK_CUR 文件当前位置 SEEK_END 文件结束位置
返回值：成功，返回0，失败返回非0值，并设置error的值，可以用perror()函数输出错误。2.ftell函数：C语言标准库函数
作用：用于得到文件位置指针当前位置相对于文件首的偏移字节数
原型：
#include <stdio.h>
long ftell(FILE *stream);
获取文件大小：首先将文件的当前位置移到文件的末尾，然后调用函数ftell()获得当前位置相对于文件首的位移，该位移值等于文件所含字节数。
fseek(fp, 0L,SEEK_END); 
len = ftell(fp); 3.lseek函数：linux下系统调用
lseek: 用于改变一个文件读写指针位置的一个系统调用。指针位置可以是绝对的或者相对的。和fseek函数类似，但lseek返回的是一个off_t数值
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
offset为正则向文件末尾移动（向前移），为负数则向文件头部（向后移）。
返回值：
正确时返回文件读写指针距文件开头的字节大小，出错返回-1
lseek的作用是打开文件下一次读写的开始位置，因此还有以下两个作用
(1)拓展文件，不过一定要一次写的操作。迅雷等下载工具在下载文件时候先扩展一个空间然后再下载的。
int fd = open("test.txt",O_RDWR);
lseek(fd,0x1000, SEEK_SET);
write(fd, "a", 1);
close(fd);(2)获取文件大小。
int fd = open("test.txt", O_RDWR);
printf("file size:%d \n", lseek(fd, 0, SEEK_END));
close(fd);总结：lseek 功能包含 ftell + fseek*//*
C++中seekp() seekg() tellp() tellg() 
seekp：设置输出文件流的文件流指针位置
tellp:：获取输出文件流的文件流指针位置
seekg：设置输入文件流的文件流指针位置
tellg：获取输入文件流的文件流指针位置
函数原型：
ostream& seekp( streampos pos );
ostream& seekp( streamoff off, ios::seek_dir dir );
istream& seekg( streampos pos );
istream& seekg( streamoff off, ios::seek_dir dir );
ostream& tellp() 
istream& tellg() 
参数含义：
pos：新的文件流指针位置值
off：需要偏移的值
dir：搜索的起始位置
dir参数用于对文件流指针的定位操作上，代表搜索的起始位置
在ios中定义的枚举类型：enum seek_dir {beg, cur, end};含义如下：ios::beg：文件流的起始位置ios::cur：文件流的当前位置ios::end：文件流的结束位置gcount(): 返回在对象上执行的最后一个未格式化的输入操作中提取的字符数，不涉及调用系统api
获取文件大小：
seekg(0, std::ios::end);
tellg();
*//*
iostream类评估流状态的4个功能函数：good()，bad()，fail()，和eof()，具体功能如下：
good()：流没有遇到错误。
bad()：数据流已遇到效应的流（即存储器分配失败，没有缓流等）。
fail()：的完整性的错误：通常可恢复的错误（格式化/解析失败）。
eof()：已达到文件结尾（EOF）字符。
*/static std::atomic<int> count_num(0);#define DB_IsBigEnd_FromRPHeaderAttr( HeaderAttr ) \((bool) (HeaderAttr & 0X00000001))//typedef unsigned int uint_t;
using uint_t = unsigned int;
typedef unsigned char Byte;
typedef unsigned short	u_short;#define DB_FORMAT_VER_MAX_LENGTH	(16)#define DB_DATA_VER_MAX_LENGTH		(16)#define DB_AUTHOR_MAX_LENGTH		(8)#define DB_DATE_MAX_LENGTH			(8)/* 小端到大端的变换宏 */
#define _V_GET_LITTLE_SHORT( x )	(short)( ( ( *((Byte*)x+1) ) << 8 ) | (*((Byte*)x)) )
#define _V_GET_LITTLE_LONG( x )		(int)( ( ( *((Byte*)x+3) ) << 24 ) | (( *((Byte*)x+2) ) << 16 ) \| (( *((Byte*)x+1) ) << 8 ) | ( *((Byte*)x) ) )
#define _V_GET_LITTLE_FLOAT( x ) (*((float *)(x)))#define _V_GET_BIGENDDIAN_SHORT( x ) (short)( ( ( *((Byte*)x) ) << 8 ) | (*((Byte*)x+1)) )
#define _V_GET_BIGENDDIAN_LONG( x )	(int)( ( ( *((Byte*)x) ) << 24 ) | (( *((Byte*)x+1) ) << 16 ) \| (( *((Byte*)x+2) ) << 8 ) |  ( *((Byte*)x+3) ) )typedef struct _DB_Header_t
{uint_t		unAttr;									uint_t		unHeaderSize;							uint_t		unFrameSize;							char		cFormatVer[DB_FORMAT_VER_MAX_LENGTH];	char		cDataVer[DB_DATA_VER_MAX_LENGTH];		char		cAuthor[DB_AUTHOR_MAX_LENGTH];		char		cDate[DB_DATE_MAX_LENGTH];			uint_t		unCalcRegionMaxSize;					uint_t		unInfoRegionMaxSize;					uint_t		unRPLinkShapePointMaxCnt;				uint_t		unNameMaxLength;						uint_t		unNameFrameOffset;						uint_t		unDistrictFrameOffset;					uint_t		unLevelCnt;								uint_t		unLevelRecordSize;						uint_t		pstLevelTable;							
} DB_Header_t ;typedef struct _DB_Level_t
{u_int	unAttr;							u_int	unCalcRegionManagerHeaderOffset;u_int	unUnCompressedCalcRegionManagerHeaderOffset;u_int	unCalcRegionManagerHeaderSize;	u_int	unInfoRegionManagerHeaderOffset;u_int	unInfoRegionManagerHeaderSize;	u_int	unAdjacentNodeManagerHeaderOffset;u_int	unAdjacentNodeManagerHeaderSize;
} DB_Level_t;typedef struct _DB_NameFrame_t
{u_int		unFrameSize;	u_int		unStructSize;	u_int		unNameIdxCnt;	u_int		unNameIdxSize;	u_int		punIdxTable;	u_int		pusNameTable;	
} DB_NameFrame_t;typedef struct _DB_DistrictFrame_t
{u_int					unFrameSize;		u_int					unStructSize;		u_int					unDistrictCnt;		u_int					unDistrictSize;		u_int					pstDistrictTable;	
} DB_DistrictFrame_t;typedef struct _DB_AbsoluteNodeID_t
{u_int 	unNodeID ;			u_int 	unNodeExtendID ;
} DB_AbsoluteNodeID_t;typedef struct _DB_AbsoluteLinkID_t
{u_int	unLinkIDPart1;	 u_int 	unLinkIDPart2;	 u_int 	unLinkAttr;	 	 	 
} DB_AbsoluteLinkID_t;typedef struct _DB_AdjacentNode_t
{u_int					nLongitude ;u_int					nLatitude ;DB_AbsoluteNodeID_t	stNodeID ;DB_AbsoluteLinkID_t	stLinkID ;bool					bIsStartNode ;
} DB_AdjacentNode_t ;typedef struct _DB_AdjacentNodeMgr_t
{u_int					unSize ;u_int					unStructSize ;u_int					unCnt ;u_int					unRecordSize;	u_int					pstAdjNodes;
} DB_AdjacentNodeMgr_t ;typedef struct _DB_CalcRegion_ManagerHeader_t
{u_int					unFrameSize;		u_int					unRankInfo;			u_int					unRegionCnt;		u_int					unRegionIdxSize;	u_int					pstRegionIdxTable;	
} DB_CalcRegion_ManagerHeader_t;typedef struct _DB_DistrictRecord_t
{u_short		usCityNameIdx;	u_short		usCountyNameIdx;
} DB_DistrictRecord_t;typedef struct _DB_CalcRegion_Idx_t
{u_short	unRegionOffset;					u_short	unUnCompressedRegionOffset ;	u_short	unRegionSize;					u_short	unCompressedRegionSize ;		
} DB_CalcRegion_Idx_t;//多个对象指向同一个文件操作
class DBFile
{
public:DBFile(std::ifstream *DBFileHandle);~DBFile();int32_t seek_cur_to_end_size(std::ifstream *m_pDBFileHandle, int32_t start_seek_pos);static int32_t seek_cur_to_end_size(std::ifstream& pFile, int32_t start_seek_pos);private:std::ifstream *m_pDBFileHandle;
};DBFile::DBFile(std::ifstream *DBFileHandle)
{if(DBFileHandle) {count_num++;m_pDBFileHandle = DBFileHandle;}
}DBFile::~DBFile()
{if(m_pDBFileHandle) {count_num--;if(count_num <= 0) {count_num = 0;}if (count_num <= 0 && m_pDBFileHandle->good() && m_pDBFileHandle->is_open()) {m_pDBFileHandle->close();}}
}int32_t DBFile::seek_cur_to_end_size(std::ifstream *m_pDBFileHandle, int32_t start_seek_pos)
{if(!m_pDBFileHandle) {return 0;}m_pDBFileHandle->seekg(0, std::ios::end);if(!m_pDBFileHandle->good()) {return 0;}int32_t seekg_end_size = m_pDBFileHandle->tellg();m_pDBFileHandle->seekg(start_seek_pos, std::ios::beg);if(!m_pDBFileHandle->good()) {return 0;}int32_t seekg_cur_size = m_pDBFileHandle->tellg();return seekg_end_size - seekg_cur_size;
}int32_t DBFile::seek_cur_to_end_size(std::ifstream& pFile, int32_t start_seek_pos)
{std::cout << 11111 << std::endl;pFile.seekg(0, std::ios::end);if(!pFile.good()) {return 0;}std::cout << 22222 << std::endl;int32_t seekg_end_size = pFile.tellg();std::cout << 33333 << std::endl;pFile.seekg(start_seek_pos, std::ios::beg);std::cout << 44444 << std::endl;if(!pFile.good()) {return 0;}std::cout << 55555 << std::endl;int32_t seekg_cur_size = pFile.tellg();std::cout << 666666 << std::endl;return seekg_end_size - seekg_cur_size;
}class DBParser
{
public:DBParser() ;virtual ~DBParser();int Init (const std::string DBPathFileName);int GetHeaderSize (u_int* punHeaderSize);int GetHeader (u_int unHeaderSize, DB_Header_t* pstHeaderInfo);int GetNameFrameSize (u_int* punNameFrameSize);int GetNameFrame (u_int unOffset,u_int unFrameSize,DB_NameFrame_t* pstNameFrame);int GetDistrictFrameSize (u_int* punDistrictFrameSize);int GetDistrictFrame (u_int unOffset,u_int unFrameSize,DB_DistrictFrame_t* pstDistrictFrame);int GetAdjacentNodeManager (u_int unMgrOffset,u_int unMgrSize,DB_AdjacentNodeMgr_t* pstAdjNodeMgr);int GetCalcRegionManagerHeader( u_int unHeaderOffset,u_int unHeaderSize,DB_CalcRegion_ManagerHeader_t* pstManagerHeader);private:std::ifstream		m_hDBFileHandle ;bool		        m_bIsBigEnd ;std::string         m_rp_db_file_path;std::atomic_bool	m_rp_db_file_install;
};DBParser::DBParser() {m_rp_db_file_path = "";m_rp_db_file_install = false;
}DBParser::~DBParser() {m_rp_db_file_install = false;if (m_hDBFileHandle.is_open()) {m_hDBFileHandle.close() ;}
}int DBParser::Init (const std::string DBPathFileName)
{m_hDBFileHandle.open(DBPathFileName , std::ios::in|std::ios::binary);if (!m_hDBFileHandle.is_open()) {return -1;}DBFile dbFile(&m_hDBFileHandle);unsigned int unHeaderAttr = 0;m_hDBFileHandle.read((char*)&unHeaderAttr, sizeof(unHeaderAttr)) ;if (!m_hDBFileHandle.good()) {return -2;}std::cout << "gcount = " << m_hDBFileHandle.gcount() << std::endl;m_bIsBigEnd = DB_IsBigEnd_FromRPHeaderAttr(unHeaderAttr) ;unsigned int unHeaderSize = 0;std::cout << 99999 << std::endl;m_hDBFileHandle.read((char*)&unHeaderSize, sizeof(unHeaderSize));std::cout << 101010 << std::endl;if (m_hDBFileHandle.gcount() != sizeof(unHeaderSize)) {return -3;}std::cout << "gcount = " << m_hDBFileHandle.gcount() << std::endl;if (unHeaderSize > 1024 * 1024) // header size 不应该大于 1M{return -4;}DB_Header_t* header = new DB_Header_t; // malloc(unHeaderSize);if (NULL == header) {return -5;}m_hDBFileHandle.seekg(0, std::ios::beg); //seek to beginm_hDBFileHandle.read((char*)header, unHeaderSize);if (m_hDBFileHandle.gcount() != unHeaderSize) {free(header);return -3;}m_hDBFileHandle.seekg(0, std::ios::end);ulong fileSize = m_hDBFileHandle.tellg();u_int frameSize = header->unFrameSize;free(header);if (frameSize != (u_int)fileSize) {return -6;}m_rp_db_file_path = DBPathFileName;m_rp_db_file_install = true;m_hDBFileHandle.close();return 0;
}int DBParser::GetHeaderSize (u_int* punHeaderSize)
{if (punHeaderSize == NULL) {return -1;}if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()){*punHeaderSize = 0;return -2 ;}int size = DBFile::seek_cur_to_end_size(m_hDBFileHandle, 4);if (!m_hDBFileHandle.good()) {return -3;}if(size < sizeof(*punHeaderSize)){return -4;}std::cout << 77777 << std::endl;m_hDBFileHandle.read ((char*)punHeaderSize, sizeof (*punHeaderSize)) ;std::cout << "gcount = " << m_hDBFileHandle.gcount() << std::endl;std::cout << 88888 << std::endl;if (!m_hDBFileHandle.good()) {return false;}if (m_bIsBigEnd){*punHeaderSize = _V_GET_LITTLE_LONG(punHeaderSize) ;}return 0;
}int DBParser::GetHeader (u_int unHeaderSize,DB_Header_t* pstHeaderInfo)
{u_int	i = 0;Byte*	pbtHeaderInfo = (Byte*)pstHeaderInfo;DB_Level_t *pstLevelTable;if (unHeaderSize == 0 || pstHeaderInfo == NULL) {return -1;}if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()) {return -2;}m_hDBFileHandle.seekg(0, std::ios::beg);if (!m_hDBFileHandle.good()) {return -3;}/*char tmpBuf[9016] = {0};m_hDBFileHandle.read(tmpBuf, unHeaderSize);*/std::cout << "begin gcount  gcount" << std::endl;m_hDBFileHandle.read((char*)pbtHeaderInfo, unHeaderSize);std::cout << "gcount = " << m_hDBFileHandle.gcount() << std::endl;std::cout << "begin gcount  gcount" << std::endl;if (!m_hDBFileHandle.good()) {return -3;} if (m_bIsBigEnd){pstHeaderInfo->unAttr = _V_GET_LITTLE_LONG (&pstHeaderInfo->unAttr) ;pstHeaderInfo->unHeaderSize = _V_GET_LITTLE_LONG (&pstHeaderInfo->unHeaderSize) ;pstHeaderInfo->unFrameSize = _V_GET_LITTLE_LONG (&pstHeaderInfo->unFrameSize) ;pstHeaderInfo->unCalcRegionMaxSize = _V_GET_LITTLE_LONG (&pstHeaderInfo->unCalcRegionMaxSize) ;pstHeaderInfo->unInfoRegionMaxSize = _V_GET_LITTLE_LONG (&pstHeaderInfo->unInfoRegionMaxSize) ;pstHeaderInfo->unRPLinkShapePointMaxCnt = _V_GET_LITTLE_LONG (&pstHeaderInfo->unRPLinkShapePointMaxCnt) ;pstHeaderInfo->unNameMaxLength = _V_GET_LITTLE_LONG (&pstHeaderInfo->unNameMaxLength) ;pstHeaderInfo->unNameFrameOffset = _V_GET_LITTLE_LONG (&pstHeaderInfo->unNameFrameOffset) ;pstHeaderInfo->unDistrictFrameOffset = _V_GET_LITTLE_LONG (&pstHeaderInfo->unDistrictFrameOffset) ;pstHeaderInfo->unLevelCnt = _V_GET_LITTLE_LONG (&pstHeaderInfo->unLevelCnt) ;pstHeaderInfo->unLevelRecordSize = _V_GET_LITTLE_LONG (&pstHeaderInfo->unLevelRecordSize) ;pstHeaderInfo->pstLevelTable = _V_GET_LITTLE_LONG (&pstHeaderInfo->pstLevelTable) ;}pstLevelTable = (DB_Level_t*)(pbtHeaderInfo + (u_int)(pstHeaderInfo->pstLevelTable));if(pstLevelTable == NULL) {return -4;}if (m_bIsBigEnd){for (i = 0; i < pstHeaderInfo->unLevelCnt ; i++){pstLevelTable->unAttr =_V_GET_LITTLE_LONG (&pstLevelTable->unAttr) ;pstLevelTable->unCalcRegionManagerHeaderOffset =_V_GET_LITTLE_LONG (&pstLevelTable->unCalcRegionManagerHeaderOffset) ;pstLevelTable->unUnCompressedCalcRegionManagerHeaderOffset =_V_GET_LITTLE_LONG (&pstLevelTable->unUnCompressedCalcRegionManagerHeaderOffset) ;pstLevelTable->unCalcRegionManagerHeaderSize =_V_GET_LITTLE_LONG (&pstLevelTable->unCalcRegionManagerHeaderSize) ;pstLevelTable->unInfoRegionManagerHeaderOffset =_V_GET_LITTLE_LONG (&pstLevelTable->unInfoRegionManagerHeaderOffset) ;pstLevelTable->unInfoRegionManagerHeaderSize =_V_GET_LITTLE_LONG (&pstLevelTable->unInfoRegionManagerHeaderSize) ;pstLevelTable->unAdjacentNodeManagerHeaderOffset =_V_GET_LITTLE_LONG (&pstLevelTable->unAdjacentNodeManagerHeaderOffset) ;pstLevelTable->unAdjacentNodeManagerHeaderSize =_V_GET_LITTLE_LONG (&pstLevelTable->unAdjacentNodeManagerHeaderSize) ;pstLevelTable = (DB_Level_t*)((Byte*)pstLevelTable + pstHeaderInfo->unLevelRecordSize );}}return 0;
}int DBParser::GetNameFrameSize(u_int* punNameFrameSize)
{DB_Header_t stHeaderInfo ;DB_NameFrame_t stNameFrame ;if (punNameFrameSize == 0) {return -1;}if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()) {return -1;}m_hDBFileHandle.seekg (0, std::ios::beg);if (!m_hDBFileHandle.good()) {return -2;}m_hDBFileHandle.read((char*)&stHeaderInfo, sizeof(stHeaderInfo)) ;if (stHeaderInfo.unNameFrameOffset == 0) {return -3;}if (m_bIsBigEnd){stHeaderInfo.unNameFrameOffset = _V_GET_LITTLE_LONG (&stHeaderInfo.unNameFrameOffset) ;}m_hDBFileHandle.seekg(stHeaderInfo.unNameFrameOffset, std::ios::beg);if (!m_hDBFileHandle.good()) {return -2;}m_hDBFileHandle.read((char*)&stNameFrame.unFrameSize, sizeof(stNameFrame.unFrameSize)) ;if (stNameFrame.unFrameSize == 0) {return -3;}*punNameFrameSize = stNameFrame.unFrameSize ;if (m_bIsBigEnd){*punNameFrameSize = _V_GET_LITTLE_LONG (punNameFrameSize) ;}return 0;
}int DBParser::GetNameFrame (u_int unOffset,u_int unFrameSize,DB_NameFrame_t* pstNameFrame)
{u_int	unTempOffset = 0;u_int	unWordsCnt ;Byte*	pbyNameFrame = (Byte*)pstNameFrame ;u_int	*punIdxTable;u_short	*pusNameTable;if (unOffset == 0 || unFrameSize == 0 || pstNameFrame == NULL) {return -1;}if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()) {return -2;}m_hDBFileHandle.seekg (unOffset, std::ios::beg);if (!m_hDBFileHandle.good()) {return -3;}m_hDBFileHandle.read ((char*)pbyNameFrame, unFrameSize) ;if (pstNameFrame->unFrameSize == 0 ||pstNameFrame->unNameIdxCnt == 0) {return -4;}if (m_bIsBigEnd){pstNameFrame->unFrameSize = _V_GET_LITTLE_LONG (&pstNameFrame->unFrameSize) ;pstNameFrame->unStructSize = _V_GET_LITTLE_LONG (&pstNameFrame->unStructSize) ;pstNameFrame->unNameIdxCnt = _V_GET_LITTLE_LONG (&pstNameFrame->unNameIdxCnt) ;pstNameFrame->unNameIdxSize = _V_GET_LITTLE_LONG (&pstNameFrame->unNameIdxSize) ;}unTempOffset = pstNameFrame->unStructSize;if (unTempOffset == 0 || unTempOffset % 4 != 0|| unTempOffset != (u_int)pstNameFrame->punIdxTable) {return -4;}punIdxTable = (u_int*)(pbyNameFrame + (u_int)pstNameFrame->punIdxTable) ;if (m_bIsBigEnd){for ( u_int i = 0; i < pstNameFrame->unNameIdxCnt; ++i ){*punIdxTable = _V_GET_LITTLE_LONG(punIdxTable);punIdxTable = (u_int*)( (Byte*)punIdxTable + pstNameFrame->unNameIdxSize );}}unTempOffset += sizeof(u_int) * pstNameFrame->unNameIdxCnt ;unWordsCnt = unFrameSize - unTempOffset ;if (unWordsCnt == 0 || unWordsCnt % sizeof(u_short) != 0) {return -4;}unWordsCnt /= sizeof(u_short) ;pusNameTable = (u_short*)(pbyNameFrame + (u_int)pstNameFrame->pusNameTable);if (m_bIsBigEnd){for ( u_int i = 0; i < unWordsCnt; ++i ){*pusNameTable = _V_GET_LITTLE_SHORT( pusNameTable );++pusNameTable;}}return 0;
}int DBParser::GetDistrictFrameSize (u_int* punDistrictFrameSize)
{DB_Header_t stHeaderInfo ;DB_DistrictFrame_t stDistrictFrame ;if (punDistrictFrameSize == 0) {return -1;}if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()) {return -4;}m_hDBFileHandle.seekg (0, std::ios::beg);if (!m_hDBFileHandle.good()) {return -4;}m_hDBFileHandle.read ((char*)&stHeaderInfo, sizeof(stHeaderInfo));if (stHeaderInfo.unDistrictFrameOffset == 0) {return -4;}if (m_bIsBigEnd){stHeaderInfo.unDistrictFrameOffset = _V_GET_LITTLE_LONG (&stHeaderInfo.unDistrictFrameOffset) ;}m_hDBFileHandle.seekg (stHeaderInfo.unDistrictFrameOffset, std::ios::beg);if (!m_hDBFileHandle.good()) {return -4;}m_hDBFileHandle.read ((char*)&stDistrictFrame.unFrameSize, sizeof(stDistrictFrame.unFrameSize)) ;if (stDistrictFrame.unFrameSize == 0) {return -4;}*punDistrictFrameSize = stDistrictFrame.unFrameSize;if (m_bIsBigEnd){*punDistrictFrameSize = _V_GET_LITTLE_LONG (punDistrictFrameSize) ;}return 0;
}int DBParser::GetDistrictFrame (u_int unOffset,u_int unFrameSize,DB_DistrictFrame_t* pstDistrictFrame)
{DB_DistrictRecord_t *pstDistrictTable;Byte*	pbyDistrictFrame = (Byte*)pstDistrictFrame ;if (unOffset == 0 || unFrameSize == 0 || pstDistrictFrame == NULL) {return  -1; } if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()) {return  -4;}m_hDBFileHandle.seekg (unOffset, std::ios::beg);if (!m_hDBFileHandle.good()) {return  -4;}m_hDBFileHandle.read ((char*)pbyDistrictFrame, unFrameSize) ;if (pstDistrictFrame->unFrameSize == 0 ||pstDistrictFrame->unDistrictCnt == 0) {return  -4;}if (m_bIsBigEnd){pstDistrictFrame->unFrameSize = _V_GET_LITTLE_LONG (&pstDistrictFrame->unFrameSize) ;pstDistrictFrame->unStructSize = _V_GET_LITTLE_LONG (&pstDistrictFrame->unStructSize) ;pstDistrictFrame->unDistrictCnt = _V_GET_LITTLE_LONG (&pstDistrictFrame->unDistrictCnt) ;pstDistrictFrame->unDistrictSize = _V_GET_LITTLE_LONG (&pstDistrictFrame->unDistrictSize) ;}if( pstDistrictFrame->unStructSize == 0|| pstDistrictFrame->unStructSize % 4 != 0|| pstDistrictFrame->unStructSize != (u_int)(pstDistrictFrame->pstDistrictTable)) {return -4;}pstDistrictTable = (DB_DistrictRecord_t*)( pbyDistrictFrame + (u_int)(pstDistrictFrame->pstDistrictTable) );if (m_bIsBigEnd){for ( u_int i = 0; i < pstDistrictFrame->unDistrictCnt; ++i ){pstDistrictTable->usCityNameIdx = _V_GET_LITTLE_SHORT( &(pstDistrictTable->usCityNameIdx) );pstDistrictTable->usCountyNameIdx = _V_GET_LITTLE_SHORT( &(pstDistrictTable->usCountyNameIdx) );pstDistrictTable = (DB_DistrictRecord_t*)( (Byte*)pstDistrictTable + pstDistrictFrame->unDistrictSize );}}return 0;
}int DBParser::GetAdjacentNodeManager (u_int unMgrOffset,u_int unMgrSize,DB_AdjacentNodeMgr_t* pstAdjNodeMgr)
{u_int	i = 0;Byte*	pbtAdjNodeMgr = (Byte*)pstAdjNodeMgr ;DB_AdjacentNode_t *pstAdjacentNode;if (unMgrOffset == 0 || unMgrSize == 0 ||pstAdjNodeMgr == NULL) {return  -1;} if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()) {return  -4;}m_hDBFileHandle.seekg(unMgrOffset, std::ios::beg);if (!m_hDBFileHandle.good()) {return  -4;}m_hDBFileHandle.read ((char*)pbtAdjNodeMgr, unMgrSize) ;if (!m_hDBFileHandle.good()) {return  -4;}if (m_bIsBigEnd){pstAdjNodeMgr->unSize = _V_GET_LITTLE_LONG (&pstAdjNodeMgr->unSize) ;pstAdjNodeMgr->unStructSize = _V_GET_LITTLE_LONG (&pstAdjNodeMgr->unStructSize) ;pstAdjNodeMgr->unCnt = _V_GET_LITTLE_LONG (&pstAdjNodeMgr->unCnt) ;pstAdjNodeMgr->unRecordSize = _V_GET_LITTLE_LONG (&pstAdjNodeMgr->unRecordSize) ;pstAdjNodeMgr->pstAdjNodes = _V_GET_LITTLE_LONG (&pstAdjNodeMgr->pstAdjNodes) ;}pstAdjacentNode = (DB_AdjacentNode_t*)( pbtAdjNodeMgr + (u_int)(pstAdjNodeMgr->pstAdjNodes) );if ( m_bIsBigEnd ){for ( i = 0; i < pstAdjNodeMgr->unCnt; ++i ){pstAdjacentNode->nLongitude = _V_GET_LITTLE_LONG (&(pstAdjacentNode->nLongitude) );pstAdjacentNode->nLatitude = _V_GET_LITTLE_LONG (&(pstAdjacentNode->nLatitude) );pstAdjacentNode->stNodeID.unNodeID = _V_GET_LITTLE_LONG (&(pstAdjacentNode->stNodeID.unNodeID) );pstAdjacentNode->stNodeID.unNodeExtendID = _V_GET_LITTLE_LONG (&(pstAdjacentNode->stNodeID.unNodeExtendID) );pstAdjacentNode = (DB_AdjacentNode_t*)( (Byte*)pstAdjacentNode + pstAdjNodeMgr->unRecordSize );}}return 0;
}int DBParser::GetCalcRegionManagerHeader (u_int unHeaderOffset,u_int unHeaderSize,DB_CalcRegion_ManagerHeader_t* pstManagerHeader)
{u_int	i = 0;Byte*	pbtCalcRegionManagerHeader = (Byte*)pstManagerHeader ;DB_CalcRegion_Idx_t *pstRegionIdxTable;if (unHeaderOffset == 0 || unHeaderSize == 0 ||pstManagerHeader == NULL) {return  -1;}if((true == m_rp_db_file_install) && (!m_hDBFileHandle.is_open())){m_hDBFileHandle.open(m_rp_db_file_path , std::ios::in|std::ios::binary);}DBFile oRpFile(&m_hDBFileHandle);if (!m_hDBFileHandle.is_open()) {return  -4;}m_hDBFileHandle.seekg (unHeaderOffset, std::ios::beg);if (!m_hDBFileHandle.good()) {return  -4;}m_hDBFileHandle.read ((char*)pbtCalcRegionManagerHeader, unHeaderSize) ;if (!m_hDBFileHandle.good()) {return  -4;}if (m_bIsBigEnd){pstManagerHeader->unFrameSize = _V_GET_LITTLE_LONG (&pstManagerHeader->unFrameSize) ;pstManagerHeader->unRankInfo = _V_GET_LITTLE_LONG (&pstManagerHeader->unRankInfo) ;pstManagerHeader->unRegionCnt = _V_GET_LITTLE_LONG (&pstManagerHeader->unRegionCnt) ;pstManagerHeader->unRegionIdxSize = _V_GET_LITTLE_LONG (&pstManagerHeader->unRegionIdxSize) ;pstManagerHeader->pstRegionIdxTable = _V_GET_LITTLE_LONG (&pstManagerHeader->pstRegionIdxTable) ;}pstRegionIdxTable = (DB_CalcRegion_Idx_t*)(pbtCalcRegionManagerHeader + (u_int)(pstManagerHeader->pstRegionIdxTable) );if (m_bIsBigEnd){for (i = 0; i < pstManagerHeader->unRegionCnt ; i++){pstRegionIdxTable->unRegionOffset = _V_GET_LITTLE_LONG (&(pstRegionIdxTable->unRegionOffset) );pstRegionIdxTable->unUnCompressedRegionOffset = _V_GET_LITTLE_LONG (&(pstRegionIdxTable->unUnCompressedRegionOffset) );pstRegionIdxTable->unRegionSize = _V_GET_LITTLE_LONG (&(pstRegionIdxTable->unRegionSize) );pstRegionIdxTable->unCompressedRegionSize = _V_GET_LITTLE_LONG (&(pstRegionIdxTable->unCompressedRegionSize) );pstRegionIdxTable = (DB_CalcRegion_Idx_t*)( (Byte*)pstRegionIdxTable + pstManagerHeader->unRegionIdxSize );}}return 0;
}int main() {std::cout << "start test ..." << std::endl;std::string usPathFileName = "/home/tiger/cpp/fstream/dbfile/103.rp";DBParser dbParser;dbParser.Init (usPathFileName);u_int unHeaderSize = 0;u_int unNameFrameSize = 0;u_int unDistrictFrameSize = 0;std::cout << dbParser.GetHeaderSize(&unHeaderSize) << " " << unHeaderSize << std::endl;if (unHeaderSize > 0) {DB_Header_t stHeaderInfo;u_int size = sizeof(stHeaderInfo);std::cout << size << "   " << unHeaderSize << std::endl;unHeaderSize > size ? unHeaderSize = size : 0;std::cout << dbParser.GetHeader(unHeaderSize, &stHeaderInfo) << std::endl;std::cout << stHeaderInfo.cFormatVer << std::endl;std::cout << stHeaderInfo.cDataVer << std::endl;std::cout << stHeaderInfo.cAuthor << std::endl;std::cout << stHeaderInfo.cDate << std::endl;}std::cout << dbParser.GetNameFrameSize(&unNameFrameSize) << " " << unNameFrameSize << std::endl;if (unNameFrameSize > 0) {DB_NameFrame_t pstNameFrame;//dbParser.GetNameFrame(, , );}std::cout << dbParser.GetDistrictFrameSize(&unDistrictFrameSize) << " " << unDistrictFrameSize << std::endl;return 0;
}

编译执行：

g++ -g -o test test.cpp 

strace -ff -o ./out  ./test 

 

1.gcount

原始代码：

 

  系统调用：

 

 结论： 

可以看到gcount是上一次读取字节数，没有对应系统api调用

2.seekg和tellg

 

 结论：

seekg和tellg对应系统调用为lseek

3.read

原始代码：

系统调用：

 结论：

read对应系统调用为read，而且可以知道即使读取4个字节，底层系统api调用是将整个文件进行读取

 


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