我的mmap代碼中的哪些設置會將所有文件數據設置為零？

Question

我正在實現流的一半，您應該能夠在其中來回跳轉X個整數。 最重要的是，當我打開文件時，它應該將數據保留在文件中。 在此實現中，我嘗試使用mmap。 我的問題是，當我第二次打開文件時，映射數據完全為零。 我的打開方法稱為createStream，如下所示：

void MapperOutForward::createStream(const char* filename)
{
    this->filename = filename;
    pFile = open(filename, O_RDWR | O_CREAT | O_SYNC, S_IRWXG|S_IRWXO|S_IRWXU);
    if ( pFile ==  -1) 
    {
        int errsv = errno;
        cout << "MappedStreamOut createStream open error: " << strerror( errno ) << endl;
    }

    //getting file size and making the file big enough
    struct stat filestatus;
    stat( filename, &filestatus );
    if(filestatus.st_size < mapSize + 1)
    {
        lseek (pFile, mapSize + 1, SEEK_SET);
        write (pFile, "", 1);
    }
    lseek (pFile, 0, SEEK_SET);

    map = (int*) mmap (0, mapSize, PROT_WRITE | PROT_READ, MAP_PRIVATE, pFile, 0);
    close (pFile);

    if ( map == (int*) -1) 
    {
        cout << "MappedStreamOut createStream mmap error: " << strerror(errno) << endl;
    }

    numberOfOffsets = 1;
    numberOfElementsInMap = 0;

    cout << "Kopirer 5 før og 4 efter i Create" << endl;
    for(int i = 0; i < numberOfElementsInMap + 5; i++)
    {
        cout << map[i] << endl;
    }
    cout << "Create numberOfOffsets:numberOfElementsInMap " << numberOfOffsets << ":" << numberOfElementsInMap << endl; 
}

最后的輸出告訴我，當我完成打開操作時，我位於文件的開頭，並且前5個整數為0。

打開命令中的O_CREAT標志僅應在不存在的情況下創建文件，並且我可以確定在我第二次調用此命令后該文件將保持其大小。 人開

PROT_WRITE | mmap命令中的PROT_READ標志似乎僅用於激活某些安全性，而不更改文件中的任何內容。 男人mmap

該課程的其余部分在這里：

#include "MapperOutForward.h"

using namespace std;

#include <cstdlib>
#include <iostream>

#include <fcntl.h>
#include <sys/stat.h>
#include <cmath>

MapperOutForward::MapperOutForward(int mapSize) //number of pages
{ 
    pageSize = getpagesize();
    this->mapSize = mapSize * pageSize; //we work in pages
    numberOfElementsInMap = 0;
    numberOfOffsets = 0;
}

MapperOutForward::~MapperOutForward() 
{ 
    munmap(map, mapSize);
}

void MapperOutForward::createStream(const char* filename)
{
    this->filename = filename;
    pFile = open(filename, O_RDWR | O_CREAT | O_SYNC, S_IRWXG|S_IRWXO|S_IRWXU);
    if ( pFile ==  -1) 
    {
        int errsv = errno;
        cout << "MappedStreamOut createStream open error: " << strerror( errno ) << endl;
    }

    //getting file size and making the file big enough
    struct stat filestatus;
    stat( filename, &filestatus );
    if(filestatus.st_size < mapSize + 1)
    {
        lseek (pFile, mapSize + 1, SEEK_SET);
        write (pFile, "", 1);
    }
    lseek (pFile, 0, SEEK_SET);

    map = (int*) mmap (0, mapSize, PROT_WRITE | PROT_READ, MAP_PRIVATE, pFile, 0);
    close (pFile);

    if ( map == (int*) -1) 
    {
        cout << "MappedStreamOut createStream mmap error: " << strerror(errno) << endl;
    }

    numberOfOffsets = 1;
    numberOfElementsInMap = 0;

    cout << "Kopirer 5 før og 4 efter i Create" << endl;
    for(int i = 0; i < numberOfElementsInMap + 5; i++)
    {
        cout << map[i] << endl;
    }
    cout << "Create numberOfOffsets:numberOfElementsInMap " << numberOfOffsets << ":" << numberOfElementsInMap << endl; 
}

void MapperOutForward::writeNext(int* data, int numberOfElements)
{
    for(int i = 0; i < numberOfElements; i++)
    {
        writeHelper(data[i]);
    }   

    cout << "Kopirer 5 før og 4 efter i writeNext" << endl;
    for(int i = numberOfElementsInMap - 5; i < numberOfElementsInMap + 5; i++)
    {
        cout << map[i] << endl;
    }
    //cout << "WriteNext numberOfOffsets:numberOfElementsInMap " << numberOfOffsets << ":" << numberOfElementsInMap << endl;
}

void MapperOutForward::writeHelper(int data)
{
    int sizeOfInt = 4; //bytes
    if(numberOfElementsInMap >= mapSize / sizeOfInt) //We need to create the next part of the file
    {
        munmap(map, mapSize);

        pFile = open(filename, O_RDWR | O_CREAT | O_SYNC, S_IRWXG|S_IRWXO|S_IRWXU);

        if ( pFile ==  -1) 
        {
            std::cout<<"filename: "<<filename<<std::endl;
            std::cout<<"mapSize: "<<mapSize<<std::endl;

            int errsv = errno;
            cout << "MappedStreamOut writeNext open error: " << strerror( errno ) << endl;
        }

        numberOfOffsets++;

        //Make the file bigger 
        lseek (pFile, mapSize * numberOfOffsets, SEEK_SET);
        write (pFile, "", 1);
        lseek (pFile, 0, SEEK_SET);

        map = (int*) mmap (0, mapSize, PROT_WRITE | PROT_READ, MAP_PRIVATE, pFile, mapSize * (numberOfOffsets - 1));
        close (pFile);

        if ( map == (int*) -1) 
        {
            cout << "MappedStreamOut writeNext mmap error: " << strerror(errno) << endl;
        }

        numberOfElementsInMap = 0;
    }

    map[numberOfElementsInMap] = data;
    numberOfElementsInMap++;
}

void MapperOutForward::jumpTo(int offset)
{
    int sizeOfInt = 4; //bytes
    //cout << "numberOfElementsInMap + offset " << numberOfElementsInMap + offset << endl;
    //cout << "mapSize / sizeOfInt " << mapSize / sizeOfInt << endl;
    if(numberOfElementsInMap + offset >= mapSize / sizeOfInt)
    {
        munmap(map, mapSize);

        pFile = open(filename, O_RDWR | O_CREAT | O_SYNC, S_IRWXG|S_IRWXO|S_IRWXU);

        if ( pFile ==  -1) 
        {
            std::cout<<"filename: "<<filename<<std::endl;
            std::cout<<"mapSize: "<<mapSize<<std::endl;

            int errsv = errno;
            cout << "MappedStreamOut writeNext open error: " << strerror( errno ) << endl;
        }

        numberOfOffsets += (int) ceil( (double) (numberOfElementsInMap + offset - (mapSize / sizeOfInt)) / (mapSize / sizeOfInt)); // rundet op ((resterende antal ints ud over mapSize) / (Antal ints i en mapsize)) = Antal offsets der skal springes

        //Make the file bigger 
        lseek (pFile, mapSize * numberOfOffsets, SEEK_SET);
        write (pFile, "", 1);
        lseek (pFile, 0, SEEK_SET);

        map = (int*) mmap (0, mapSize, PROT_WRITE | PROT_READ, MAP_PRIVATE, pFile, mapSize * (numberOfOffsets - 1));
        close (pFile);

        if ( map == (int*) -1) 
        {
            cout << "MappedStreamOut writeNext mmap error: " << strerror(errno) << endl;
        }

        numberOfElementsInMap = (numberOfElementsInMap + offset - (mapSize / sizeOfInt)) % (mapSize / sizeOfInt); //(resterende antal ints ud over mapSize) % (Antal ints i en mapsize) = Antal elementer ind i offsettet
        //cout << "JumpTo offsetForward numberOfOffsets:numberOfElementsInMap " << numberOfOffsets << ":" << numberOfElementsInMap << endl;
    }
    else if(numberOfElementsInMap + offset < 0)
    {
        munmap(map, mapSize);

        pFile = open(filename, O_RDWR | O_CREAT | O_SYNC, S_IRWXG|S_IRWXO|S_IRWXU);

        if ( pFile ==  -1) 
        {
            std::cout<<"filename: "<<filename<<std::endl;
            std::cout<<"mapSize: "<<mapSize<<std::endl;

            int errsv = errno;
            cout << "MappedStreamOut writeNext open error: " << strerror( errno ) << endl;
        }

        numberOfOffsets += (int) floor( (double) (numberOfElementsInMap + offset) / (mapSize / sizeOfInt)); // rundet op ((resterende antal ints ud over mapSize) / (Antal ints i en mapsize)) = Antal offsets der skal springes

        //Make the file bigger 
        lseek (pFile, mapSize * numberOfOffsets, SEEK_SET);
        write (pFile, "", 1);
        lseek (pFile, 0, SEEK_SET);

        map = (int*) mmap (0, mapSize, PROT_WRITE | PROT_READ, MAP_PRIVATE, pFile, mapSize * (numberOfOffsets - 1));
        close (pFile);

        if ( map == (int*) -1) 
        {
            cout << "MappedStreamOut writeNext mmap error: " << strerror(errno) << endl;
        }

        numberOfElementsInMap = (mapSize + (numberOfElementsInMap + offset)) % (mapSize / sizeOfInt); //(resterende antal ints ud over mapSize) % (Antal ints i en mapsize) = Antal elementer ind i offsettet
        //cout << "JumpTo offsetBacward numberOfOffsets:numberOfElementsInMap " << numberOfOffsets << ":" << numberOfElementsInMap << endl;
    }
    else
    {
        numberOfElementsInMap += offset;
        //cout << "JumpTo else numberOfOffsets:numberOfElementsInMap " << numberOfOffsets << ":" << numberOfElementsInMap << endl;
    }
}

void MapperOutForward::closeStream()
{
    munmap(map, mapSize);
}

主文件在這里：

#include "MapperInForward.h"
#include "MapperOutForward.h"
//#include "MapperOutBackward.h"
//#include "MapperInBackward.h"
//#include "MapperStreamFactory.h"
#include <functional>

int main( int argc, const char* argv[] )
{
/*********************Tester Buffer Stream*************************************/
    close(open((char*) "mapperTest", O_WRONLY | O_CREAT | O_SYNC | O_TRUNC, S_IRWXG|S_IRWXO|S_IRWXU));
    //BufferStreamFactory* bsf = new BufferStreamFactory(3);

    MapperOutForward* mso = new MapperOutForward(1);
    mso->createStream((char*) "mapperTest");

    int data[4000];

    for(int i = 0; i < 4000; i++)
    {
        data[i] = i + 1;
    }

    mso->writeNext(data, 4000);
    mso->closeStream();

    MapperOutForward* mso1 = new MapperOutForward(1);
    mso1->createStream((char*) "mapperTest");

    data[0] = 9999;

    mso1->jumpTo(0);

    mso1->jumpTo(100);
    mso1->writeNext(data, 1);

    mso1->jumpTo(1000);
    mso1->writeNext(data, 1);

    mso1->jumpTo(-900);
    mso1->writeNext(data, 1);

    mso1->closeStream();
}

它們都在Ubuntu Linux上運行並使用g ++編譯。

感謝您抽出寶貴的時間閱讀所有這些內容，希望您能對我有所幫助。

Answer 1

您可能需要MAP_SHARED而不是MAP_PRIVATE否則更改將不會寫入到標准所指定的文件中。

MAP_SHARED和MAP_PRIVATE描述對內存對象的寫引用的配置。 如果指定了MAP_SHARED ，則寫引用應更改基礎對象。 如果指定了MAP_PRIVATE ，則調用過程對映射數據的修改僅對調用過程可見，並且不得更改基礎對象。