在VC ++中读取一个boost内存映射文件的结尾

Question

I am writing a program in C++ using VS2010 to read a text file and extract certain information from it. I completed the code using filestream and it worked well. However now I am asked to map the file to memory and use it rather than the file operations.

I am absolutely a newbie in case of memory mapping. A part of the code I have written is as follows.

boost::iostreams::mapped_file_source apifile;

apifile.open(LogFileName,LogFileSize);

if(!apifile.is_open()) 

return FILE_OPEN_ERROR;

// Get pointer to the data.

PBYTE Buffer = (PBYTE)apifile.data();

while(//read till end of the file)
{
     // read a line and check if it contains a specific word
}

While using filestream I would have used eof and getline and string::find for performing the operations. But I don't have any idea on how to do it using memory mapped file.

EDIT 1:

int ProcessLogFile(string file_name)
{
    LogFileName = file_name;

    apifile.open(LogFileName);//boost::iostreams::mapped_file_source apifile(declared globally)

    streamReader.open(apifile, std::ios::binary);//boost::iostreams::stream <boost::iostreams::mapped_file_source> streamReader(declared globally)

    streamoff Curr_Offset = 0;

    string read_line;

    int session_id = 0;

    int device_id = 0;

    while(!streamReader.eof())
    {
        \\COLLECT OFFSETS OF DIFFERENT SESSIONS
    }

    streamReader.close();
}

This function worked and i got the offsets to the required structure.

Now after calling this function, I call yet another function as follows:

int GetSystemDetails()
{   
    streamReader.open(apifile, std::ios::binary);

    string read_line;

    getline(streamReader,read_line);

    cout << "LINE : " << read_line;

    streamReader.close();
}

I don't get any data in read_line. Is that memory mapping only for a single function? How can I use the same memory mapped file across different functions?

Answer 1

I agree with people questioning the use of a mmap if you just read through the file sequentially.

boost::mapped_file_source models a Device. There's two approaches to use such a Device:

1. use it raw (using data() as you try)
2. using a stream wrapper

1. Using the raw Device source

You can use the mapped_file_source to report the actual size ( m.data()+m.size() ).

Let's take a sample to count lines:

#include <boost/iostreams/device/mapped_file.hpp> // for mmap
#include <algorithm>  // for std::find
#include <iostream>   // for std::cout
#include <cstring>

int main()
{
    boost::iostreams::mapped_file mmap("input.txt", boost::iostreams::mapped_file::readonly);
    auto f = mmap.const_data();
    auto l = f + mmap.size();

    uintmax_t m_numLines = 0;
    while (f && f!=l)
        if ((f = static_cast<const char*>(memchr(f, '\n', l-f))))
            m_numLines++, f++;

    std::cout << "m_numLines = " << m_numLines << "\n";
}

You could possibly adapt this. I have several more complicated parsing examples based on memory mapped files:

• Fast textfile reading in c++
  _{Note how in the updates you can see that indeed, open() + read() was faster than the memory map due the sequential access nature}
• How to parse space-separated floats in C++ quickly?

---

2. Wrapping the source device in a istream

This gives you all the usual stream-based operations of c++ standard streams, so you can detect the end of the file like you would always:

#include <boost/iostreams/device/mapped_file.hpp> // for mmap
#include <boost/iostreams/stream.hpp>             // for stream
#include <algorithm>                              // for std::find
#include <iostream>                               // for std::cout
#include <cstring>

int main()
{
    using boost::iostreams::mapped_file_source;
    using boost::iostreams::stream;
    mapped_file_source mmap("test.cpp");
    stream<mapped_file_source> is(mmap, std::ios::binary);

    std::string line;

    uintmax_t m_numLines = 0;
    while (std::getline(is, line))
    {
        m_numLines++;
    }

    std::cout << "m_numLines = " << m_numLines << "\n";
}