Read till end of a boost memory mapped file in VC++ - c++

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:
use it raw (using data() as you try)
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";
}