How to read a growing text file in C++?

user_235472 0
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I am trying to read from a file which is growing (something similar to what tail -F does), but there must be some problems with my code:

string   log, logFile("test.log");
size_t   p = 0;

while(true)
{
    ifstream ifs(logFile.c_str());

    ifs.seekg(p);  //*1

    while(ifs.eof() == false)
    {
        getline(ifs, log);

        cout << log << endl;

        p = ifs.tellg();  //*2
    }

    nanosleep(&pause, NULL);
}

Without the lines //*1 and //*2, the log file is correctly read up to its end, but if new lines are added nothing happens.

With seekg and tellg I am trying to store the current end position of the file, so that when I reopen it I can go strait there and read what has been added.

I would like to know what is wrong in my code, and if it is really necessary to close and reopen the same file for this purpose.

Thank you.

seekg fstream logging stl c++
4 Answers

Since none of these answers worked, i came up with one that does work...

#include <iostream>
#include <string>
#include <fstream>

using namespace std;

int main()
{
    string   log, logFile("test.txt");
    std::streamoff   p = 0;
    ifstream ifs(logFile.c_str());

    while(true)
    {

        ifs.seekg(p);  //*1
        while (getline(ifs, log))
        {
            cout << log << endl;
            if(ifs.tellg() == -1) p = p + log.size();
            else p = ifs.tellg();
        }
        ifs.clear();

    }
}
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