CODEWITHSUNDEEP
c++filenonblockingostream
A.D
A.D

Reputation: 438

Non-blocking way to check if there is data on a iofstream

I need to have a way to check if there is data to read on a file (fifo) in a non-blocking way. I have tried using peek; but it is blocking, I have tried to get and then unget a character in order to check the file without altering the contents; but once again get is blocking...

The only non-blocking solution I have found is to use std::getline(file, line_str) and check if the string is empty; however this does not suit my needs as it alters the data on the file. (The data is a serialized object I will read once I detect there is something to read).

Note: I need this to be non-blocking: I have multiple file streams and need to check all of them regularly to see if there is an object to read/deserialize.

Here is a simple example of what I am trying to achieve:

Sender.cpp:

#include <fstream>
#include <iostream>
#include <string>
extern "C"{
    #include <sys/stat.h>      // S_IRUSR, S_IWUSR, mkfifo
}
#include <cerrno>              // errno

int main(int, char** argv) {
    std::string pipe = "foobar";
    if(mkfifo(pipe.c_str(), S_IRUSR | S_IWUSR) < 0){
        if (errno != EEXIST){
            std::cerr << errno;
        }
    }
    std::ofstream file{pipe.c_str()};
    file.write("boop", 4); // Simulated object serialization

}

Reader.cpp:

#include <fstream>
#include <iostream>
#include <string>
extern "C"{
    #include <sys/stat.h>      // S_IRUSR, S_IWUSR, mkfifo
}
#include <cerrno>              // errno

int main(int, char** argv) {
    std::string pipe = "foobar";
    if(mkfifo(pipe.c_str(), S_IRUSR | S_IWUSR) < 0){
        if (errno != EEXIST){
            std::cerr << errno;
        }
    }
    std::ifstream file{pipe.c_str()};
    // ...
    /* Do check for data and read/deserialize if any data */
    // This is in some sort of loop that goes over the different
    // filestreams and checks to see if they have data to treat
}

Any help is really appreciated...

EDIT: Following Zoso's answer I tried using the file size to determine if the file had been changed; however attempeting to get the size of a fifo named pipe is not possible : filesystem error: cannot get file size: Operation not supported [myFilePath]

Upvotes: 0

Views: 459

Answers (1)

Zoso
Zoso

Reputation: 3465

I'm not sure if this would work for your particular use case but you could use the filesystem APIs. A simple example is

#include <iostream>
#include <fstream>
#include <filesystem>

namespace fs = std::filesystem;
int main()
{
    while (true) {
        auto path = fs::current_path().append("test");
        std::cout <<"Press enter to know file size of "<<path.c_str() <<'\n';
        char c= getchar();
        try {
            std::cout<<"Size of "<<path.c_str()<<"is "<<fs::file_size(path)<<'\n';
        } catch(fs::filesystem_error& e) {
            std::cout << e.what() << '\n';
        }        
    }
}

As and when the file gets data, that can be kept track of based on the increasing size and the data to be processed can be tracked as and when that data is consumed.

Upvotes: 1

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