Unable to read contents of a file into std::string

Question

I am trying to read the output of an executed command into a std::string but for some reason, only the first lines of the file are being assigned.

Here is what I am trying to do

/*SomeClient.cpp*/

#include <iostream>
#include <string>
#include <sstream>
#include <fstream>
#include <cstdlib>
#include <iterator>


void SomeClient::triggerProcessOutput() const {
   std::string runningProcessInfo = getRunningProcessInfo();
   // USER       PID

}

const std::string SomeClient::getRunningProcessInfo() const {
    return getStdOutFromCommand("ps aux");
}

const std::string SomeClient::getStdOutFromCommand(std::string cmd) const {
    std::string tmpFileName = std::tmpnam(NULL);
    std::system( ( cmd + " > " + tmpFileName ).c_str() ); // I can see the output of the command in the created tmp file.
    std::stringstream data;
    std::ifstream processInfo(tmpFileName.c_str());
    data << processInfo.rdbuf();

    return data.str();

    /*
    //Did not work - Same output

    std::string result(std::istreambuf_iterator<char>(std::ifstream(tmpFileName.c_str())
                                                         >> std::skipws),
                  std::istreambuf_iterator<char>());
    return result
    */

    /*
    //Did not work - Same output

    std::ifstream file(tmpFileName.c_str());
    std::string str;
    std::string file_contents;
    while (std::getline(file, str))
    {
       file_contents += str;
       file_contents.push_back('\n');
    }  
    return file_contents;
    */

}

After logging the runningProcessInfo variable

Output : USER PID,

Expected output :

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.3   3312  1820 ?        Ss   Sep20   0:04 /sbin/ss.exe
root         2  0.0  0.0      0     0 ?        S    Sep20   0:00 [xxxx]
root         3  0.0  0.0      0     0 ?        S    Sep20   0:02 [xxxrd/0]
root         7  0.0  0.0      0     0 ?        S    Sep20   0:09 [dfxxxxx]
.
.
.
.
.
.

I have also tried other approaches that are mentioned here, here and here but no success. All the approaches produce the same output mentioned above ( USER PID).

I am new to C++ and might be missing something obvious. Also, I won't be able to use boost and popen for this problem.

Some elementary error checking, e.g. checking `if ( processInfo ) { ... } else { std::cout << "Failed to open file";`, would we helpful to narrow down where, exactly, your code fails. Also, it shouldn't take much work to factor out the usage of `Log.h`, `SomeClient` and `PowerClient` to make the example self-contained and reproducable... a [mcve], perhaps? — DevSolar, Sep 30 '19 at 07:13
I'd debug that by putting a breakpoint on the return statement and look into the content of each variable. — nada, Sep 30 '19 at 07:16

DevSolar · Accepted Answer · 2019-09-30T07:29:56.410

0

I made your code example self-contained by removing outside references...

#include <iostream>
#include <string>
#include <sstream>
#include <fstream>
#include <cstdlib>

const std::string getStdOutFromCommand(std::string cmd) {
    std::string tmpFileName = std::tmpnam(NULL);
    std::system( ( cmd + " > " + tmpFileName ).c_str() );
    std::stringstream data;
    std::ifstream processInfo(tmpFileName.c_str());

    // Some basic error checking
    if ( processInfo )
    {
        data << processInfo.rdbuf();
        // CLEAN UP after yourself!
        processInfo.close();
        std::remove( tmpFileName.c_str() );
    }
    else
    {
        return "Failed to get stdout from command.";
    }

    return data.str();   
}

int main() {
    std::cout << getStdOutFromCommand( "ps aux" );
}

...and was unable to reproduce any problem. The code does show the output of the command allright. I assume your problem is with your Log implementation, not with the code shown.

edited Sep 30 '19 at 07:29

answered Sep 30 '19 at 07:23

DevSolar

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@SachinKariyattin: Basic debugging technique -- check your assumptions. Like the check whether that file was actually opened successfully, that your function returned that string, that your `Log` implementation actually generates output. That helps to narrow down *where* exactly your code breaks. – DevSolar Sep 30 '19 at 08:02
Looks like the problem is with the system that I am working on. It's not Log module either. Thanks for the help – Sachin Kariyattin Sep 30 '19 at 08:26
It was indeed the Log module. – Sachin Kariyattin Oct 03 '19 at 06:30

Unable to read contents of a file into std::string

1 Answers1