CODEWITHSUNDEEP
c++cqtpipe
user667430
user667430

Reputation: 1547

Qt GUI app unexpectedly ending

Hi I am working on Linux and I am trying to create a GUI app to go with my executable I have made.

For some reason it unexpectedly ends. There is no error message, it just says in the Qt console window it unexpectedly ended with exit code 0.

Can someone please have a look at it for me. I am working on Linux.

I will also paste the code here.

void MainWindow::on_pushButton_clicked()
{
    QString stringURL = ui->lineEdit->text();

    ui->labelError->clear();
    if(stringURL.isEmpty() || stringURL.isNull()) {
        ui->labelError->setText("You have not entered a URL.");
        stringURL.clear();
        return;
    }

    std::string cppString = stringURL.toStdString();
    const char* cString = cppString.c_str();

    char* output;

    //These arrays will hold the file id of each end of two pipes
    int fidOut[2];
    int fidIn[2];

    //Create two uni-directional pipes
    int p1 = pipe(fidOut);                //populates the array fidOut with read/write fid
    int p2 = pipe(fidIn);                 //populates the array fidIn  with read/write fid
    if ((p1 == -1) || (p2 == -1)) {
        printf("Error\n");
        return;
    }

    //To make this more readable - I'm going to copy each fileid
    //into a semantically more meaningful name
    int parentRead  = fidIn[0];
    int parentWrite = fidOut[1];
    int childRead   = fidOut[0];
    int childWrite  = fidIn[1];

    //////////////////////////
    //Fork into two processes/
    //////////////////////////
    pid_t processId = fork();

    //Which process am I?
    if (processId == 0) {
        /////////////////////////////////////////////////
        //CHILD PROCESS - inherits file id's from parent/
        /////////////////////////////////////////////////
        ::close(parentRead);      //Don't need these
        ::close(parentWrite);     //

        //Map stdin and stdout to pipes
        dup2(childRead,  STDIN_FILENO);
        dup2(childWrite, STDOUT_FILENO);

        //Exec - turn child into sort (and inherit file id's)
        execlp("htmlstrip", "htmlstrip", "-n", NULL);

    } else {
        /////////////////
        //PARENT PROCESS/
        /////////////////
        ::close(childRead);       //Don't need this
        ::close(childWrite);      //

        //Write data to child process
        //char strMessage[] = cString;
        write(parentWrite, cString, strlen(cString));
        ::close(parentWrite);     //this will send an EOF and prompt sort to run

        //Read data back from child
        char charIn;
        while ( read(parentRead, &charIn, 1) > 0 ) {
            output = output + (charIn);
            printf("%s", output);
        }
        ::close(parentRead);      //This will prompt the child process to quit
    }

    return;
}

EDIT:: DEBUGGING RESULTS

I ran the debugger and this is the error I received:

The inferior stopped because it received a signal from the Operating System.

Signal name : SIGSEGV
Signal meaning : Segmentation fault

Upvotes: 1

Views: 562

Answers (1)

mfontanini
mfontanini

Reputation: 21910

You haven't initialized the "output" variable. On the last lines of your code, you do this:

while ( read(parentRead, &charIn, 1) > 0 ) {
    output = output + (charIn);
    printf("%s", output);
}

Which will do nasty things, since you are adding a byte read from your child process, to the output variable, which is a pointer that contains garbage, and then printing the contents of the "output" variable's address as a string. You probably want "output" to be a std::string, that way your code could make sense:

std::string output;
/* ... */
while ( read(parentRead, &charIn, 1) > 0 ) {
    output += (charIn);
}
std::cout << output;

Once you have read all the data your child process has generated, you can write it to stdout.

EDIT: since you want to set the contents of "output" to a QPlainTextEdit, you can use QPlainTextEdit::setPlainText:

while ( read(parentRead, &charIn, 1) > 0 ) {
    output += (charIn);
}
plainTextEdit.setPlainText(output.c_str());

Upvotes: 5

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