qtxlxs segfault when reading an empty excel file using xlsx.cellAt(0,0)->value();

cdwijs

Hi All,

I've made a program that segfaults when I run this line on an empty excel file:
variant = xlsx.cellAt(0,0)->value();
But it does not crash using this line: (although it yields an invalid variant)
variant = xlsx.read(0,0);

I've traced the problem back to QVariant Cell::value(), d is not accessible.

This problem is not mentioned in the bugtracker:
https://github.com/dbzhang800/QtXlsxWriter/issues?page=1&q=is%3Aissue+is%3Aopen

Is this the correct place to ask this question? What can I do to fix this?

Cheers,
Cedric

My versions:
Windows 7 Enterprise SP1
Qt 5.11.1 mingw53_32
Xlsx: http://qtxlsx.debao.me/

My program:
.pro

#-------------------------------------------------
#
# Project created by QtCreator 2018-10-30T17:59:42
#
#-------------------------------------------------

QT       += core gui widgets

TARGET = xlsx-crash
TEMPLATE = app

# The following define makes your compiler emit warnings if you use
# any feature of Qt which has been marked as deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if you use deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

include(3rdparty/QtXlsxWriter/qtxlsx.pri)

CONFIG += c++11

SOURCES += \
        main.cpp \
        mainwindow.cpp

HEADERS += \
        mainwindow.h

FORMS += \
        mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include "xlsxdocument.h"
#include "xlsxchart.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    QString filename = "u:/test.xlsx";

    ui->setupUi(this);

    QXlsx::Document xlsx1;
    xlsx1.write(1,1,"Hi there!");
    xlsx1.saveAs(filename); //create empty file


    QXlsx::Document xlsx(filename);
    QVariant variant;
    variant = xlsx.cellAt(0,0)->value(); //crash
    variant = xlsx.read(0,0); //no crash
}

MainWindow::~MainWindow()
{
    delete ui;
}

JonB

@cdwijs

Is this the correct place to ask this question? What can I do to fix this?

http://qtxlsx.debao.me/qtxlsx-module.html looks like a third-party piece of code, developed in 2013/14. It's not any part of Qt itself.

As such you need to refer to that author about this. Though given the time period I wonder if he is not around.

Here you are hoping someone is using this XSLX package, and I haven't seen anyone, so best of luck. Someone may be kind enough to look at the lines of code in this Qtxslsx package and see if they can see what's going on, but that's not guaranteed.

Just a heads-up about how this question stands on this forum.

P.S.
To try to workaround from your own code if that's what you'd like to do:

xlsx.cellAt(0,0)

maybe this Qtxslx has another call to check whether the cell exists which doesn't fall over when it's "empty"? Or maybe you can safely ask it how many rows/columns are valid in the sheet, and look at that before referencing a particular cell?

jsulm

@cdwijs said in qtxlxs segfault when reading an empty excel file using xlsx.cellAt(0,0)->value();:

variant = xlsx.cellAt(0,0)->value();

My guess is that

xlsx.cellAt(0,0)

returns a nullptr and you're then dereferencing it...

cdwijs

Thanks jsulm,
You are correct. It indeed returns 0 when an empty cell is encountered. For future reference, here's the way to first test for a null pointer:

    QXlsx::Cell *cell;
    cell = xlsx.cellAt(2,2); //cell = 0 on empty cell
    if (cell)
    {
        variant = cell->value();
    }

(Also, 1,1 is the cell at the top-left position, 0,0 is an invalid position)

Cheers,
Cedric

JonB

@cdwijs
OK, now I see that your Qtxslx does have documentation, which you should look at as you write your code. To use xlsx.cellAt() you should have read http://qtxlsx.debao.me/document.html#cellAt, which tells you:

Cell * Document::cellAt(int row, int col) const
Returns the cell at the position pos. If there is no cell at the specified position, the function returns 0.

I advise you to use the docs for your future work! Also note there are a bunch of examples at http://qtxlsx.debao.me/qtxlsx-examples.html, which can doubtless be useful to your coding.