QT String to char * adds extra characters












1















I have a qTextEdit that I grab the text from (QString) and convert to a char* with this code:



QString msgQText = ui->textMsg->toPlainText();
size_t textSize = (size_t)msgQText.size();

if (textSize > 139) {
textSize = 139;
}

unsigned char * msgText = (unsigned char *)malloc(textSize);

memcpy(msgText, msgQText.toLocal8Bit().data(), textSize);

msgText[textSize] = '';

if (textSize > 0) {
Msg * newTextMsg = new Msg;
newTextMsg->type = 1; // text message type
newTextMsg->bitrate = 0;
newTextMsg->samplerate = 0;
newTextMsg->bufSize = (int)textSize;
newTextMsg->len = 0;
newTextMsg->buf = (char *)malloc(textSize);
memcpy((char *)newTextMsg->buf, (char *)msgText, textSize);

lPushToEnd(sendMsgList, newTextMsg, sizeof(Msg));

ui->sendRecList->addItem((char *)newTextMsg->buf);

ui->textMsg->clear();
}


I put the text into a qListBox, but it shows up like wrong text



However, the character array, if I print it out, does not have the extra characters.



I have tried checking the "compile using UTF-8" option, but it doesn't make a difference.



Also, I send the text using RS232, and the receiver side also displays the extra characters.



The receiver code is here:



m_serial->waitForReadyRead(200);
const QByteArray data = m_serial->readAll();
if (data.size() > 0) {
qDebug() << "New serial data: " << data;

QString str = QString(data);
if (str.contains("0x6F8C32E90A")) {
qDebug() << "TEST SUCCESSFUL!";
}

return data.data();
} else {
return NULL;
}









share|improve this question

























  • Note that you're writing off the end of the allocated buffer when you do msgText[textSize] = '';. That's undefined behaviour.

    – G.M.
    Nov 13 '18 at 14:32
















1















I have a qTextEdit that I grab the text from (QString) and convert to a char* with this code:



QString msgQText = ui->textMsg->toPlainText();
size_t textSize = (size_t)msgQText.size();

if (textSize > 139) {
textSize = 139;
}

unsigned char * msgText = (unsigned char *)malloc(textSize);

memcpy(msgText, msgQText.toLocal8Bit().data(), textSize);

msgText[textSize] = '';

if (textSize > 0) {
Msg * newTextMsg = new Msg;
newTextMsg->type = 1; // text message type
newTextMsg->bitrate = 0;
newTextMsg->samplerate = 0;
newTextMsg->bufSize = (int)textSize;
newTextMsg->len = 0;
newTextMsg->buf = (char *)malloc(textSize);
memcpy((char *)newTextMsg->buf, (char *)msgText, textSize);

lPushToEnd(sendMsgList, newTextMsg, sizeof(Msg));

ui->sendRecList->addItem((char *)newTextMsg->buf);

ui->textMsg->clear();
}


I put the text into a qListBox, but it shows up like wrong text



However, the character array, if I print it out, does not have the extra characters.



I have tried checking the "compile using UTF-8" option, but it doesn't make a difference.



Also, I send the text using RS232, and the receiver side also displays the extra characters.



The receiver code is here:



m_serial->waitForReadyRead(200);
const QByteArray data = m_serial->readAll();
if (data.size() > 0) {
qDebug() << "New serial data: " << data;

QString str = QString(data);
if (str.contains("0x6F8C32E90A")) {
qDebug() << "TEST SUCCESSFUL!";
}

return data.data();
} else {
return NULL;
}









share|improve this question

























  • Note that you're writing off the end of the allocated buffer when you do msgText[textSize] = '';. That's undefined behaviour.

    – G.M.
    Nov 13 '18 at 14:32














1












1








1








I have a qTextEdit that I grab the text from (QString) and convert to a char* with this code:



QString msgQText = ui->textMsg->toPlainText();
size_t textSize = (size_t)msgQText.size();

if (textSize > 139) {
textSize = 139;
}

unsigned char * msgText = (unsigned char *)malloc(textSize);

memcpy(msgText, msgQText.toLocal8Bit().data(), textSize);

msgText[textSize] = '';

if (textSize > 0) {
Msg * newTextMsg = new Msg;
newTextMsg->type = 1; // text message type
newTextMsg->bitrate = 0;
newTextMsg->samplerate = 0;
newTextMsg->bufSize = (int)textSize;
newTextMsg->len = 0;
newTextMsg->buf = (char *)malloc(textSize);
memcpy((char *)newTextMsg->buf, (char *)msgText, textSize);

lPushToEnd(sendMsgList, newTextMsg, sizeof(Msg));

ui->sendRecList->addItem((char *)newTextMsg->buf);

ui->textMsg->clear();
}


I put the text into a qListBox, but it shows up like wrong text



However, the character array, if I print it out, does not have the extra characters.



I have tried checking the "compile using UTF-8" option, but it doesn't make a difference.



Also, I send the text using RS232, and the receiver side also displays the extra characters.



The receiver code is here:



m_serial->waitForReadyRead(200);
const QByteArray data = m_serial->readAll();
if (data.size() > 0) {
qDebug() << "New serial data: " << data;

QString str = QString(data);
if (str.contains("0x6F8C32E90A")) {
qDebug() << "TEST SUCCESSFUL!";
}

return data.data();
} else {
return NULL;
}









share|improve this question
















I have a qTextEdit that I grab the text from (QString) and convert to a char* with this code:



QString msgQText = ui->textMsg->toPlainText();
size_t textSize = (size_t)msgQText.size();

if (textSize > 139) {
textSize = 139;
}

unsigned char * msgText = (unsigned char *)malloc(textSize);

memcpy(msgText, msgQText.toLocal8Bit().data(), textSize);

msgText[textSize] = '';

if (textSize > 0) {
Msg * newTextMsg = new Msg;
newTextMsg->type = 1; // text message type
newTextMsg->bitrate = 0;
newTextMsg->samplerate = 0;
newTextMsg->bufSize = (int)textSize;
newTextMsg->len = 0;
newTextMsg->buf = (char *)malloc(textSize);
memcpy((char *)newTextMsg->buf, (char *)msgText, textSize);

lPushToEnd(sendMsgList, newTextMsg, sizeof(Msg));

ui->sendRecList->addItem((char *)newTextMsg->buf);

ui->textMsg->clear();
}


I put the text into a qListBox, but it shows up like wrong text



However, the character array, if I print it out, does not have the extra characters.



I have tried checking the "compile using UTF-8" option, but it doesn't make a difference.



Also, I send the text using RS232, and the receiver side also displays the extra characters.



The receiver code is here:



m_serial->waitForReadyRead(200);
const QByteArray data = m_serial->readAll();
if (data.size() > 0) {
qDebug() << "New serial data: " << data;

QString str = QString(data);
if (str.contains("0x6F8C32E90A")) {
qDebug() << "TEST SUCCESSFUL!";
}

return data.data();
} else {
return NULL;
}






qt






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edited Nov 14 '18 at 5:33









AAEM

726318




726318










asked Nov 13 '18 at 14:11









HengyHengy

324




324













  • Note that you're writing off the end of the allocated buffer when you do msgText[textSize] = '';. That's undefined behaviour.

    – G.M.
    Nov 13 '18 at 14:32



















  • Note that you're writing off the end of the allocated buffer when you do msgText[textSize] = '';. That's undefined behaviour.

    – G.M.
    Nov 13 '18 at 14:32

















Note that you're writing off the end of the allocated buffer when you do msgText[textSize] = '';. That's undefined behaviour.

– G.M.
Nov 13 '18 at 14:32





Note that you're writing off the end of the allocated buffer when you do msgText[textSize] = '';. That's undefined behaviour.

– G.M.
Nov 13 '18 at 14:32












1 Answer
1






active

oldest

votes


















1














There is a difference between the size of a QString and the size of the QByteArray returned by toLocal8Bit(). A QString contains unicode text stored as UTF-16, while a QByteArray is "just" a char.



A QByteArray is null-terminated, so you do not need to add it manually.



As @GM pointed out: msgText[textSize] = ''; is undefined behavior. You are writing to the textSize + 1 position of the msgText array.
This position may be owned by something else and may be overwritten, so you end up with a non null terminated string.



This should work:



QByteArray bytes = msgQText.toLocal8Bit();
size_t textSize = (size_t)bytes.size() + 1; // Add 1 for the final ''
unsigned char * msgText = (unsigned char *) malloc(textSize);
memcpy(msgText, bytes.constData(), textSize);


Additional tips:




  • Prefer using const functions on Qt types that are copy-on-write, e.g. use QBytearray::constData() instead of QByteArray::data(). The non-const functions can cause a deep-copy of the object.


  • Do not use malloc() and other C-style functions if possible. Here you could do:
    unsigned char * msgText = new unsigned char[textSize]; and later delete msgText;.


  • Prefer using C++ casts (static_cast, reinterpret_cast, etc.) instead of C-style casts.


  • You are making 2 copies of the text (2 calls to memcpy), given your code only 1 seem to be enough.






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    1 Answer
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    active

    oldest

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    oldest

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    active

    oldest

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    1














    There is a difference between the size of a QString and the size of the QByteArray returned by toLocal8Bit(). A QString contains unicode text stored as UTF-16, while a QByteArray is "just" a char.



    A QByteArray is null-terminated, so you do not need to add it manually.



    As @GM pointed out: msgText[textSize] = ''; is undefined behavior. You are writing to the textSize + 1 position of the msgText array.
    This position may be owned by something else and may be overwritten, so you end up with a non null terminated string.



    This should work:



    QByteArray bytes = msgQText.toLocal8Bit();
    size_t textSize = (size_t)bytes.size() + 1; // Add 1 for the final ''
    unsigned char * msgText = (unsigned char *) malloc(textSize);
    memcpy(msgText, bytes.constData(), textSize);


    Additional tips:




    • Prefer using const functions on Qt types that are copy-on-write, e.g. use QBytearray::constData() instead of QByteArray::data(). The non-const functions can cause a deep-copy of the object.


    • Do not use malloc() and other C-style functions if possible. Here you could do:
      unsigned char * msgText = new unsigned char[textSize]; and later delete msgText;.


    • Prefer using C++ casts (static_cast, reinterpret_cast, etc.) instead of C-style casts.


    • You are making 2 copies of the text (2 calls to memcpy), given your code only 1 seem to be enough.






    share|improve this answer




























      1














      There is a difference between the size of a QString and the size of the QByteArray returned by toLocal8Bit(). A QString contains unicode text stored as UTF-16, while a QByteArray is "just" a char.



      A QByteArray is null-terminated, so you do not need to add it manually.



      As @GM pointed out: msgText[textSize] = ''; is undefined behavior. You are writing to the textSize + 1 position of the msgText array.
      This position may be owned by something else and may be overwritten, so you end up with a non null terminated string.



      This should work:



      QByteArray bytes = msgQText.toLocal8Bit();
      size_t textSize = (size_t)bytes.size() + 1; // Add 1 for the final ''
      unsigned char * msgText = (unsigned char *) malloc(textSize);
      memcpy(msgText, bytes.constData(), textSize);


      Additional tips:




      • Prefer using const functions on Qt types that are copy-on-write, e.g. use QBytearray::constData() instead of QByteArray::data(). The non-const functions can cause a deep-copy of the object.


      • Do not use malloc() and other C-style functions if possible. Here you could do:
        unsigned char * msgText = new unsigned char[textSize]; and later delete msgText;.


      • Prefer using C++ casts (static_cast, reinterpret_cast, etc.) instead of C-style casts.


      • You are making 2 copies of the text (2 calls to memcpy), given your code only 1 seem to be enough.






      share|improve this answer


























        1












        1








        1







        There is a difference between the size of a QString and the size of the QByteArray returned by toLocal8Bit(). A QString contains unicode text stored as UTF-16, while a QByteArray is "just" a char.



        A QByteArray is null-terminated, so you do not need to add it manually.



        As @GM pointed out: msgText[textSize] = ''; is undefined behavior. You are writing to the textSize + 1 position of the msgText array.
        This position may be owned by something else and may be overwritten, so you end up with a non null terminated string.



        This should work:



        QByteArray bytes = msgQText.toLocal8Bit();
        size_t textSize = (size_t)bytes.size() + 1; // Add 1 for the final ''
        unsigned char * msgText = (unsigned char *) malloc(textSize);
        memcpy(msgText, bytes.constData(), textSize);


        Additional tips:




        • Prefer using const functions on Qt types that are copy-on-write, e.g. use QBytearray::constData() instead of QByteArray::data(). The non-const functions can cause a deep-copy of the object.


        • Do not use malloc() and other C-style functions if possible. Here you could do:
          unsigned char * msgText = new unsigned char[textSize]; and later delete msgText;.


        • Prefer using C++ casts (static_cast, reinterpret_cast, etc.) instead of C-style casts.


        • You are making 2 copies of the text (2 calls to memcpy), given your code only 1 seem to be enough.






        share|improve this answer













        There is a difference between the size of a QString and the size of the QByteArray returned by toLocal8Bit(). A QString contains unicode text stored as UTF-16, while a QByteArray is "just" a char.



        A QByteArray is null-terminated, so you do not need to add it manually.



        As @GM pointed out: msgText[textSize] = ''; is undefined behavior. You are writing to the textSize + 1 position of the msgText array.
        This position may be owned by something else and may be overwritten, so you end up with a non null terminated string.



        This should work:



        QByteArray bytes = msgQText.toLocal8Bit();
        size_t textSize = (size_t)bytes.size() + 1; // Add 1 for the final ''
        unsigned char * msgText = (unsigned char *) malloc(textSize);
        memcpy(msgText, bytes.constData(), textSize);


        Additional tips:




        • Prefer using const functions on Qt types that are copy-on-write, e.g. use QBytearray::constData() instead of QByteArray::data(). The non-const functions can cause a deep-copy of the object.


        • Do not use malloc() and other C-style functions if possible. Here you could do:
          unsigned char * msgText = new unsigned char[textSize]; and later delete msgText;.


        • Prefer using C++ casts (static_cast, reinterpret_cast, etc.) instead of C-style casts.


        • You are making 2 copies of the text (2 calls to memcpy), given your code only 1 seem to be enough.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Nov 13 '18 at 15:29









        Benjamin TBenjamin T

        5,1031027




        5,1031027






























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