[SOLVED]SIGSEGV QArrayData::deallocate(QArrayData*, unsigned int, unsigned int)

topix93

@QByteArray buffer; //declared as public in the header file

void MyClass::run(){

while(!bQuit){
    mutex.lock();
    buffer = readData(); // On this line i get the SIGSEGV
    mutex.unlock();
}

}@

the method readData() return a QByteArray

the backtrace is:
@>&"bt\n"

~"#0 0xb7fdd424 in __kernel_vsyscall ()\n"
~"#1 0xb6c011df in __GI_raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64\n"
~"#2 0xb6c04825 in __GI_abort () at abort.c:91\n"
~"#3 0xb6c3e39a in __libc_message (do_abort=2, fmt=0xb6d39888 "*** glibc detected *** %s: %s: 0x%s **\n") at ../sysdeps/unix/sysv/linux/libc_fatal.c:201\n"
~"#4 0xb6c48ee2 in malloc_printerr (action=<optimized out>, str=<optimized out>, ptr=0xaeb5a2a8) at malloc.c:5039\n"
~"#5 0xb6f25eb5 in QArrayData::deallocate(QArrayData, unsigned int, unsigned int) () from /home/tizio/Qt5.1.1/5.1.1/gcc/lib/libQt5Core.so.5\n"
~"#6 0xb6f27cd0 in QByteArray::operator=(QByteArray const&) () from /home/tizio/Qt5.1.1/5.1.1/gcc/lib/libQt5Core.so.5\n"
~"#7 0x0806a68f in MyClass::run (this=0x81a20c0) at ../Project/myclass.cpp:60\n"
~"#8 0xb6f253a9 in ?? () from /home/tizio/Qt5.1.1/5.1.1/gcc/lib/libQt5Core.so.5\n"
~"#9 0xb6856d4c in start_thread (arg=0xafcfdb40) at pthread_create.c:308\n"
~"#10 0xb6cc2bae in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130\n"
197^done@

This error don't appears always, only sometimes

JKSH

Hi,

What does MyClass inherit?

How did you implement readData()?

How are you using your threads?

I recommend reading "this page":http://doc-snapshot.qt-project.org/qt5-stable/threads-synchronizing.html for info about transferring data between threads. Also see "this page":http://doc-snapshot.qt-project.org/qt5-stable/threads-technologies.html for recommendations how to use threads.

topix93

1 - MyClass inherits QThread.

2 - i have a class Utility with the public method readData(), for this class i use the extern keyword : "extern Utility mUtility;"
in the read data method i write the following code:

@int iPlcConnResult = 0;

QByteArray buff;
buff.clear();

unsigned char data = (unsigned char)malloc(length);

//mutexPlcSocket.lock();
iPlcConnResult = readBytes(conn, tipoArea, numDB, startAddress, length, data);
//mutexPlcSocket.unlock();

bPlcConnError = (iPlcConnResult != 0);
if (!bPlcConnError)
buff = buff.setRawData((char*) data, length);
else
sPlcConnError = "Error " + QString::number(iPlcConnResult) + " - " + daveStrerror(iPlcConnResult);

return buff;@

3 - i start the thread with myThread.start(QThread::HighestPriority);

JKSH

Hi,

There isn't enough code to know for sure what's the cause, but you really shouldn't be using malloc in C++. There are safer ways to allocate memory.

Using C-style casts can be dangerous too. Use static_cast<> instead. http://stackoverflow.com/questions/103512/in-c-why-use-static-castintx-instead-of-intx

Questions:

Where did you call free?
Is Utility thread-safe?

Try this, instead of calling malloc and QByteArray::setRawData():
@
// Create an heap-allocated array, filled with null bytes
QByteArray buff(length, '\0');

iPlcConnResult = readBytes(conn, tipoArea, numDB, startAddress,
length, static_cast<unsigned char*>(buff.data()));
@

carallen9988

I have been through with this same problem too. Thanks for the help!