Need help with a segfault problem

[jsulm]

@Qjay Is it possible that you're calling deallocate() several times, so you free same memory several times?
Do

void MultiSegmentLogic::deallocate()
{
    if (segs) {
        delete[] segs;
        segs = nullptr;
    }
}

[Qjay]

@jsulm said in Need help with a segfault problem:

void MultiSegmentLogic::deallocate()
{
if (segs) {
delete[] segs;
segs = nullptr;
}
}

hey @jsulm thanks for quick reply but there is still same error

Program received signal SIGSEGV, Segmentation fault. 0x00401983 in MultiSegmentLogic::deallocate (this=0x61fe7c) at deepmem.cpp:234 234 delete[] segs; (gdb)

[jsulm]

@Qjay Then please put a break point at delete[] segs; and see the status (segs pointing to valid chunk of memory?)

[Paul Colby]

I'd strongly recommend initialising segs to nullptr in the MultiSegmentLogic constructor. Your current logic certainly allows segs to be accessed uninitialised, given certain values for numSegs.

Try:

MultiSegmentLogic::MultiSegmentLogic(int numSegs = 0) : numSegs(0), segs(nullptr)
{
   ...
}

[Qjay]

hey , i tried still same error

here is updated code : https://ghostbin.com/paste/q8d5e

changes

MultiSegmentLogic::MultiSegmentLogic(int numSegs = 0) : numSegs(0), segs(nullptr){
    
   if (!setNumSegs(numSegs))
      numSegs = 0;
}

[Paul Colby]

Good. Now you also have to do the same in BooleanFunc::deallocate() as @jsulm suggested for MultiSegmentLogic::deallocate(), ie

void BooleanFunc::deallocate()
{
    if (truthTable) {
        delete[] truthTable;
        truthTable = nullptr;
    }
}

This is related to the crash point, because the delete in MultiSegmentLogic invokes the BooleanFunc destructors, and those destructors in turn call BooleanFunc::deallocate().

Also, don't forget to set the breakpoint as @jsulm suggested. After that, try valgrind.

Cheers.

[Qjay]

@jsulm i did breakpoint with this

break MultiSegmentLogic::deallocate

output

Breakpoint 1, MultiSegmentLogic::deallocate (this=0x61fe6c) at deepmem.cpp:234
234         if (segs) {
(gdb) step
235             delete[] segs;
(gdb) step

Program received signal SIGSEGV, Segmentation fault.
0x00401995 in MultiSegmentLogic::deallocate (this=0x61fe6c) at deepmem.cpp:235
235             delete[] segs;
(gdb)

[Qjay]

@Paul-Colby

updated code here : https://ghostbin.com/paste/cvfq7

changes

void BooleanFunc::deallocate()
{
    if (truthTable) {
        delete[] truthTable;
        truthTable = nullptr;
    }
}

[Paul Colby]

The (next) problem is with your copy constructor. (valgrind show'd it immediately)

> g++ foo.cpp && valgrind ./a.out
...
==9657== Conditional jump or move depends on uninitialised value(s)
==9657==    at 0x109314: MultiSegmentLogic::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109405: MultiSegmentLogic::operator=(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x1093D8: MultiSegmentLogic::MultiSegmentLogic(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109FB3: main (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657== 
==9657== Conditional jump or move depends on uninitialised value(s)
==9657==    at 0x109325: MultiSegmentLogic::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109405: MultiSegmentLogic::operator=(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x1093D8: MultiSegmentLogic::MultiSegmentLogic(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109FB3: main (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657== 
==9657== Use of uninitialised value of size 8
==9657==    at 0x10933B: MultiSegmentLogic::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109405: MultiSegmentLogic::operator=(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x1093D8: MultiSegmentLogic::MultiSegmentLogic(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109FB3: main (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657== 
==9657== Conditional jump or move depends on uninitialised value(s)
==9657==    at 0x10935A: MultiSegmentLogic::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109405: MultiSegmentLogic::operator=(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x1093D8: MultiSegmentLogic::MultiSegmentLogic(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109FB3: main (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657== 
==9657== Use of uninitialised value of size 8
==9657==    at 0x108F76: BooleanFunc::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x108CFF: BooleanFunc::~BooleanFunc() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109367: MultiSegmentLogic::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109405: MultiSegmentLogic::operator=(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x1093D8: MultiSegmentLogic::MultiSegmentLogic(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109FB3: main (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657== 
==9657== Invalid read of size 8
==9657==    at 0x108F76: BooleanFunc::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x108CFF: BooleanFunc::~BooleanFunc() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109367: MultiSegmentLogic::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109405: MultiSegmentLogic::operator=(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x1093D8: MultiSegmentLogic::MultiSegmentLogic(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109FB3: main (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==  Address 0x7fbb2fe8 is not stack'd, malloc'd or (recently) free'd
==9657== 
==9657== 
==9657== Process terminating with default action of signal 11 (SIGSEGV)
==9657==  Access not within mapped region at address 0x7FBB2FE8
==9657==    at 0x108F76: BooleanFunc::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x108CFF: BooleanFunc::~BooleanFunc() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109367: MultiSegmentLogic::deallocate() (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109405: MultiSegmentLogic::operator=(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x1093D8: MultiSegmentLogic::MultiSegmentLogic(MultiSegmentLogic const&) (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==    by 0x109FB3: main (in /home/paul/src/extern/forum-qt-io/segfault/a.out)
==9657==  If you believe this happened as a result of a stack
==9657==  overflow in your program's main thread (unlikely but
==9657==  possible), you can try to increase the size of the
==9657==  main thread stack using the --main-stacksize= flag.
==9657==  The main thread stack size used in this run was 8388608.
test 1test 2==9657== 
==9657== HEAP SUMMARY:
==9657==     in use at exit: 357 bytes in 8 blocks
==9657==   total heap usage: 11 allocs, 4 frees, 74,101 bytes allocated
==9657== 
==9657== LEAK SUMMARY:
==9657==    definitely lost: 0 bytes in 0 blocks
==9657==    indirectly lost: 0 bytes in 0 blocks
==9657==      possibly lost: 0 bytes in 0 blocks
==9657==    still reachable: 357 bytes in 8 blocks
==9657==                       of which reachable via heuristic:
==9657==                         newarray           : 80 bytes in 1 blocks
==9657==         suppressed: 0 bytes in 0 blocks
==9657== Rerun with --leak-check=full to see details of leaked memory
==9657== 
==9657== For counts of detected and suppressed errors, rerun with: -v
==9657== Use --track-origins=yes to see where uninitialised values come from
==9657== ERROR SUMMARY: 10 errors from 10 contexts (suppressed: 0 from 0)

At a quick glance it looks like your copy constructor is deallocating before initialising segs.

Cheers.

PS I highly recommend you explore valgrind - such a handy tool for things like this.

[Qjay]

this section right . but the deallocate() will only be called if this != rhs . still not sure how to fix it

MultiSegmentLogic::MultiSegmentLogic(const MultiSegmentLogic &rhs)
{
   *this = rhs;
}
MultiSegmentLogic & MultiSegmentLogic::operator=(const MultiSegmentLogic &rhs)
{
   if (this != &rhs)
   {
      deallocate();
      this->numSegs = rhs.numSegs;
      allocateSegsArray(numSegs);
      for (int i = 0; i < numSegs; i++)
         this->segs[i] = rhs.segs[i];
   }

   return *this;
}

Thanks i will look and learn more about valgrind .

[Qjay]

alright i got it .

thanks @Paul-Colby , @jsulm .

the problem was indeed deallocate() . i have removed it . program completes without any errors

[aha_1980]

Hi @jsulm and @Paul-Colby,

if (segs) {
    delete[] segs;
    segs = nullptr;
}

This is an anti-pattern! Just make sure segs is nullptr before you call new and set it to nullptr after delete. No need to check for nullptr before delete:

delete segs;
segs = nullptr;

Quoting the C++ Reference:

"If this is a null-pointer, the function does nothing."

Regards