5

I got segfault when trying to call "new" to create a pointer and push it into a vector. The code that I push the element in the vector is:

queue->push_back(new Box(gen_id, Interval(x_mid, x_end), Interval(y_mid-y_halfwidth, y_mid+y_halfwidth)));

Basically Box is a Class and the constructor just take 3 arguments, generation_id, and 2 Intervals. I printed out the content in vector before and after this "push", before: 

[ -0.30908203125, -0.3087158203125 ] , [ -0.951416015625, -0.9510498046875 ]
[ -0.3087158203125, -0.308349609375 ] , [ -0.951416015625, -0.9510498046875 ]
[ -0.30908203125, -0.3087158203125 ] , [ -0.9510498046875, -0.95068359375 ]
[ -0.3087158203125, -0.308349609375 ] , [ -0.9510498046875, -0.95068359375 ]

after:

[ -0.30908203125, -0.3087158203125 ] , [ -0.951416015625, -0.9510498046875 ]
[ -0.3087158203125, -0.308349609375 ] , [ -0.951416015625, -0.9510498046875 ]
[ 8.9039208750109844342e-243, 6.7903818933216500424e-173 ] , [ -0.9510498046875, -0.95068359375 ]
[ -0.3087158203125, -0.308349609375 ] , [ -0.9510498046875, -0.95068359375 ]
[ -0.3087158203125, -0.308349609375 ] , [ -0.95123291015625, -0.95086669921875 ]

I have no clue why does this happen, but apparently, there's one element got corrupted. There's no other codes between these two sets of output except that "push", and I used gdb to confirm that. Also, I checked those 2 Intervals variables, both give me a result that make sense.

My questions is: in what situation does "new" get segfault? Or is my problem caused because of other stuff? Thanks.

Oliver Charlesworth
  • 267,707
  • 33
  • 569
  • 680
Shang Wang
  • 24,909
  • 20
  • 73
  • 94
  • 2
    When does the segfault occur? Please show us the code for `Box`. – Oliver Charlesworth Sep 06 '11 at 23:18
  • 1
    Why are you pushing pointers? –  Sep 06 '11 at 23:20
  • I don't think new can normally segfualt. I would look for stack corruption, starting with `Box(int, Interval, Interval)`, and `Interval(int, int)` and `Interval`s copy/move constructor/operator – Mooing Duck Sep 06 '11 at 23:21
  • 1
    @Moo, `new` can conceivably segfault if there is heap corruption. – hmakholm left over Monica Sep 06 '11 at 23:23
  • @Henning Makholm: anything can happen if there is heap corruption. http://stackoverflow.com/questions/2766731/what-exactly-do-ib-and-ub-mean/2766749#2766749, so yes. Still: probably array bounds or pointer errors. – Mooing Duck Sep 06 '11 at 23:24
  • I was surprised that the segfault happened at the second time I do "push" (I have a total of 4 "pushes"). The first time it prints out the above wired result, and the second time it immediately throws segfault :( – Shang Wang Sep 06 '11 at 23:52
  • Have you implemented correct copy constructors for Box and Interval ?? – parapura rajkumar Sep 06 '11 at 23:31

2 Answers2

13

Assuming it really is new generating the segfault, the most common cause would be a corrupted heap, typically a result of overwriting memory you don't own and/or a double delete.

Valgrind will be your friend if you can run on a Linux system.

Mark B
  • 95,107
  • 10
  • 109
  • 188
2

I doubt that new itself is giving you the segfault; the problem is probably in one of the constructors. Try splitting up that giant line, and put in some print statements to see exactly where the problem is. 

printf("Creating the first interval...\n");
Interval a(x_mid, x_end);
printf("Creating the second interval...\n");
Interval b(y_mid-y_halfwidth, y_mid + y_halfwidth);
printf("Creating the box...\n");
Box* box_to_enqueue = new Box(gen_id, a, b);
printf("Enqueueing the box...\n");
// Do you really want to enqueue a pointer instead of a Box?
queue->push_back(box_to_enqueue);
Alan
  • 192
  • 1
  • 8
  • 1
    And don't forget to put an fflush() in this sequence, or else error messages might end up in an unwritten buffer. Even better, use a debugger. – thiton Sep 17 '11 at 18:45