I have an library that don't give correct output. I guess it is possibly an write violation, and focused it on this section of code:
void Page::build_default_frame(PosType genome_display_length)
{
Frame* frame = new Frame(*this,
margin_left,
margin_top,
width - margin_left - margin_right,
genome_display_length);
default_frame = frame;
frames.insert(default_frame);
}
The default_frame is a boost intrusive_ptr<Frame>.
Before execute the sentence default_frame = frame, the content of object frame was all right, but after that, its contents were modified to weird value. So I set two watches on two member variables of frame object:
(gdb) watch -l frame->genome_scale.genome_display_length
Hardware watchpoint 4: -location frame->genome_scale.genome_display_length
(gdb) watch -l frame->genome_scale.frame_width
Hardware watchpoint 5: -location frame->genome_scale.frame_width
and then continue. It suddenly reports write operation on these address:
(gdb) c
Continuing.
Hardware watchpoint 4: -location frame->genome_scale.genome_display_length
Old value = 1000
New value = 16
_dl_runtime_resolve () at ../sysdeps/x86_64/dl-trampoline.S:39
39 ../sysdeps/x86_64/dl-trampoline.S: No such file or directory.
(gdb) bt
#0 _dl_runtime_resolve () at ../sysdeps/x86_64/dl-trampoline.S:39
#1 0x00007ffff7b93dd0 in geno_eye::Page::build_default_frame (this=0x6071b0, genome_display_length=1000)
at /home/yangxi/projects/GenoEye/src/geno_eye/Page.cpp:127
#2 0x00007ffff7b93cc1 in geno_eye::Page::Page (this=0x6071b0, context=0x607750, width=300, height=300,
genome_display_length=1000) at /home/yangxi/projects/GenoEye/src/geno_eye/Page.cpp:29
#3 0x00000000004016b8 in geno_eye::__tester__::run (this=0x7fffffffe1c8)
at /home/yangxi/projects/GenoEye/t/t_page.cpp:15
#4 0x00000000004015d1 in main () at /home/yangxi/projects/GenoEye/t/t_page.cpp:36
(gdb) c
Continuing.
Hardware watchpoint 5: -location frame->genome_scale.frame_width
Old value = 240
New value = 3.1228427039313504e-317
_dl_runtime_resolve () at ../sysdeps/x86_64/dl-trampoline.S:40
40 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) bt
#0 _dl_runtime_resolve () at ../sysdeps/x86_64/dl-trampoline.S:40
#1 0x00007ffff7b93dd0 in geno_eye::Page::build_default_frame (this=0x6071b0, genome_display_length=1000)
at /home/yangxi/projects/GenoEye/src/geno_eye/Page.cpp:127
#2 0x00007ffff7b93cc1 in geno_eye::Page::Page (this=0x6071b0, context=0x607750, width=300, height=300,
genome_display_length=1000) at /home/yangxi/projects/GenoEye/src/geno_eye/Page.cpp:29
#3 0x00000000004016b8 in geno_eye::__tester__::run (this=0x7fffffffe1c8)
at /home/yangxi/projects/GenoEye/t/t_page.cpp:15
#4 0x00000000004015d1 in main () at /home/yangxi/projects/GenoEye/t/t_page.cpp:36
The two old values are the correct values for that two member variables. This write operation is happened before executing the = function of boost intrusive_ptr, as I pressed tens of "next", and the code is still in file dl-trampoline.S.
(gdb) n
41 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
42 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
43 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
44 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
45 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
46 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
47 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
48 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
49 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
50 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
51 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
52 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
53 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
54 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
56 in ../sysdeps/x86_64/dl-trampoline.S
(gdb) n
boost::intrusive_ptr<geno_eye::Frame>::operator= (this=0x6071b0, rhs=0x3e8)
at /usr/include/boost/smart_ptr/intrusive_ptr.hpp:134
134 {
What is dl-trampoline.S ? Why it silently write on the memory of my object?
In addition of that, I also run valgrind:
$ valgrind ./t_page
However, instead of invalid write, it reports invalid read to that object, which is happened after the object creation is finished.
This is caused by an reference-to-stack bug.
Object genome_scale holds two references to two member variables of frame object. When I reconstruct my code, it accidentally reference to two stack variables...
So, maybe I should avoid the use of reference types in this situation, as you can easily provide stack stuffs to them and don't get any warns.