CODEWITHSUNDEEP
c++stlcrashgdbbreakpoints
Hind Forsum
Hind Forsum

Reputation: 10527

STL type/function used in gdb conditional break, will crash the program?

I wish to test in my program below: when s="abc", break inside "f()" and see the value if "i".

#include<string>
using namespace std;
int i=0;
void f(const string& s1)
{
  ++i; // line 6
}
int main()
{
  string s="a";
  s+="b";
  s+="c";
  s+="d";
  s+="e";
  s+="f";
  return 0;
}

Compile and run a.out, no problem. I then debug it

g++ 1.cpp -g
gdb a.out
...
(gdb) b main if strcmp(s.c_str(),"abc")==0
Breakpoint 1 at 0x400979: file 1.cpp, line 9.
(gdb) r
Starting program: /home/dev/a.out 

Program received signal SIGSEGV, Segmentation fault.
__strcmp_sse2_unaligned () at ../sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S:31
31  ../sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S: No such file or directory.
Error in testing breakpoint condition:
The program being debugged was signaled while in a function called from GDB.
GDB remains in the frame where the signal was received.
To change this behavior use "set unwindonsignal on".
Evaluation of the expression containing the function
(__strcmp_sse2_unaligned) will be abandoned.
When the function is done executing, GDB will silently stop.

Program received signal SIGSEGV, Segmentation fault.

Breakpoint 1, __strcmp_sse2_unaligned () at ../sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S:31
31  in ../sysdeps/x86_64/multiarch/strcmp-sse2-unaligned.S

If I change the break point declaration into:

(gdb) b main:6 if s.compare("abc")==0
Breakpoint 1 at 0x400979: file 1.cpp, line 9.

Then I get another kind of crash, seems:

(gdb) r
Starting program: /home/dev/a.out 

Program received signal SIGSEGV, Segmentation fault.
__memcmp_sse4_1 () at ../sysdeps/x86_64/multiarch/memcmp-sse4.S:1024
1024    ../sysdeps/x86_64/multiarch/memcmp-sse4.S: No such file or directory.
Error in testing breakpoint condition:
The program being debugged was signaled while in a function called from GDB.
GDB remains in the frame where the signal was received.
To change this behavior use "set unwindonsignal on".
Evaluation of the expression containing the function
(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::compare(char const*) const) will be abandoned.
When the function is done executing, GDB will silently stop.

Program received signal SIGSEGV, Segmentation fault.

Breakpoint 1, __memcmp_sse4_1 () at ../sysdeps/x86_64/multiarch/memcmp-sse4.S:1024
1024    in ../sysdeps/x86_64/multiarch/memcmp-sse4.S

Is this crash caused by gdb, or my command? If my command has runtime problem, why gdb doesn't simply report an error, by rather crash the program?

Hope to get some explanations, as I didn't get this error cause.

Upvotes: 0

Views: 474

Answers (1)

Tom Tromey
Tom Tromey

Reputation: 22569

What is going on here is that your command:

(gdb) break main:6

... is interpreted by gdb as the same as break main. You can see this by typing the latter as well:

(gdb) b main:6
Breakpoint 1 at 0x400919: file q.cc, line 10.
(gdb) b main
Note: breakpoint 1 also set at pc 0x400919.
Breakpoint 2 at 0x400919: file q.cc, line 10.

Now, this is peculiar because gdb presumably ought to warn you that the trailing :6 is ignored. (I'd recommend filing a bug asking that this be made a syntax error.)

If you want to break at a certain line in a file you must use the source file name. Presumably you meant to type:

(gdb) break main.cc:6

Upvotes: 1

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