using electric fence in a c++ program

Question

I've been experimenting with Electric Fence lately and I can't figure out how to use it with c++ code.

Here's an example:

// test.cpp
#include <cstdlib>                                                                                                                                         

using namespace std;                                                                                                                                       

int main()                                                                                                                                                 
{                                                                                                                                                                                                                                                                                                     
        int *a = new int(10);                                                                                                                              
        delete a;                                                                                                                              
}

I compiled it with

g++ ./test.cpp -o test -lefence -L/home/bor/efence_x86_64/lib -lpthread

And I don't see Electric Fence banner at the start and can't find EF symbols in the executable (using nm command).

But if I modify a program like so:

// test.cpp
#include <cstdlib>                                                                                                                                         

using namespace std;                                                                                                                                       

int main()                                                                                                                                                 
{                                                                                                                                                          
        char *p = (char*)malloc(20);                                                                                                                       
        free(p);                                                                                                                                           
        int *a = new int(10);                                                                                                                              
        delete a;
}

everything is good - EF appears. I know it kinda solves the problem, I know :). I just want to understand why it didn't work in the first place, because new() should call malloc(), and delete() calls free(), no?

The reason I got into this is a big project using boost libraries and several others. And this program never calls malloc() or free() directly. And when I build it with EF not I only linked EF to the final executable but rebuilt all the libraries trying to link EF to them. And I can't find EF symbols in either one of them. Is it the right approach? Or is it wrong and EF only should be linked to the executable in the end, the libs should be left intact? But again I can't find EF symbols in the executable then.

because I need to debug a program that runs on ARMv5 (http://stackoverflow.com/q/9456194/4378). Valgrind isn't prorted there. — Nick Borodulin, Feb 29 '12 at 07:17

score 4 · Accepted Answer · answered Feb 29 '12 at 07:52

You are assuming that the compiler is compiling the code behind new, but that code generally resides somewhere in a pre-compiled RT.

new also doesn't generally call malloc directly (on some systems like Windows, it doesn't call malloc at all), it has a few tasks of its own that are performed, before and after it handles the allocation. for something like this you might have to go the semi-evil route of globally overloading new and delete to force it to directly call malloc and free from your code.

score 1 · Answer 2 · answered Mar 13 '14 at 02:51

From the slackware docs at http://slackbuilds.org/repository/13.1/libraries/electric-fence/

In order to debug a program it needs to be linked with Electric Fence's
library or dynamic linking needs to be used; README.Debian explains that
in detail.


If you're using c++, and you and want to statically link your c++
programs, you shouldn't use g++ to link libefence.a, but rather:
gcc -o myprog myprog.o -lstdc++ -lg++ -lefence
(if you use g++, the order is different, and efence's malloc doesn't
get used)

Be sure to read the libefence manpage which describes how to set various environment variables which alter lebefence's behavior

score 1 · Answer 3 · answered Mar 14 '16 at 11:43

For people who are looking for a quick way to "debug" C++ code with electric fence on armv5:

It is not necessary to statically compile and change the linker command. For me it was enough to install electric fence and run:

LD_PRELOAD=libefence.so ./your-buggy-program

(from the same docs as quoted above)

score 0 · Answer 4 · answered Nov 28 '15 at 20:35

Only if the code of yours contains "mallocs", will the "-libefence" be visible in the build binary with "ldd" command. Otherwise, if no "mallocs" and only "new" then, you might not see the "-libefence" library in the linked-library list of the build-binary.

using electric fence in a c++ program

4 Answers4