Segmentation fault calling std::map::clear

Question

I have been struggling with a segmentation fault for months, now I'm here to ask for help. The segmentation fault appears when I call the following function

void foo(..., std::map<MyClass*, double> & x) {
if ( !x.empty() ) x.clear();
...
}

Class A {
private:
map<MyClass*, double> _N;
public:
void f(...) {
foo(..., _N);
...
}
};

//in main routine, the function is called in a loop
A a;
while(...) {
a.f(...);
}

Using gdb, I tacked the error to the line calling the clear() function, it shows "double free or corruption" error, and the program aborts at calling c++/4.1.2/ext/new_allocator.h:94 delete(__P) which further calls free() from the gnu library /lib64/libc.so.6. But since the elements in the map are not allocated by new, why it still calls free() to clear it up. I would really appreciate your comments. Thank you.

The error must be elsewhere. You can call `clear()` irrespective of whether the map is empty or not anyway. Show us some more ambient code. — Kerrek SB, Jul 26 '11 at 19:45
what happen if you pass the map by value, the program semantics would be wrong, but does it segf'? — 111111, Jul 26 '11 at 19:45
is the object passed as parameter to `foo` allocated on the heap ? If so, is it possible that it had already been de-allocated (or moved elsewhere) before ? — Sander De Dycker, Jul 26 '11 at 19:46
Thank you guys. Actually the object passed to function foo is a member of another class. Here is a more complete description of my code, please take a look. — cool machine, Jul 26 '11 at 19:55
I think we need more code than that, to me it looks ok. A should be alive at the any point where foo is called. — 111111, Jul 26 '11 at 20:09
Could you create a destructor for A and set a break/printf there and see if that occurs prior to segfault? — Jesse Vogt, Jul 26 '11 at 20:24

score 2 · Answer 1 · answered Jul 26 '11 at 20:17

Given that the map is owned by another object it suspiciously sounds that the map-owning object was already deleted when the clear was called.

Also note that names starting with underscore and a capital letter are reserved for the implementation - you aren't allowed to use them.

score 1 · Answer 2 · answered Jul 26 '11 at 19:48

The code looks fine to me. At least with the limited context you have provided. Usually when I run into issues like this I will simply run the valgrind memcheck tool to find the place were the first "delete" happened. Once you know that, these issues can be pretty simple to solve.

Segmentation fault calling std::map::clear

2 Answers2