C++ valgrind possible leaks on STL string

Question

Hi,

I do not see the reason of the leak below.

#include <iostream>
#include <cstdlib>

int fail(const std::string str)
{
    std::cerr<< str << std::endl;
    exit(1);
}

const std::string usage()
{
    std::string a = "a";
    return a;
}   

int main()
{
    fail(usage());
    return 0;
}

Valgrind says:

==7238== 14 bytes in 1 blocks are possibly lost in loss record 1 of 1
==7238==    at 0x402377E: operator new(unsigned) (vg_replace_malloc.c:224)
==7238==    by 0x40E7C03: std::string::_Rep::_S_create(unsigned, unsigned, 
std::allocator<char> const&) (in /usr/lib/libstdc++.so.6.0.10)
==7238==    by 0x40E8864: (within /usr/lib/libstdc++.so.6.0.10)
==7238==    by 0x40E89D5: std::string::string(char const*, std::allocator<char> const&) 
(in /usr/lib/libstdc++.so.6.0.10)
==7238==    by 0x80488EC: usage() (main.cpp:12)
==7238==    by 0x804897C: main (main.cpp:18)
==7238== LEAK SUMMARY:
==7238==    definitely lost: 0 bytes in 0 blocks.
==7238==      possibly lost: 14 bytes in 1 blocks.
==7238==    still reachable: 0 bytes in 0 blocks.
==7238==         suppressed: 0 bytes in 0 blocks.

The problem is in the fail() function. As it exits(), the memory is leaked.

If I comment out exit(1); then there is no possible leak.

Also, if I change the signature from int fail(const std::string str) to int fail(const char* str)

then there is no possible leak as well. I don't like this solution, as I am using fail(string + (LINE)) type of things, but regardless, what is going on here?

I will be happy if someone can explain.

Thanks!

(upps. same question asked before I guess, sorry! http://stackoverflow.com/questions/1901322/valgrind-report-memory-leak-when-assign-a-value-to-a-string)

Answer 1

When you call exit(), the destructors of automatic objects (local variables) do not get called.

In your specific example, the std::string destructor is not called, so the memory owned by the std::string is never deallocated.

The reason there is no leak if you have fail() take a const char* is that there is no destructor for const char*; nothing is deallocated when a pointer is destroyed. If the pointer points to dynamically allocated memory, then that memory must be deallocated (by you) before the program exits, otherwise you have a memory leak. If it points to a string literal, then there is no memory leak because string literals have static storage duration (that is, they exist for the entire lifetime of your program).

Answer 2

James McNellis already wrote a correct answer. But I'd like to add some things:

1. It is always a good thing to write software in a way that it does not have to call exit() - this helps you improve the overall design, specify and understand object lifetimes (except in very special - rather low level - cases..).

2. As you see here, this is important when using tools like valgrind! A "clean" shutdown procedure makes you feel safe, then everything worked fine, as you expected ;) A clean shutdown procedure in exceptional cases should be a requirement of every software.

You should consider to throw an exception instead of calling some fail() function. When an exception is thrown, the stack will be unwound, so the std::string destructor in your case would be called.