Memory leak in gcc 4.8.1 when using thread_local? - c++11

Question

Valgrind is reporting leaked blocks, apparently one per thread, in the following code:
#include <iostream>
#include <thread>
#include <mutex>
#include <list>
#include <chrono>
std::mutex cout_mutex;
struct Foo
{
Foo()
{
std::lock_guard<std::mutex> lock( cout_mutex );
std::cout << __PRETTY_FUNCTION__ << '\n';
}
~Foo()
{
std::lock_guard<std::mutex> lock( cout_mutex );
std::cout << __PRETTY_FUNCTION__ << '\n';
}
void
hello_world()
{
std::lock_guard<std::mutex> lock( cout_mutex );
std::cout << __PRETTY_FUNCTION__ << '\n';
}
};
void
hello_world_thread()
{
thread_local Foo foo;
// must access, or the thread local variable may not be instantiated
foo.hello_world();
// keep the thread around momentarily
std::this_thread::sleep_for( std::chrono::milliseconds( 100 ) );
}
int main()
{
for ( int i = 0; i < 100; ++i )
{
std::list<std::thread> threads;
for ( int j = 0; j < 10; ++j )
{
std::thread thread( hello_world_thread );
threads.push_back( std::move( thread ) );
}
while ( ! threads.empty() )
{
threads.front().join();
threads.pop_front();
}
}
}
Compiler version:
$ g++ --version
g++ (GCC) 4.8.1
Copyright (C) 2013 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
GCC build options:
--enable-shared
--enable-threads=posix
--enable-__cxa_atexit
--enable-clocale=gnu
--enable-cxx-flags='-fno-omit-frame-pointer -g3'
--enable-languages=c,c++
--enable-libstdcxx-time=rt
--enable-checking=release
--enable-build-with-cxx
--disable-werror
--disable-multilib
--disable-bootstrap
--with-system-zlib
Program compilation options:
g++ -std=gnu++11 -Og -g3 -Wall -Wextra -fno-omit-frame-pointer thread_local.cc
valgrind version:
$ valgrind --version
valgrind-3.8.1
Valgrind options:
valgrind --leak-check=full --verbose ./a.out > /dev/null
Tail-end of valgrind output:
==1786== HEAP SUMMARY:
==1786== in use at exit: 24,000 bytes in 1,000 blocks
==1786== total heap usage: 3,604 allocs, 2,604 frees, 287,616 bytes allocated
==1786==
==1786== Searching for pointers to 1,000 not-freed blocks
==1786== Checked 215,720 bytes
==1786==
==1786== 24,000 bytes in 1,000 blocks are definitely lost in loss record 1 of 1
==1786== at 0x4C29969: operator new(unsigned long, std::nothrow_t const&) (vg_replace_malloc.c:329)
==1786== by 0x4E8E53E: __cxa_thread_atexit (atexit_thread.cc:119)
==1786== by 0x401036: hello_world_thread() (thread_local.cc:34)
==1786== by 0x401416: std::thread::_Impl<std::_Bind_simple<void (*())()> >::_M_run() (functional:1732)
==1786== by 0x4EE4830: execute_native_thread_routine (thread.cc:84)
==1786== by 0x5A10E99: start_thread (pthread_create.c:308)
==1786== by 0x573DCCC: clone (clone.S:112)
==1786==
==1786== LEAK SUMMARY:
==1786== definitely lost: 24,000 bytes in 1,000 blocks
==1786== indirectly lost: 0 bytes in 0 blocks
==1786== possibly lost: 0 bytes in 0 blocks
==1786== still reachable: 0 bytes in 0 blocks
==1786== suppressed: 0 bytes in 0 blocks
==1786==
==1786== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)
--1786--
--1786-- used_suppression: 2 dl-hack3-cond-1
==1786==
==1786== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 2 from 2)
Constructors and destructors were run once for each thread:
$ ./a.out | grep 'Foo::Foo' | wc -l
1000
$ ./a.out | grep hello_world | wc -l
1000
$ ./a.out | grep 'Foo::~Foo' | wc -l
1000
Notes:
If you change the number of threads created, the number of leaked blocks matches the number of threads.
The code is structured in such a way that might permit resource reuse (i.e. the leaked block) if GCC were so implemented.
From the valgrind stacktrace, thread_local.cc:34 is the line: thread_local Foo foo;
Due to the sleep_for() call, a program run takes about 10 seconds or so.
Any idea if this memory leak is in GCC, a result of my config options, or is some bug in my program?

Answer 1

It seems that the leak comes from the dynamic initialization.
Here is an example with an int :
thread_local int num=4; //static initialization
The last example does not leak. I tried it with 2 threads and no leak at all.
But now :
int func()
{
return 4;
}
thread_local int num2=func(); //dynamic initialization
This one leak ! With 2 threads it gives total heap uage: 8 allocs, 6 frees, 428 bytes allocated...
I would suggest you to use a workaround like :
thread_local Foo *foo = new Foo; //dynamic initialization
No forget at the end of the thread execution to do :
delete foo;
But the last example as one problem : What if the thread exit with error before your delete ? Leak again...
It seems that there is no great solution. Maybe we should report that to the g++ developers about that ?

Answer 2

try removing thread_local and using the following code
void
hello_world_thread()
{
Foo foo;
// must access, or the thread local variable may not be instantiated
foo.hello_world();
// keep the thread around momentarily
std::this_thread::sleep_for( std::chrono::milliseconds( 100 ) );
}
foo within hello_world_thread should be in the local stack for every thread. so every thread will maintain its own copy of foo. no need to explicitly marking it as thread_local. A thread_local should be used in a context when you have something like static or namespace level variable but you want each variable to maintain its own copy for every thread.
Regards
Kajal