Passing variable by reference to several threads corrupts heap

This is part of a test for thread safety. I'm running an anonymous lambda in different threads.

I use the variable i as thread id.

Originally I passed every variable from main scope by using [&], but this corrupts the heap.

Solved it now by passing i by value, but for the life of me I can't figure out why this would cause problems on the heap since the threads are only reading i.

Can anyone explain?

---

Minimal compilable example producing error:

#include <thread>
#include <vector>

using namespace std;

int main(int argc, char** argv) {
    vector<thread> threads;
    vector<string> vec1;
    vector<string> vec2;
    for (int i = 0; i < 2; i++) {
        threads.push_back(
            thread([&vec1, &vec2, &i]() {
                for (int j = 0; j < 10; j++) {
                    const string str = "foo";
                    if (i == 0) {
                        vec1.push_back(str);
                    } else {
                        vec2.push_back(str);
                    }
                }
            })
        );
    }

    for (auto& thread : threads) {
        thread.join();
    }

    return 0;
}

Output:

*** Error in `/vagrant/bin/TempFileTest': double free or corruption (fasttop): 0x00007f00240008c0 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x777e5)[0x7f002a0e97e5]
/lib/x86_64-linux-gnu/libc.so.6(+0x8037a)[0x7f002a0f237a]
/lib/x86_64-linux-gnu/libc.so.6(cfree+0x4c)[0x7f002a0f653c]
/vagrant/bin/TempFileTest(_ZNSt6vectorINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESaIS5_EE19_M_emplace_back_auxIJRKS5_EEEvDpOT_+0x1a3)[0x4021b3]
/vagrant/bin/TempFileTest[0x401e83]
/usr/lib/x86_64-linux-gnu/libstdc++.so.6(+0xb8c80)[0x7f002a70ac80]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x76ba)[0x7f002a9db6ba]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d)[0x7f002a17941d]
======= Memory map: ========
00400000-00403000 r-xp 00000000 08:02 2622834                            /vagrant/bin/TempFileTest
00602000-00603000 r--p 00002000 08:02 2622834                            /vagrant/bin/TempFileTest
00603000-00604000 rw-p 00003000 08:02 2622834                            /vagrant/bin/TempFileTest
02182000-021b4000 rw-p 00000000 00:00 0                                  [heap]
7f001c000000-7f001c021000 rw-p 00000000 00:00 0 
7f001c021000-7f0020000000 ---p 00000000 00:00 0 
7f0024000000-7f0024021000 rw-p 00000000 00:00 0 
7f0024021000-7f0028000000 ---p 00000000 00:00 0 
7f0028d67000-7f0028d68000 ---p 00000000 00:00 0 
7f0028d68000-7f0029568000 rw-p 00000000 00:00 0 
7f0029568000-7f0029569000 ---p 00000000 00:00 0 
7f0029569000-7f0029d69000 rw-p 00000000 00:00 0 
7f0029d69000-7f0029e71000 r-xp 00000000 00:32 313                        /lib/x86_64-linux-gnu/libm-2.23.so
7f0029e71000-7f002a070000 ---p 00108000 00:32 313                        /lib/x86_64-linux-gnu/libm-2.23.so
7f002a070000-7f002a071000 r--p 00107000 00:32 313                        /lib/x86_64-linux-gnu/libm-2.23.so
7f002a071000-7f002a072000 rw-p 00108000 00:32 313                        /lib/x86_64-linux-gnu/libm-2.23.so
7f002a072000-7f002a232000 r-xp 00000000 00:32 45                         /lib/x86_64-linux-gnu/libc-2.23.so
7f002a232000-7f002a432000 ---p 001c0000 00:32 45                         /lib/x86_64-linux-gnu/libc-2.23.so
7f002a432000-7f002a436000 r--p 001c0000 00:32 45                         /lib/x86_64-linux-gnu/libc-2.23.so
7f002a436000-7f002a438000 rw-p 001c4000 00:32 45                         /lib/x86_64-linux-gnu/libc-2.23.so
7f002a438000-7f002a43c000 rw-p 00000000 00:00 0 
7f002a43c000-7f002a452000 r-xp 00000000 00:32 314                        /lib/x86_64-linux-gnu/libgcc_s.so.1
7f002a452000-7f002a651000 ---p 00016000 00:32 314                        /lib/x86_64-linux-gnu/libgcc_s.so.1
7f002a651000-7f002a652000 rw-p 00015000 00:32 314                        /lib/x86_64-linux-gnu/libgcc_s.so.1
7f002a652000-7f002a7c4000 r-xp 00000000 00:32 311                        /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f002a7c4000-7f002a9c4000 ---p 00172000 00:32 311                        /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f002a9c4000-7f002a9ce000 r--p 00172000 00:32 311                        /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f002a9ce000-7f002a9d0000 rw-p 0017c000 00:32 311                        /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.21
7f002a9d0000-7f002a9d4000 rw-p 00000000 00:00 0 
7f002a9d4000-7f002a9ec000 r-xp 00000000 00:32 61                         /lib/x86_64-linux-gnu/libpthread-2.23.so
7f002a9ec000-7f002abeb000 ---p 00018000 00:32 61                         /lib/x86_64-linux-gnu/libpthread-2.23.so
7f002abeb000-7f002abec000 r--p 00017000 00:32 61                         /lib/x86_64-linux-gnu/libpthread-2.23.so
7f002abec000-7f002abed000 rw-p 00018000 00:32 61                         /lib/x86_64-linux-gnu/libpthread-2.23.so
7f002abed000-7f002abf1000 rw-p 00000000 00:00 0 
7f002abf1000-7f002ac17000 r-xp 00000000 00:32 42                         /lib/x86_64-linux-gnu/ld-2.23.so
7f002adf6000-7f002adfc000 rw-p 00000000 00:00 0 
7f002ae15000-7f002ae16000 rw-p 00000000 00:00 0 
7f002ae16000-7f002ae17000 r--p 00025000 00:32 42                         /lib/x86_64-linux-gnu/ld-2.23.so
7f002ae17000-7f002ae18000 rw-p 00026000 00:32 42                         /lib/x86_64-linux-gnu/ld-2.23.so
7f002ae18000-7f002ae19000 rw-p 00000000 00:00 0 
7fff30694000-7fff306b5000 rw-p 00000000 00:00 0                          [stack]
7fff30761000-7fff30763000 r--p 00000000 00:00 0                          [vvar]
7fff30763000-7fff30765000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
Aborted (core dumped)

Minimal compilable example without error (notice no & on i):

#include <thread>
#include <vector>

using namespace std;

int main(int argc, char** argv) {
    vector<thread> threads;
    vector<string> vec1;
    vector<string> vec2;
    for (int i = 0; i < 2; i++) {
        threads.push_back(
            thread([&vec1, &vec2, i]() {
                for (int j = 0; j < 10; j++) {
                    const string str = "foo";
                    if (i == 0) {
                        vec1.push_back(str);
                    } else {
                        vec2.push_back(str);
                    }
                }
            })
        );
    }

    for (auto& thread : threads) {
        thread.join();
    }

    return 0;
}

---

I'm using:

Ubuntu 16.04

gcc 5.4.0