Valgrind detected wrong allocation on heap?

Question

I wrote a C++ program that ran correctly over valgrind but I have notices something really strange:

==23369== HEAP SUMMARY:
==23369==     in use at exit: 0 bytes in 0 blocks
==23369==   total heap usage: 1 allocs, 1 frees, 72,704 bytes allocated

Where this allocation came from? I only called sbrk() and mmap() but never used malloc or new so how can I know what is causing this?

---

Update: I removed #include <iostream> and it worked! Still I didn't get why is this. I just included a library which I didn't use... (I have cassert and removing #include <iostream> caused no problems

Answer 1

You may try to debug your program(We need to install the debug symbol for libstdc++ at first, see this):

1. start the program with gdb

gdb $YOUR_PROGRAM

2. Add debug breakpoint

b malloc

3. Run the program

r

4. Get the call trace after hitting breakpoint:

bt

On my PC with this nearly empty program:

#include <iostream>
int main(int argc, char* argv[]) { return 0; }

We get the call trace only once (Same as the OP, and the size 72,704 is exactly the same!):

#0  __GI___libc_malloc (bytes=bytes@entry=72704) at malloc.c:3043
#1  0x00007ffff7e2eb8a in (anonymous namespace)::pool::pool (
    this=0x7ffff7fc3b40 <(anonymous namespace)::emergency_pool>)
    at ../../../../src/libstdc++-v3/libsupc++/eh_alloc.cc:123
#2  __static_initialization_and_destruction_0 (__priority=65535, __initialize_p=1)
    at ../../../../src/libstdc++-v3/libsupc++/eh_alloc.cc:262
#3  _GLOBAL__sub_I_eh_alloc.cc(void) ()
    at ../../../../src/libstdc++-v3/libsupc++/eh_alloc.cc:338
#4  0x00007ffff7fdfdbe in ?? () from /lib64/ld-linux-x86-64.so.2
#5  0x00007ffff7fdfea8 in ?? () from /lib64/ld-linux-x86-64.so.2
#6  0x00007ffff7fcf10a in ?? () from /lib64/ld-linux-x86-64.so.2

If we remove the #include <iostream>, we won't hit the malloc again! Now we confirmed that the malloc is called by libstdc++(The STL) library itself.

There is a global object emergency_pool in STL, which will call malloc during destructing to reserve memory, if we include iostream it will be constructed before main.