[c++] How to track down a "double free or corruption" error

When I run my (C++) program it crashes with this error.

* glibc detected * ./load: double free or corruption (!prev): 0x0000000000c6ed50 ***

How can I track down the error?

I tried using print (std::cout) statements, without success. Could gdb make this easier?

There are at least two possible situations:

1. you are deleting the same entity twice
2. you are deleting something that wasn't allocated

For the first one I strongly suggest NULL-ing all deleted pointers.

You have three options:

1. overload new and delete and track the allocations
2. yes, use gdb -- then you'll get a backtrace from your crash, and that'll probably be very helpful
3. as suggested -- use Valgrind -- it isn't easy to get into, but it will save you time thousandfold in the future...

Three basic rules:

1. Set pointer to NULL after free
2. Check for NULL before freeing.
3. Initialise pointer to NULL in the start.

Combination of these three works quite well.

I know this is a very old thread, but it is the top google search for this error, and none of the responses mention a common cause of the error.

Which is closing a file you've already closed.

If you're not paying attention and have two different functions close the same file, then the second one will generate this error.

If you're using glibc, you can set the MALLOC_CHECK_ environment variable to 2, this will cause glibc to use an error tolerant version of malloc, which will cause your program to abort at the point where the double free is done.

You can set this from gdb by using the set environment MALLOC_CHECK_ 2 command before running your program; the program should abort, with the free() call visible in the backtrace.

see the man page for malloc() for more information

With modern C++ compilers you can use sanitizers to track.

Sample example :

My program:

$cat d_free.cxx 
#include<iostream>

using namespace std;

int main()

{
   int * i = new int();
   delete i;
   //i = NULL;
   delete i;
}

Compile with address sanitizers :

# g++-7.1 d_free.cxx -Wall -Werror -fsanitize=address -g

Execute :

# ./a.out 
=================================================================
==4836==ERROR: AddressSanitizer: attempting double-free on 0x602000000010 in thread T0:
    #0 0x7f35b2d7b3c8 in operator delete(void*, unsigned long) /media/sf_shared/gcc-7.1.0/libsanitizer/asan/asan_new_delete.cc:140
    #1 0x400b2c in main /media/sf_shared/jkr/cpp/d_free/d_free.cxx:11
    #2 0x7f35b2050c04 in __libc_start_main (/lib64/libc.so.6+0x21c04)
    #3 0x400a08  (/media/sf_shared/jkr/cpp/d_free/a.out+0x400a08)

0x602000000010 is located 0 bytes inside of 4-byte region [0x602000000010,0x602000000014)
freed by thread T0 here:
    #0 0x7f35b2d7b3c8 in operator delete(void*, unsigned long) /media/sf_shared/gcc-7.1.0/libsanitizer/asan/asan_new_delete.cc:140
    #1 0x400b1b in main /media/sf_shared/jkr/cpp/d_free/d_free.cxx:9
    #2 0x7f35b2050c04 in __libc_start_main (/lib64/libc.so.6+0x21c04)

previously allocated by thread T0 here:
    #0 0x7f35b2d7a040 in operator new(unsigned long) /media/sf_shared/gcc-7.1.0/libsanitizer/asan/asan_new_delete.cc:80
    #1 0x400ac9 in main /media/sf_shared/jkr/cpp/d_free/d_free.cxx:8
    #2 0x7f35b2050c04 in __libc_start_main (/lib64/libc.so.6+0x21c04)

SUMMARY: AddressSanitizer: double-free /media/sf_shared/gcc-7.1.0/libsanitizer/asan/asan_new_delete.cc:140 in operator delete(void*, unsigned long)
==4836==ABORTING

To learn more about sanitizers you can check this or this or any modern c++ compilers (e.g. gcc, clang etc.) documentations.

You can use valgrind to debug it.

#include<stdio.h>
#include<stdlib.h>

int main()
{
 char *x = malloc(100);
 free(x);
 free(x);
 return 0;
}

[sand@PS-CNTOS-64-S11 testbox]$ vim t1.c
[sand@PS-CNTOS-64-S11 testbox]$ cc -g t1.c -o t1
[sand@PS-CNTOS-64-S11 testbox]$ ./t1
*** glibc detected *** ./t1: double free or corruption (top): 0x00000000058f7010 ***
======= Backtrace: =========
/lib64/libc.so.6[0x3a3127245f]
/lib64/libc.so.6(cfree+0x4b)[0x3a312728bb]
./t1[0x400500]
/lib64/libc.so.6(__libc_start_main+0xf4)[0x3a3121d994]
./t1[0x400429]
======= Memory map: ========
00400000-00401000 r-xp 00000000 68:02 30246184                           /home/sand/testbox/t1
00600000-00601000 rw-p 00000000 68:02 30246184                           /home/sand/testbox/t1
058f7000-05918000 rw-p 058f7000 00:00 0                                  [heap]
3a30e00000-3a30e1c000 r-xp 00000000 68:03 5308733                        /lib64/ld-2.5.so
3a3101b000-3a3101c000 r--p 0001b000 68:03 5308733                        /lib64/ld-2.5.so
3a3101c000-3a3101d000 rw-p 0001c000 68:03 5308733                        /lib64/ld-2.5.so
3a31200000-3a3134e000 r-xp 00000000 68:03 5310248                        /lib64/libc-2.5.so
3a3134e000-3a3154e000 ---p 0014e000 68:03 5310248                        /lib64/libc-2.5.so
3a3154e000-3a31552000 r--p 0014e000 68:03 5310248                        /lib64/libc-2.5.so
3a31552000-3a31553000 rw-p 00152000 68:03 5310248                        /lib64/libc-2.5.so
3a31553000-3a31558000 rw-p 3a31553000 00:00 0
3a41c00000-3a41c0d000 r-xp 00000000 68:03 5310264                        /lib64/libgcc_s-4.1.2-20080825.so.1
3a41c0d000-3a41e0d000 ---p 0000d000 68:03 5310264                        /lib64/libgcc_s-4.1.2-20080825.so.1
3a41e0d000-3a41e0e000 rw-p 0000d000 68:03 5310264                        /lib64/libgcc_s-4.1.2-20080825.so.1
2b1912300000-2b1912302000 rw-p 2b1912300000 00:00 0
2b191231c000-2b191231d000 rw-p 2b191231c000 00:00 0
7ffffe214000-7ffffe229000 rw-p 7ffffffe9000 00:00 0                      [stack]
7ffffe2b0000-7ffffe2b4000 r-xp 7ffffe2b0000 00:00 0                      [vdso]
ffffffffff600000-ffffffffffe00000 ---p 00000000 00:00 0                  [vsyscall]
Aborted
[sand@PS-CNTOS-64-S11 testbox]$


[sand@PS-CNTOS-64-S11 testbox]$ vim t1.c
[sand@PS-CNTOS-64-S11 testbox]$ cc -g t1.c -o t1
[sand@PS-CNTOS-64-S11 testbox]$ valgrind --tool=memcheck ./t1
==20859== Memcheck, a memory error detector
==20859== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==20859== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==20859== Command: ./t1
==20859==
==20859== Invalid free() / delete / delete[]
==20859==    at 0x4A05A31: free (vg_replace_malloc.c:325)
==20859==    by 0x4004FF: main (t1.c:8)
==20859==  Address 0x4c26040 is 0 bytes inside a block of size 100 free'd
==20859==    at 0x4A05A31: free (vg_replace_malloc.c:325)
==20859==    by 0x4004F6: main (t1.c:7)
==20859==
==20859==
==20859== HEAP SUMMARY:
==20859==     in use at exit: 0 bytes in 0 blocks
==20859==   total heap usage: 1 allocs, 2 frees, 100 bytes allocated
==20859==
==20859== All heap blocks were freed -- no leaks are possible
==20859==
==20859== For counts of detected and suppressed errors, rerun with: -v
==20859== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
[sand@PS-CNTOS-64-S11 testbox]$


[sand@PS-CNTOS-64-S11 testbox]$ valgrind --tool=memcheck --leak-check=full ./t1
==20899== Memcheck, a memory error detector
==20899== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==20899== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==20899== Command: ./t1
==20899==
==20899== Invalid free() / delete / delete[]
==20899==    at 0x4A05A31: free (vg_replace_malloc.c:325)
==20899==    by 0x4004FF: main (t1.c:8)
==20899==  Address 0x4c26040 is 0 bytes inside a block of size 100 free'd
==20899==    at 0x4A05A31: free (vg_replace_malloc.c:325)
==20899==    by 0x4004F6: main (t1.c:7)
==20899==
==20899==
==20899== HEAP SUMMARY:
==20899==     in use at exit: 0 bytes in 0 blocks
==20899==   total heap usage: 1 allocs, 2 frees, 100 bytes allocated
==20899==
==20899== All heap blocks were freed -- no leaks are possible
==20899==
==20899== For counts of detected and suppressed errors, rerun with: -v
==20899== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 4 from 4)
[sand@PS-CNTOS-64-S11 testbox]$

One possible fix:

#include<stdio.h>
#include<stdlib.h>

int main()
{
 char *x = malloc(100);
 free(x);
 x=NULL;
 free(x);
 return 0;
}

[sand@PS-CNTOS-64-S11 testbox]$ vim t1.c
[sand@PS-CNTOS-64-S11 testbox]$ cc -g t1.c -o t1
[sand@PS-CNTOS-64-S11 testbox]$ ./t1
[sand@PS-CNTOS-64-S11 testbox]$

[sand@PS-CNTOS-64-S11 testbox]$ valgrind --tool=memcheck --leak-check=full ./t1
==20958== Memcheck, a memory error detector
==20958== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==20958== Using Valgrind-3.5.0 and LibVEX; rerun with -h for copyright info
==20958== Command: ./t1
==20958==
==20958==
==20958== HEAP SUMMARY:
==20958==     in use at exit: 0 bytes in 0 blocks
==20958==   total heap usage: 1 allocs, 1 frees, 100 bytes allocated
==20958==
==20958== All heap blocks were freed -- no leaks are possible
==20958==
==20958== For counts of detected and suppressed errors, rerun with: -v
==20958== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 4 from 4)
[sand@PS-CNTOS-64-S11 testbox]$

Check out the blog on using Valgrind Link

You can use gdb, but I would first try Valgrind. See the quick start guide.

Briefly, Valgrind instruments your program so it can detect several kinds of errors in using dynamically allocated memory, such as double frees and writes past the end of allocated blocks of memory (which can corrupt the heap). It detects and reports the errors as soon as they occur, thus pointing you directly to the cause of the problem.

Are you using smart pointers such as Boost shared_ptr? If so, check if you are directly using the raw pointer anywhere by calling get(). I've found this to be quite a common problem.

For example, imagine a scenario where a raw pointer is passed (maybe as a callback handler, say) to your code. You might decide to assign this to a smart pointer in order to cope with reference counting etc. Big mistake: your code doesn't own this pointer unless you take a deep copy. When your code is done with the smart pointer it will destroy it and attempt to destroy the memory it points to since it thinks that no-one else needs it, but the calling code will then try to delete it and you'll get a double free problem.

Of course, that might not be your problem here. At it's simplest here's an example which shows how it can happen. The first delete is fine but the compiler senses that it's already deleted that memory and causes a problem. That's why assigning 0 to a pointer immediately after deletion is a good idea.

int main(int argc, char* argv[])
{
    char* ptr = new char[20];

    delete[] ptr;
    ptr = 0;  // Comment me out and watch me crash and burn.
    delete[] ptr;
}

Edit: changed delete to delete[], as ptr is an array of char.