C++ libevent usage (memory leak and delete operator)

Question

I have two files:

// event_test_delete.cpp
#include <event.h>

int main() {
        event_base* ev;

        ev = event_init();
        delete ev;

        return 0; 
}

And

// event_test_free.cpp
#include <event.h>
#include <cstdlib>

int main() {
        event_base* ev;

        ev = event_init();
        free(ev);

        return 0; 
}

When I compile (g++ event_test_delete.cpp -levent -o event_test_delete.o) event_test_delete.cpp I get an error:

event_test_delete.cpp: In function ‘int main()’:
event_test_delete.cpp:8:9: warning: possible problem detected in invocation of delete operator:
event_test_delete.cpp:5:14: warning: ‘ev’ has incomplete type
/usr/include/event.h:211:8: warning: forward declaration of ‘struct event_base’
event_test_delete.cpp:8:9: note: neither the destructor nor the class-specific operator delete will be called, even if they are declared when the class is defined.

But when I compile g++ event_test_free.cpp -levent -o event_test_free.o event_test_free.cpp I don`t get the same error why?

And the second question is (using valgrind) why there is a memory leak?

Valgrind output for first file: (why here is an Mismatched free() / delete / delete [] ?)

azat:~/Desktop/event_test$ valgrind --show-reachable=yes --leak-check=full ./event_test_delete.o 
==4135== Memcheck, a memory error detector
==4135== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==4135== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==4135== Command: ./event_test_delete.o
==4135== 
==4135== Mismatched free() / delete / delete []
==4135==    at 0x4023881: operator delete(void*) (vg_replace_malloc.c:387)
==4135==    by 0x8048571: main (in /home/azat/Desktop/event_test/event_test_delete.o)
==4135==  Address 0x4323028 is 0 bytes inside a block of size 944 alloc'd
==4135==    at 0x402328F: calloc (vg_replace_malloc.c:467)
==4135==    by 0x4047DA7: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x8048561: main (in /home/azat/Desktop/event_test/event_test_delete.o)
==4135== 
==4135== 
==4135== HEAP SUMMARY:
==4135==     in use at exit: 672 bytes in 5 blocks
==4135==   total heap usage: 6 allocs, 1 frees, 1,616 bytes allocated
==4135== 
==4135== 8 bytes in 1 blocks are indirectly lost in loss record 1 of 5
==4135==    at 0x4023F50: malloc (vg_replace_malloc.c:236)
==4135==    by 0x4047C7D: event_base_priority_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047E8B: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x8048561: main (in /home/azat/Desktop/event_test/event_test_delete.o)
==4135== 
==4135== 12 (4 direct, 8 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 5
==4135==    at 0x402328F: calloc (vg_replace_malloc.c:467)
==4135==    by 0x4047C2D: event_base_priority_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047E8B: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x8048561: main (in /home/azat/Desktop/event_test/event_test_delete.o)
==4135== 
==4135== 256 bytes in 1 blocks are indirectly lost in loss record 3 of 5
==4135==    at 0x402328F: calloc (vg_replace_malloc.c:467)
==4135==    by 0x4056192: ??? (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047E46: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x8048561: main (in /home/azat/Desktop/event_test/event_test_delete.o)
==4135== 
==4135== 384 bytes in 1 blocks are indirectly lost in loss record 4 of 5
==4135==    at 0x4023F50: malloc (vg_replace_malloc.c:236)
==4135==    by 0x405616C: ??? (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047E46: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x8048561: main (in /home/azat/Desktop/event_test/event_test_delete.o)
==4135== 
==4135== 660 (20 direct, 640 indirect) bytes in 1 blocks are definitely lost in loss record 5 of 5
==4135==    at 0x402328F: calloc (vg_replace_malloc.c:467)
==4135==    by 0x4056157: ??? (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047E46: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4135==    by 0x8048561: main (in /home/azat/Desktop/event_test/event_test_delete.o)
==4135== 
==4135== LEAK SUMMARY:
==4135==    definitely lost: 24 bytes in 2 blocks
==4135==    indirectly lost: 648 bytes in 3 blocks
==4135==      possibly lost: 0 bytes in 0 blocks
==4135==    still reachable: 0 bytes in 0 blocks
==4135==         suppressed: 0 bytes in 0 blocks
==4135== 
==4135== For counts of detected and suppressed errors, rerun with: -v
==4135== ERROR SUMMARY: 3 errors from 3 contexts (suppressed: 28 from 7)

And for second file

azat:~/Desktop/event_test$ valgrind --show-reachable=yes --leak-check=full ./event_test_free.o 
==4140== Memcheck, a memory error detector
==4140== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==4140== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==4140== Command: ./event_test_free.o
==4140== 
==4140== 
==4140== HEAP SUMMARY:
==4140==     in use at exit: 672 bytes in 5 blocks
==4140==   total heap usage: 6 allocs, 1 frees, 1,616 bytes allocated
==4140== 
==4140== 8 bytes in 1 blocks are indirectly lost in loss record 1 of 5
==4140==    at 0x4023F50: malloc (vg_replace_malloc.c:236)
==4140==    by 0x4047C7D: event_base_priority_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047E8B: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x8048531: main (in /home/azat/Desktop/event_test/event_test_free.o)
==4140== 
==4140== 12 (4 direct, 8 indirect) bytes in 1 blocks are definitely lost in loss record 2 of 5
==4140==    at 0x402328F: calloc (vg_replace_malloc.c:467)
==4140==    by 0x4047C2D: event_base_priority_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047E8B: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x8048531: main (in /home/azat/Desktop/event_test/event_test_free.o)
==4140== 
==4140== 256 bytes in 1 blocks are indirectly lost in loss record 3 of 5
==4140==    at 0x402328F: calloc (vg_replace_malloc.c:467)
==4140==    by 0x4056192: ??? (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047E46: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x8048531: main (in /home/azat/Desktop/event_test/event_test_free.o)
==4140== 
==4140== 384 bytes in 1 blocks are indirectly lost in loss record 4 of 5
==4140==    at 0x4023F50: malloc (vg_replace_malloc.c:236)
==4140==    by 0x405616C: ??? (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047E46: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x8048531: main (in /home/azat/Desktop/event_test/event_test_free.o)
==4140== 
==4140== 660 (20 direct, 640 indirect) bytes in 1 blocks are definitely lost in loss record 5 of 5
==4140==    at 0x402328F: calloc (vg_replace_malloc.c:467)
==4140==    by 0x4056157: ??? (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047E46: event_base_new (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x4047EF6: event_init (in /usr/lib/libevent-1.4.so.2.1.3)
==4140==    by 0x8048531: main (in /home/azat/Desktop/event_test/event_test_free.o)
==4140== 
==4140== LEAK SUMMARY:
==4140==    definitely lost: 24 bytes in 2 blocks
==4140==    indirectly lost: 648 bytes in 3 blocks
==4140==      possibly lost: 0 bytes in 0 blocks
==4140==    still reachable: 0 bytes in 0 blocks
==4140==         suppressed: 0 bytes in 0 blocks
==4140== 
==4140== For counts of detected and suppressed errors, rerun with: -v
==4140== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 28 from 7)

Can you show us implementation of event_init(). Note, that delete can be called only on memory, allocated by new. It seems that valgrind is pointing you to this error in first file. Maybe compiler is doing the same. — beduin
– beduin, Commented May 6, 2011 at 9:02

Bo Persson · Accepted Answer · 2011-05-06 09:34:54Z

2

When using delete the compiler needs to see the type of the object pointed to, to determine if there are any destructors it needs to call at that point.

On the other hand, valgrind seems to say that the memory is allocated using malloc and calloc. In that case, you should not use delete at all, but perhaps free.

In the second case, when using free, valgrind still complains on memory leaks. One possibility here is that the event object contains pointers to other allocations that also need to be freed.

In that case, there should be another function event_free or event_release you should call, to return the event object. Do you have one of those?

answered May 6, 2011 at 9:34

Bo Persson

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3 Comments

azat Over a year ago

event_init is a libevent function, monkey.org/~provos/libevent/doxygen-1.4.10

azat Over a year ago

memcached (memcached.org) us using libevent too, but I try to profile it with valgrind, and there is no memory leaks. But in example that give me libevent official page, there is a memory leaks. But the question is, how can I avoid this memory leaks in my case?

Bo Persson Over a year ago

@azat - The link goes to a typical doxygen page, it describes each function independently, but says nothing about what order you should call them. But as you get an event_base* from event_init(), one could guess that event_base_free would be a good call to free it again. But that's just a guess! I don't know for sure.

Ferenc Deak · Accepted Answer · 2011-05-06 09:26:15Z

1

First Question: delete needs to know the type of the pointer it deletes, since it might need to call a destructor.

Seconds Question: see the comment below the question. We need to know what event_init does and how it allocates the memory to advice about existing memory leaks. However, a good advice: Trust Valgrind.

answered May 6, 2011 at 9:26

Ferenc Deak

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1 Comment

azat Over a year ago

see comments to the next anwser

Joakim · Accepted Answer · 2012-11-15 13:24:38Z

0

Libevent is not written in C++, and hence it does not use destructors. You should never use delete on code that hasn't been allocated using new.

If you read the libevent manual under "deallocating an event base" it states that you should use:

void event_base_free(struct event_base *base);

Also the event_new function for allocating an event base is deprecated (since it's not thread safe), you should instead use:

struct event_base *event_base_new(void);

answered Nov 15, 2012 at 13:24

Joakim

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