Memory leak detected in Getline() using Valgrind

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Solution 1:

Upon invoking the

exit()

function to exit a C++ program, the destructors of objects may not be executed, leading to potential memory leaks that may be flagged by Valgrind.


Solution 2:


Although this post is almost a year old, I stumbled upon it while searching for a solution to a getline() memory leak problem. To reproduce this particular issue, it seems sufficient to simply define a std::string variable and not properly exit the program. To help others who may encounter this issue, I will provide concise instructions.

Given the following:

leaky.cpp:

#include 
int main(int argc, char ** argv) {
    std::string x = "x";
    exit(1);
    return 0;
}

Compile string:

g++ -g -Wall -Wpedantic --std=gnu++11 leaky.cpp -o leaky

Valgrind invocation:

valgrind --tool=memcheck --leak-check=full ./leaky

This action will expose the presence of a leak.

==4434== Memcheck, a memory error detector
==4434== Copyright (C) 2002-2013, and GNU GPL'd, by Julian Seward et al.
==4434== Using Valgrind-3.10.0.SVN and LibVEX; rerun with -h for copyright info
==4434== Command: ./leaky
==4434== 
==4434== 
==4434== HEAP SUMMARY:
==4434==     in use at exit: 16 bytes in 1 blocks
==4434==   total heap usage: 1 allocs, 0 frees, 16 bytes allocated
==4434== 
==4434== 16 bytes in 1 blocks are possibly lost in loss record 1 of 1
==4434==    at 0x402A6DC: operator new(unsigned int) (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==4434==    by 0x40F8213: std::string::_Rep::_S_create(unsigned int, unsigned int, std::allocator const&) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==4434==    by 0x40FA125: char* std::string::_S_construct(char const*, char const*, std::allocator const&, std::forward_iterator_tag) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==4434==    by 0x40FA7AF: std::basic_string, std::allocator >::basic_string(char const*, std::allocator const&) (in /usr/lib/i386-linux-gnu/libstdc++.so.6.0.19)
==4434==    by 0x804875E: main (leaky.cpp:3)
==4434== 
==4434== LEAK SUMMARY:
==4434==    definitely lost: 0 bytes in 0 blocks
==4434==    indirectly lost: 0 bytes in 0 blocks
==4434==      possibly lost: 16 bytes in 1 blocks
==4434==    still reachable: 0 bytes in 0 blocks
==4434==         suppressed: 0 bytes in 0 blocks
==4434== 
==4434== For counts of detected and suppressed errors, rerun with: -v
==4434== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

To address the query, it is worth mentioning that invoking exit(1) will terminate the program without calling any destructors, to my limited knowledge. I am relatively new to using C++ and cannot claim to be an authority on the subject.



Solution 3:


It’s possible that the behavior of the C++ class in question with pointers is indicating a potential leak, according to Valgrind.

In a certain project, I receive diagnostic information when allocating arrays and adjusting the pointer to the third element. This is done to store meta-information in indices [-2] and [-1]. However, I ensure there is no leakage by properly restoring the pointer and freeing the arrays.

Valgrind detects that the objects are being referenced through interior pointers instead of the pointers to their base address, indicating a potential leak as there is no pointer available for freeing the object.

In programs that allocate a large object and distribute pieces of it to other modules via pointers, there is a possibility of encountering leaky situations where the big object is leaked. The presence of interior pointers can serve as an indication of a leak.

Perhaps

getline

has formed a close relationship with the representation of

basic::string<>

and engages in similar behavior. Upon duplicating the object, the newly created object does not exhibit any unusual behavior, but rather simply refers to the string data using the original address. The previous object is then discarded, freeing up the associated data.

Just a hypothesis.

In the program mentioned earlier, I made modifications to accommodate Valgrind by adding an additional pointer to the base address in the vector objects that manage the arrays. This extra pointer is exclusively available when the software is built for Valgrind debugging, along with other features like the Valgrind client request API.

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