xerces-c: Xml parsing multiple files - c++

Question

I'm atempting to learn xerces-c and was following this tutorial online.
http://www.yolinux.com/TUTORIALS/XML-Xerces-C.html
I was able to get the tutorial to compile and run through a memory checker (valgrind) with no problems however when I made alterations to the program slightly, the memory checker returned some potential leak bytes. I only added a few extra lines to main to allow the program to read two files instead of one.
int main()
{
string configFile="sample.xml"; // stat file. Get ambigious segfault otherwise.
GetConfig appConfig;
appConfig.readConfigFile(configFile);
cout << "Application option A=" << appConfig.getOptionA() << endl;
cout << "Application option B=" << appConfig.getOptionB() << endl;
// Added code
configFile = "sample1.xml";
appConfig.readConfigFile(configFile);
cout << "Application option A=" << appConfig.getOptionA() << endl;
cout << "Application option B=" << appConfig.getOptionB() << endl;
return 0;
}
I was wondering why is it when I added the extra lines of code to read in another xml file, it would result in the following output?
==776== Using Valgrind-3.6.0 and LibVEX; rerun with -h for copyright info
==776== Command: ./a.out
==776==
Application option A=10
Application option B=24
Application option A=30
Application option B=40
==776==
==776== HEAP SUMMARY:
==776== in use at exit: 6 bytes in 2 blocks
==776== total heap usage: 4,031 allocs, 4,029 frees, 1,092,045 bytes allocated
==776==
==776== 3 bytes in 1 blocks are definitely lost in loss record 1 of 2
==776== at 0x4C28B8C: operator new(unsigned long) (vg_replace_malloc.c:261)
==776== by 0x5225E9B: xercesc_3_1::MemoryManagerImpl::allocate(unsigned long) (MemoryManagerImpl.cpp:40)
==776== by 0x53006C8: xercesc_3_1::IconvGNULCPTranscoder::transcode(unsigned short const*, xercesc_3_1::MemoryManager*) (IconvGNUTransService.cpp:751)
==776== by 0x4038E7: GetConfig::readConfigFile(std::string&) (in /home/bonniehan/workspace/test/a.out)
==776== by 0x403B13: main (in /home/bonniehan/workspace/test/a.out)
==776==
==776== 3 bytes in 1 blocks are definitely lost in loss record 2 of 2
==776== at 0x4C28B8C: operator new(unsigned long) (vg_replace_malloc.c:261)
==776== by 0x5225E9B: xercesc_3_1::MemoryManagerImpl::allocate(unsigned long) (MemoryManagerImpl.cpp:40)
==776== by 0x53006C8: xercesc_3_1::IconvGNULCPTranscoder::transcode(unsigned short const*, xercesc_3_1::MemoryManager*) (IconvGNUTransService.cpp:751)
==776== by 0x40393F: GetConfig::readConfigFile(std::string&) (in /home/bonniehan/workspace/test/a.out)
==776== by 0x403B13: main (in /home/bonniehan/workspace/test/a.out)
==776==
==776== LEAK SUMMARY:
==776== definitely lost: 6 bytes in 2 blocks
==776== indirectly lost: 0 bytes in 0 blocks
==776== possibly lost: 0 bytes in 0 blocks
==776== still reachable: 0 bytes in 0 blocks
==776== suppressed: 0 bytes in 0 blocks
==776==
==776== For counts of detected and suppressed errors, rerun with: -v
==776== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 2 from 2)

Answer 1

Looks like the example code has some shortcomings for your use case. It contains this code:
m_OptionA = XMLString::transcode(xmlch_OptionA);
From the documentation we can see that transcode requires its caller to deallocate the returned (C-style) string with XMLString::release(). We can see that this is done in the GetConfig destructor:
if(m_OptionA) XMLString::release( &m_OptionA );
But this code does not exist in readConfig(). You should add it there. You may also want to initialize those C-style string members to NULL in the constructor, or you will face another memory problem (potentially a crash bug) if you call readConfig() zero times instead of one or two.