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.