LI75559: ISSUES WITH STL STD::STRING COPY ON WRITE

June 2011 Update for XL C/C++ for Linux, V10.1
XL C/C++ for Linux Fix Pack 8 (January 2014 Update) for 10.1
August 2010 Update for XL C/C++ for Linux, V10.1
January 2011 Update for XL C/C++ for Linux, V10.1
January 2012 Update for XL C/C++ for Linux, V10.1
July 2012 Update for XL C/C++ for Linux, V10.1

APAR status

• Closed as program error.

Error description

• The following test case demonstrates an issue with the
  copy-on-write optimization performed by the string class:
  
  ===== COMPILE COMMAND:
  xlC test.cxx
  
  ===== TESTCASE:
  $ cat test.cxx
  #include <cstdio>
  #include <string>
  #include <vector>
  
  int main()
  {
    std::string const x = "x";
  
    int const n = 256;
    std::vector<std::string> v;
    v.reserve(n);
  
    for(int i=0; i < n; ++i) {
      v.push_back(x);
      std::string xy = x + "y";
      const char* p = x.c_str();
      std::printf("x = [%s], xy = [%s]", p, xy.c_str());
  #if defined(COUNT)
      std::printf(", count = %d", int(p[-1]));
  #endif
      std::printf("\n");
    }
  
    return 0;
  }
  
  
  ===== ACTUAL OUTPUT:
    $ xlC test.cxx && ./a.out
    x = [x], xy = [xy]
    ...
    x = [x], xy = [xy]
    x = [x], xy = [xy]
    x = [xy], xy = [xy]
    x = [xy], xy = [xy]
    ...
    x = [xy], xy = [xy]
    $
  
  ===== EXPECTED OUTPUT:
    $ xlC test.cxx && ./a.out
    x = [x], xy = [xy]
    ...
    x = [x], xy = [xy]
    x = [x], xy = [xy]
    x = [x], xy = [xy]
    x = [x], xy = [xy]
    ...
    x = [x], xy = [xy]
    $
  

Local fix

• Use xlC_r instead of xlC, as xlC_r disables the copy-on-write
  optimization.
  

Problem summary

• USERS AFFECTED:
  Users who use the same string many times in assign or append
  string operations may be affected by this issue.
  
  PROBLEM DESCRIPTION:
  When the same string is copied many times, the compiler uses
  the Copy On Write (COW) algorithm in the implementation,
  which only updates the reference count in the original string
  in the operation of assigning the original string to another
  string.
  When the reference count approaches to the maximum number for
  the string, instead of creating a copy of the original string,
  it reuses the original string, and when other operations like
  append happens, the original string also gets updated.
  This causes the test case start to fail after a certain number
  of copies.
  

Problem conclusion

• The compiler has been changed to create a new copy of the
  original string, instead of reusing the string.
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

  IZ70285

• APAR is sysrouted TO one or more of the following:

Fix information

• Fixed component name

  XL C/C++ SLES10

• Fixed component ID

  5724U8300

Applicable component levels

• RA10 PSN IZ70285

     UP06/09/13