IV96373: ISSUES WITH XLC COPY ON WRITE

XL C/C++ for AIX Fix Pack 9 (June 2017 PTF) for 13.1
XL C/C++ Runtime for AIX, V13.1.3.3 Fix Pack (June 2017)
XL C/C++ for AIX Fix Pack 10 (February 2018 PTF) for 13.1
XL C/C++ for AIX Fix Pack 11 (November 2018 PTF) for 13.1

APAR status

• Closed as program error.

Error description

• The following test case demonstrates an issue with the
  copy-on-write string implementation that is enabled with
  single-threaded builds:
  
  
  ===== COMPILE COMMAND:
  xlC test.cpp
  
  ===== TESTCASE:
  $ cat test.cpp
  #include <cassert>
  #include <iostream>
  #include <string>
  #include <vector>
  
  int main()
  {
  std::string s("abc");
  
  // this vector should never be modified
  const std::vector<std::string> v(256, s);
  
  // modify the string
  s[0] = 'z';
  
  std::cout << "s = " << s << '\n';
  std::cout << "v[ 0] = " << v[ 0] << '\n';
  std::cout << "v[ 1] = " << v[ 1] << '\n';
  std::cout << "v[253] = " << v[253] << '\n';
  std::cout << "v[254] = " << v[254] << '\n';
  std::cout << "v[255] = " << v[255] << '\n';
  
  assert(0 == s.compare("zbc"));
  assert(0 == v[ 0].compare("abc"));
  assert(0 == v[ 1].compare("abc"));
  assert(0 == v[253].compare("abc"));
  assert(0 == v[254].compare("abc"));
  assert(0 == v[255].compare("abc"));
  }
  $
  
  ===== ACTUAL OUTPUT:
  $ xlC test.cpp
  $ ./a.out
  s = zbc
  v[ 0] = zbc
  v[ 1] = zbc
  v[253] = zbc
  v[254] = abc
  v[255] = abc
  Assertion failed: 0 == v[ 0].compare("abc"), file  test.cpp,
  line 24
  IOT/Abort trap
  $
  
  
  ===== EXPECTED OUTPUT:
  $ xlC test.cpp
  $ ./a.out
  s = zbc
  v[ 0] = abc
  v[ 1] = abc
  v[253] = abc
  v[254] = abc
  v[255] = abc
  $
  

Local fix

• Use xlC_r instead of xlC invocation as a workaround.
  

Problem summary

• USERS AFFECTED:
  Users who have a string referenced 254 or more times and modify
  the string afterwards are affected by this issue.
  
  PROBLEM DESCRIPTION:
  The runtime fails to perform copy on write when a string's
  reference count
  is 254 and as a result, the write affects items where the string
  is referenced.
  

Problem conclusion

• The issue has been fixed.
  

Temporary fix

Comments

APAR Information

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

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

  PI86240

Fix information

• Fixed component name

  XL C/C++ RUNTIM

• Fixed component ID

  5725C7201

Applicable component levels

• RD13 PSY

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