PH48307: BACK OUT OF METAL C __PTR32/__PTR64 AMODE PARAMETER CHANGES

Obtain the fix for this APAR.

APAR status

• Closed as program error.

Error description

• A number of APARs developed to fix issues with Metal C-generated
  code for passing parameters between functions with different
  AMODEs are causing cascading code generation problems in
  subsequent PTFs.
  
  Code generation issues are seen with usage of
  the __ptr32/__ptr64 pointer qualifiers and the
  __attribute__((amode31))/__attribute__((amode64)) function
  attributes.
  
  There are a number of ways that the issues present.  Typically
  the issues are related to function parameters with incorrect
  memory offset values, missing dereferences, and incorrect
  pointer sizes.
  
  ================================================================
  
  The initial APAR introducing issues was PH18005 in the November
  2019 V2R4 PTF, with additional APARs PH27049 and PH35278 in the
  February 2021 V2R4 PTF.
  
  
  Fix list for z/OS XL C/C++
  https://www.ibm.com/support/pages/fix-list-zos-xl-cc#24
  
  2.4 November 2019 PTF
  APAR PH18005 - AMODE dependency in function parameter list
  formatio using LP64.
  
  2.4 February 2021 PTF
  APAR PH27049 - __ptr64 pointer treated as 31-bit pointer when
  passed as function parameter with O2
  APAR PH35278 Incorrect struct array function parameter when
  accessed via __PTR32 POINTER
  
  ================================================================
  
  It was determined that a complete fix for all function
  parameter problem scenarios was not possible in a PTF release,
  and that the compiler behaviour will need to be considered as a
  limitation.
  
  It was decided to back out the 3 APAR fixes and revert the
  compiler's behaviour back to the GA behaviour for all cases.
  This will re-introduce the issue in APAR PH18005, however the
  problem can be worked around by adding a __ptr32 qualifier cast
  to the parameter:
  
  -----------------------
  void g(int, ...) __attribute__((amode31));
  
  void f()
  { g(0, ""); }
  -----------------------
  
  modified to:
  
  -----------------------
  void g(int, ...) __attribute__((amode31));
  void f() {
    char * __ptr32 test = "";
    g(0, test);
  }
  -----------------------
  

Local fix

• For users who are required to use a specific PTF level and are
  unable to update to the latest PTF, a potential solution is to
  provide AMODE conversion information to the compiler via
  pointer casting.
  
  Whenever there is pointer usage involving AMODE conversion, a
  __ptr32/__ptr64 cast must be used to make sure of the correct
  pointer value.  There are many different code scenarios where
  this may happen, but the essential rule is to cast the pointer
  to the expected AMODE, for example:
  
  -----------------------
  #include <metal.h>
  
  typedef struct {
     int i;
     union {
        unsigned char ar[16];
     } u;
  } X;
  
  int ext(void * p) __attribute__((amode31));
  void* f(void* buffer) {
     X* __ptr32 rec = (X*)buffer;
  
     //ext(rec->u.ar); // INCORRECT
  
     //ext((void*__ptr32 )(((X* __ptr64 )rec)->u.ar)); //CORRECT
  }
  -----------------------
  
  How to properly cast the ext() __ptr32 parameter, 'rec', is
  shown.  The 'rec' pointer is originally assigned to the
  64-bit pointer 'buffer' - this is OK - but access and
  operations to the 64-bit struct being pointed at, via the
  __ptr32 'rec' pointer, requires a 64-bit cast.
  
  Therefore the -> dereference in the param list needs to be cast
  to 64-bit, (X* __ptr64).  Then, that 64-bit dereferenced value
  needs to be passed to the AMODE31 ext() function, so the value
  needs to be cast again to (void* __ptr32).
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: Uses accessing array members in structs      *
  *                 using alternate amode pointers to the        *
  *                 structure                                    *
  ****************************************************************
  * PROBLEM DESCRIPTION: Access to array struct members via a    *
  *                      ptr32 qualified pointer in 64-bit mode  *
  *                      may give incorrect offsets into the     *
  *                      structure.                              *
  ****************************************************************
  * RECOMMENDATION: Install the PTF                              *
  ****************************************************************
  N/A
  

Problem conclusion

• Install the PTF or use workaround
  

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:

  PH49634

Modules/Macros

• CCNECICS CCNECWI  CCNED240 CCNEDSCT CCNEDWRT CCNEFILT CCNEHIFC
  CCNEIPA3 CCNEIPAO CCNEMDEP CCNEOPTP CCNEP    CCNEPP   CCNETBY
  CCNMSGE  CCNMSGT  CCNQIPA
  

Fix information

• Fixed component name

  C/C++ FOR MVS

• Fixed component ID

  56551210A

Applicable component levels

• R7C0 PSY UI81828

     UP22/09/07 P F209

Fix is available

• Select the PTF appropriate for your component level. You will be required to sign in. Distribution on physical media is not available in all countries.