Pointer Conversions

The IBM® C and C++ compilers convert __ptr128 to __ptr64 and vice versa as needed, based on function and variable declarations. However, interfaces with pointer-to-pointer parameters require special handling.

The compilers automatically insert pointer conversions to match pointer lengths. For example, conversions are inserted when the pointer arguments to a function do not match the length of the pointer parameters in the prototype for the function. Or, if pointers of different lengths are compared, the compiler will implicitly convert the 8-byte pointer to a 16-byte pointer for the comparison. The compilers also allow explicit conversions to be specified, as casts. Keep these points in mind if adding pointer casts:
  • A conversion from a 16-byte pointer to an 8-byte pointer works only if the 16-byte pointer contains a teraspace address or a null pointer value. Otherwise, an MCH0609 exception is signalled.
  • 16-byte pointers cannot have types converted from one to another, but a 16-byte OPEN pointer can contain any pointer type. In contrast, no 8-byte OPEN pointer exists, but 8-byte pointers can be logically converted between a space pointer and a procedure pointer. Even so, an 8-byte pointer conversion is just a view of the pointer type, so it doesn't allow a space pointer to actually be used as a procedure pointer unless the space pointer was set to point to a procedure.

When adding explicit casts between pointers and binary values, remember that 8-byte and 16-byte pointers behave differently. An 8-byte pointer can retain a full 8-byte binary value, while a 16-byte pointer can only retain a 4-byte binary value. While holding a binary value, the only operation defined for a pointer is a conversion back to a binary field. All other operations are undefined, including use as a pointer, conversion to a different pointer length and pointer comparison. So, for example, if the same integer value were assigned to an 8-byte pointer and to a 16-byte pointer, then the 8-byte pointer were converted to a 16-byte pointer and a 16-byte pointer comparison were done, the comparison result would be undefined and likely would not produce an equal result.

Mixed-length pointer comparisons are defined only when a 16-byte pointer holds a teraspace address and an 8-byte pointer does, too (that is, the 8-byte pointer does not contain a binary value). Then it is valid to convert the 8-byte pointer to a 16-byte pointer and compare the two 16-byte pointers. In all other cases, comparison results are undefined. So, for example, if a 16-byte pointer were converted to an 8-byte pointer and then compared with an 8-byte pointer, the result is undefined.