THE DESIGN OF THE MEMORY POOL SYSTEM INTERFACE TO C
                         design.mps.interface.c
                             incomplete doc
                           richard 1996-07-29

INTRODUCTION


Scope

.scope: This document is the design for the Memory Pool System (MPS) interface 
to the C Language, impl.h.mps.


Background

.bg: See mail.richard.1996-07-24.10-57.


Document History

.hist.0: The first draft of this document was generated in response to 
review.impl.h.mps.10 which revealed the lack of a detailed design document and 
also the lack of conventions for external interfaces.  The aim of the draft was 
to record this information, even if it isn't terribly well structured.


ANALYSIS


Goals

.goal.c: The file impl.h.mps is the C external interface to the MPS.  It is the 
default interface between client code written in C and the MPS.  .goal.cpp: 
impl.h.mps is not specifically designed to be an interface to C++, but should 
be usable from C++.


Requirements

.req: The interface must provide an interface from client code written in C to 
the functionality of the MPS required by the product (see req.product), Dylan 
(req.dylan), and the Core RIP (req.epcore).

mps.h may not include internal MPS header files such as "pool.h" etc.

It is essential that the interface cope well with change, in order to avoid 
restricting possible future MPS developments.  This means that the interface 
must be "open ended" in its definitions.  This accounts for some of the 
apparently tortuous methods of doing things (fmt_A, for example).  The 
requirement is that the MPS should be able to add new functionality, or alter 
the implementation of existing functionality, without affecting existing client 
code.  A stronger requirement is that the MPS should be able to change without 
_recompiling_ client code.  This is not always possible.

[.naming.global wqas presumable done in response to an unwritten requirement 
regarding the use of the name spaces in C, perhaps something like 
".req.name.iso: The interface shall not conflict in terms of naming with any 
interfaces specified by ISO C and all reasonable future versions" and 
".req.name.general: The interface shall use a documented and reasonably small 
portion of the namespace so that clients can interoperate easily"  drj 
1998-10-01]


ARCHITECTURE

.fig.arch: The architecture of the MPS Interface



Just behind mps.h is the file mpsi.c, the "MPS interface layer" which does the 
job of converting types and checking parameters before calling through to the 
MPS proper, using internal MPS methods.


GENERAL CONVENTIONS


.naming: The external interface names should adhere to the documented interface 
conventions; these are found in doc.mps.ref-man.if-conv(0).naming.  
(paraphrased/recreated here)  .naming.unixy: The external interface does not 
follow the same naming conventions as the internal code.  The interface is 
designed to resemble a more conventional C, Unix, or Posix naming convention.  
.naming.case: Identifiers are in lower case, except non-function-like macros, 
which are in upper case.  .naming.global: All publicised identifiers are 
prefixed "mps_" or "MPS_".  .naming.all: All identifiers defined by the MPS 
should begin "mps_" or "MPS_" or "_mps_".  .naming.type: Types are suffixed 
"_t".  .naming.struct: Structure types and tags are suffixed "_s".  
.naming.union: Unions types and tags are suffixed "_u".

.naming.scope: The naming conventions apply to all identifiers (see ISO C 
clause 6.1.2); this includes names of functions, variables, types (through 
typedef), structure and union tags, enumeration members, structure and union 
members, macros, macro parameters, labels.  .naming.scope.labels: labels (for 
goto statements) should be rare, only in special block macros and probably not 
even then.  .naming.scope.other: The naming convention would also extend to 
enumeration types and parameters in functions prototypes but both of those are 
prohibited from having names in an interface file.

.type.gen: The interface defines memory addresses as "void *" and sizes as 
"size_t" for compatibility with standard C (in particular, with malloc etc.).  
These types must be binary compatible with the internal types "Addr" and "Size" 
respectively.  Note that this restricts the definitions of the internal types 
"Addr" and "Size" when the MPS is interfaced with C, but does not restrict the 
MPS in general.

.type.opaque: Opaque types are defined as pointers to structures which are 
never defined.  These types are cast to the corresponding internal types in 
mpsi.c.

.type.trans:  Some transparent structures are defined.  The client is expected 
to read these, or poke about in them, under restrictions which should be 
documented.  The most important is probably the allocation point (mps_ap_s) 
which is part of allocation buffers.  The transparent structures must be binary 
compatible with corresponding internal structures.  For example, the fields of 
mps_ap_s must correspond with APStruct internally.  This is checked by mpsi.c 
in mps_check().

.type.pseudo: Some pseudo-opaque structures are defined.  These only exist so 
that code can be inlined using macros.  The client code shouldn't mess with 
them.  The most important case of this is the scan state (mps_ss_s) which is 
accessed by the in-line scanning macros, MPS_SCAN_* and MPS_FIX*.

.type.enum: There should be no enumeration types in the interface.  Note that 
enum specifiers (to declare integer constants) are fine as long as no type is 
declared.  See guide.impl.c.misc.enum.type.

.type.fun: Whenever function types or derived function types (such as pointer 
to function) are declared a prototype should be used and the parameters to the 
function should not be named.  This includes the case where you are declaring 
the prototype for an interface function.  .type.fun.example: So use "extern 
mps_res_t mps_alloc(mps_addr_t *, mps_pool_t, size_t, ...);" rather than 
"extern mps_res_t mps_alloc(mps_addr_t *addr_return, mps_pool_t pool , size_t 
size, ...);"  and "typedef mps_addr_t (*mps_fmt_class_t)(mps_addr_t);" rather 
then "typedef mps_addr_t (*mps_fmt_class_t)(mps_addr_t object);".  See 
guide.impl.c.misc.prototype.parameters.


Checking

.check.space: When the space needs to be recovered from a parameter it is check 
using AVER(CHECKT(Foo, foo)) before any attempt to access FooSpace(foo).  
CHECKT (impl.h.assert) performs simple thread-safe checking of foo, so it can 
be called outside of SpaceEnter/SpaceLeave.  [perhaps this should be a special 
macro.  "AVER(CHECKT(" can look like the programmer made a mistake.  drj 
1998-11-05]

.check.types: We use definitions of types in both our external interface and 
our internal code, and we want to make sure that they are compatible.  (The 
external interface changes less often and hides more information.)  At first, 
we were just checking their sizes, which wasn't very good, but I've come up 
with some macros which check the assignment compatibility of the types too.  
This is a sufficiently useful trick that I thought I'd send it round.  It may 
be useful in other places where types and structures need to be checked for 
compatibility at compile time.

These macros don't generate warnings on the compilers I've tried.

This macro checks the assignment compatibility of two lvalues.  The hack is 
that it uses sizeof to guarantee that the assignments have no effect.

#define check_lvalues(_e1, _e2) \
  (sizeof((_e1) = (_e2)), sizeof((_e2) = (_e1)), sizeof(_e1) == sizeof(_e2))

This macro checks that two types are compatible and equal in size.  The hack 
here is that it generates an lvalue for each type by casting zero to a pointer 
to the type.

#define check_types(_t1, _t2)   check_lvalues(*((_t1 *)0), *((_t2 *)0))

This macro just checks that the offset and size of two fields are the same.

#define check_fields_approx(_s1, _f1, _s2, _f2) \
  (offsetof(_s1, _f1) == offsetof(_s2, _f2) && \
   sizeof(((_s1 *)0)->_f1) == sizeof(((_s2 *)0)->_f2))

This macro checks the offset, size, and compatibility of fields.

#define check_fields(_s1, _f1, _s2, _f2) \
  (check_lvalues(((_s1 *)0)->_f1, ((_s2 *)0)->_f2) && \
   check_fields_approx(_s1, _f1, _s2, _f2))


Binary Compatibility Issues

As in Enumeration types are not allowed (see mail.richard.1995-09-08.09-28).

There are two main aspects to run-time compatibility: binary interface and 
protocol.  The binary interface is all the information needed to correctly use 
the library, and includes external symbol linkage, calling conventions, type 
representation compatibility, structure layouts, etc.  The protocol is how the 
library is actually used by the client code -- whether this is called before 
that -- and determines the semantic correctness of the client with respect to 
the library.

The binary interface is determined completely by the header file and the 
target.  The header file specifies the external names and the types, and the 
target platform specifies calling conventions and type representation.  There 
is therefore a many-to-one mapping between the header file version and the 
binary interface.

The protocol is determined by the implementation of the library.

  
Constraints

.cons: The MPS C Interface constrains the MPS in order to provide useful memory 
management services to a C or C++ program.

.cons.addr: The interface constrains the MPS address type, Addr 
(design.mps.type.addr), to being the same as C's generic pointer type, void *, 
so that the MPS can manage C objects in the natural way.

.pun.addr: We pun the type of mps_addr_t (which is void *) into Addr (an 
incomplete type, see design.mps.type.addr).  This happens in the call to the 
scan state's fix function, for example.

.cons.size: The interface constrains the MPS size type, Size 
(design.mps.type.size), to being the same as C's size type, size_t, so that the 
MPS can manage C objects in the natural way.
 
.pun.size: We pun the type of size_t in mps.h into Size in the MPM, as an 
argument to the format methods.  We assume this works.

.cons.word: The MPS assumes that Word (design.mps.type.word) and Addr 
(design.mps.type.addr) are the same size, and the interface constrains Word to 
being the same size as C's generic pointer type, void *.


NOTES

The file mpstd.h is the MPS target detection header.  It decodes
preprocessor symbols which are predefined by build environments in order to
determine the target platform, and then defines uniform symbols, such as
MPS_ARCH_I3, for use internally by the MPS.

There is a design document for the mps interface, design.mps.interface, but
it was written before we had the idea of having a C interface layer.  It is
quite relevant, though, and could be updated.  We should use it during the
review.

All exported identifiers and file names should begin with mps_ or mps so
that they don't clash with other systems.

We should probably have a specialized set of rules and a special checklist
for this interface.

.fmt.extend: This paragraph should be an explanation of why mps_fmt_A is so 
called.  The underlying reason is future extensibility.

.thread-safety: Most calls through this interface lock the space and therefore 
make the MPM single-threaded.  In order to do this they must recover the space 
from their parameters.  Methods such as ThreadSpace() must therefore be 
callable when the space is _not_ locked.  These methods are tagged with the tag 
of this note.

.lock-free: Certain functions inside the MPM are thread-safe and do not need to 
be serialized by using locks.  They are marked with the tag of this note.

.form: Almost all functions in this implementation simply cast their arguments 
to the equivalent internal types, and cast results back to the external type, 
where necessary.  Only exceptions are noted in comments.