12. C interface design

12.1. Introduction

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

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

12.2. Analysis

12.2.1. 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++.

12.2.2. 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).

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 (mps_fmt_A_t, 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.

Note

.naming.global was presumably done in response to unwritten requirements regarding the use of the name spaces in C, perhaps these:

  • .req.name.iso: The interface shall not conflict in terms of naming with any interfaces specified by ISO C and all reasonable future versions.

  • .req.name.general: The interface shall use a documented and reasonably small portion of the namespace so that clients can interoperate easily.

David Jones, 1998-10-01.

12.3. Architecture

.fig.arch: The architecture of the MPS Interface

[missing figure]

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.

12.4. 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. They are 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 §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.

Note

This principle is not adhered to in the source code, which uses goto for handling error cases. Gareth Rees, 2013-05-27.

.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()). 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 than:

typedef mps_addr_t (*mps_fmt_class_t)(mps_addr_t object);

See guide.impl.c.misc.prototype.parameters.

12.5. Checking

.check.space: When the arena needs to be recovered from a parameter it is check using AVERT(Foo, foo) before any attempt to call FooArena(foo). The macro AVERT() in impl.h.assert performs simple thread-safe checking of foo, so it can be called outside of ArenaEnter() and ArenaLeave().

.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.

COMPATLVALUE(lvalue1, lvalue2)

This macro checks the assignment compatibility of two lvalues. It uses sizeof() to ensure that the assignments have no effect.

#define COMPATLVALUE(lv1, lv2) \
  ((void)sizeof((lv1) = (lv2)), (void)sizeof((lv2) = (lv1)), TRUE)
COMPATTYPE(type1, type2)

This macro checks that two types are assignment-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. The use of sizeof() avoids the undefined behaviour that would otherwise result from dereferencing a null pointer.

#define COMPATTYPE(t1, t2) \
  (sizeof(t1) == sizeof(t2) && \
   COMPATLVALUE(*((t1 *)0), *((t2 *)0)))
COMPATFIELDAPPROX(structure1, field1, structure2, field2)

This macro checks that the offset and size of two fields in two structure types are the same.

#define COMPATFIELDAPPROX(s1, f1, s2, f2) \
  (sizeof(((s1 *)0)->f1) == sizeof(((s2 *)0)->f2) && \
   offsetof(s1, f1) == offsetof(s2, f2))
COMPATFIELD(structure1, field1, structure2, field2)

This macro checks the offset, size, and assignment-compatibility of two fields in two structure types.

#define COMPATFIELD(s1, f1, s2, f2) \
  (COMPATFIELDAPPROX(s1, f1, s2, f2) && \
   COMPATLVALUE(((s1 *)0)->f1, ((s2 *)0)->f2))

12.6. 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.

12.7. 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*.

12.8. 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_t 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.