18. C interface design

18.1. 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.

18.2. Analysis

18.2.1. 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++.

18.2.2. 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), and Open Dylan (req.dylan).

.req.separation: The external interface may not include internal MPS header files (such as pool.h).

.req.flexibility: 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 (such as the keyword argument mechanism; see design.mps.keyword-arguments). 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.

.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 use the MPS C interface in combination with other interfaces without name conflicts.

18.3. 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.

18.4. Naming conventions

.naming: The external interface names should adhere to the documented interface conventions; these are found in the “Interface conventions” chapter of the Reference Manual. They are paraphrased/recreated here.

.naming.file: All files in the external interface have names starting with mps.

.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 documented identifiers begin mps_ or MPS_.

.naming.all: All identifiers defined by the MPS begin mps_ or MPS_ or _mps_.

.naming.type: Types are suffixed _t, except for structure and union types.

.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.

.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.

18.5. Type conventions

.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 documented restrictions. The most important is the allocation point structure (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 are 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.

18.6. Checking

.check.testt: Before any use of a parameter foo belonging to a pointer type Foo, it is checked using TESTT(Foo, foo). The macro TESTT() in impl.h.check performs simple thread-safe checking of foo, so it can be called outside of ArenaEnter() and ArenaLeave().

.check.avert: With the arena lock held, foo is checked using AVERT(Foo, foo). This macro has different definitions depending on how the MPS is compiled (see design.mps.config.def.var). It may expand to TESTT(), or it may call the full checking function for the type.

.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.) This checking uses the following macros, originally from mail.richard.1996-08-07.09-49.

COMPATLVALUE(lvalue1, lvalue2)

.check.types.compat.lvalue: 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)

.check.types.compat.type: 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)

.check.types.compat.field.approx: 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)

.check.types.compat.field: 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))

18.7. Binary compatibility issues

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

.compat: There are two main aspects to run-time compatibility: binary interface and protocol.

.compat.binary: 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.

.compat.binary.unneeded: Binary compatibility is not required by the open source MPS: we expect (and indeed, recommend) that a client program is compiled against the MPS sources. Nonetheless we try to maintain binary compatibility in case the capability is required in future.

.compat.binary.dependencies: 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.

.compat.protocol: 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.

.compat.protocol.dependencies: The protocol is determined by the implementation of the library.

18.8. 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*.

18.9. Implementation

.impl: The external interface consists of the following header files:

.impl.mps: mps.h is the main external interface, containing of type and function declarations needed by all clients of the MPS.

.impl.mpstd: 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 (see design.mps.config), and then defines uniform symbols, such as MPS_ARCH_I3, for use externally and internally by the MPS. mpstd.h is not included by any of the other external headers, as it relies on exact set of preprocessor constants defined by compilers.

.impl.mpsio: mpsio.h is the interface to the MPS I/O subsystem, part of the plinth. See design.mps.io.

.impl.mpslib: mpslib.h is the interface to the MPS Library Interface, part of the plinth. See design.mps.lib.

.impl.mpsa: Interfaces to arena classes are in files with names starting mpsa: for example, the interface to the Virtual Memory arena class is in mpsavm.h.

.impl.mpsc: Interfaces to pool classes are in files with names starting mpsc: for example, the interface to the MVFF pool class is in mpscmvff.h.

18.10. Notes

.fmt.extend: mps_fmt_A_t is so called because new pool classes might require new format methods, but these methods cannot be added to the format structure without breaking binary compatibility. Therefore these new pool classes would use new format structures named mps_fmt_B_t and so on.

.thread-safety: Most calls through this interface lock the arena and therefore make the MPM single-threaded. In order to do this they must recover the arena from their parameters. Methods such as FormatArena() and ThreadArena() must therefore be callable when the arena 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.