8. Keyword arguments in the MPS

8.1. Introduction

Up to version 1.111, the Memory Pool System used varags to pass arguments to arena and pool classes, because the general MPS interface can’t specify what arguments those classes might need in C prototypes. This mechanism was error-prone and did not allow for any optional arguments, meaning that the client had to specify or predict esoteric tuning parameters.

Starting with version 1.112, the MPS uses an idiom for keyword arguments.

8.2. Overview

The basic design is not specific to the MPS. The keyword argument list is passed as an array of argument structures which look like this:

typedef struct mps_key_s *mps_key_t;
typedef struct mps_arg_s {
  mps_key_t key;
  union {
    int i;
    char c;
    void *p;
    size_t size;
    /* etc. */
  } val;
} mps_arg_s;

The argument list is assembled and passed like this:

mps_arg_s args[3];
args[0].key = MPS_KEY_MIN_SIZE;
args[0].val.size = 32;
args[1].key = MPS_KEY_MAX_SIZE;
args[1].val.size = 1024;
args[2].key = MPS_KEY_ARGS_END;
mps_pool_create_k(&pool, some_pool_class(), args);

This can be written quite concisely in C99:

mps_pool_create_k(&pool, some_pool_class(),
        (mps_arg_s []){{MPS_KEY_MIN_SIZE, {.size = 32}},
                       {MPS_KEY_MAX_SIZE, {.size = 1024}},
                       {MPS_KEY_ARGS_END}});

The arguments that are recognised and used by the function are removed from the array (and the subsequent arguments moved up) so that if they are all consumed the array has MPS_KEY_ARGS_END in slot zero on return. This can be checked by the caller.

• It’s not a static error to pass excess arguments. This makes it easy to substitute one pool or arena class for another (which might ignore some arguments). The caller can check that args[0].key is MPS_KEY_ARGS_END if desired.

• NULL is not a valid argument list. This is in line with general MPS design principles to avoid accidental omissions. For convenience, we provide mps_args_none as a static empty argument list.

• NULL is not a valid argument key. This is in line with general MPS design principles to avoid accidental omissions. Every key points to a structure with a signature that can be checked. This makes it virtually impossible to get an argument list with bad keys or that is unterminated past MPS checking.

8.3. Internals

Internally, keys are static constant structures which are signed and contain a checking method for the argument, like this:

typedef struct mps_arg_s *Arg;
typedef struct mps_key_s {
  Sig sig;              /* Always KeySig */
  const char *name;
  Bool check(Arg arg);
} KeyStruct;

They are mostly declared in the modules that consume them, except for a few common keys. Declarations look like:

const KeyStruct _mps_key_extend_by = {KeySig, "extend_by", ArgCheckSize};

but arg.h provides a macro for this:

ARG_DEFINE_KEY(extend_by, Size);

We define keys as static structures (rather than, say, an enum) because:

• The set of keys can be extended indefinitely.

• The set of keys can be extended by indepdently linked modules.

• The structure contents allow strong checking of argument lists.

In the MPS Interface, we declare keys like this:

extern const struct mps_key_s _mps_key_extend_by;
#define MPS_KEY_EXTEND_BY (&_mps_key_extend_by)

The underscore on the symbol requests that client code doesn’t reference it, but instead uses the macro. This gives us adaptability to change the design and replace keys with, say, magic numbers.

8.4. The varargs legacy

For backward compatibility, varargs to arena and pool creation are converted into keyword arguments by position, using a method in the arena or pool class. For example:

static void MVVarargs(ArgStruct args[], va_list varargs)
{
  args[0].key = MPS_KEY_EXTEND_BY;
  args[0].val.size = va_arg(varargs, Size);
  args[1].key = MPS_KEY_MEAN_SIZE;
  args[1].val.size = va_arg(varargs, Size);
  args[2].key = MPS_KEY_MAX_SIZE;
  args[2].val.size = va_arg(varargs, Size);
  args[3].key = MPS_KEY_ARGS_END;
  AVER(ArgListCheck(args));
}

This leaves the main body of code, and any future code, free to just handle keyword arguments only.

The use of varargs is deprecated in the manual and the interface and these methods can be deleted at some point in the future.

8.5. References

RB_2012-05-24

Richard Brooksby. Ravenbrook Limited. 2012-05-24. “Keyword and optional arguments”.