doc/runtime.tex, lib/keyword.3: Explain the other benefit of `NO_KWARGS'.

[sod] / lib / keyword.3

.\" -*-nroff-*-
.\"
.\" Keyword argument support
.\"
.\" (c) 2015 Straylight/Edgeware
.\"
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.\"--------------------------------------------------------------------------
.TH keyword 3 "16 December 2015" "Straylight/Edgeware" "Sensible Object Design"
.
.SH NAME
keyword \- keyword argument support library
.
.\"--------------------------------------------------------------------------
.SH SYNOPSIS
.B #include <sod/keyword.h>
.PP
.B "struct kwval { const char *kw; const void *val; };"
.br
.B "struct kwtab { const struct kwval *v; size_t n; };"
.br
.BI "typedef void kw_unkhookfn(const char *" set ", const char *" kw ");"
.PP
.BI "#define " set "_KWSET(_) \e"
.in +4m
.BI   "_(" name ", " type ", " default ") \e"
.br
\&...
.in
.IB declaration-specifiers " KWSET_STRUCT(" set ");"
.br
.IB declaration-specifiers " KWSET_PARSEFN(" set ")"
.PP
.B KWCALL
.IB type0 " " func "(" type1 " " arg1 ,
.RB ... ,
.IB typen " " argn ,
.B "KWTAIL);"
.br
.BI "KWDECL(" set ", " kw ");"
.br
.BI "KW_PARSE(" set ", " kw ", " kwfirst ");"
.br
.BI "KW_PARSE_EMPTY(" set ", " kwfirst ");"
.br
.BI "KWPARSE(" set ");"
.br
.BI "KWPARSE_EMPTY(" set ");"
.PP
.I val
.B =
.IB func "(" arg1 ,
.RB ... ,
.IB argn ,
.BI "KWARGS(" \c
.t(
.BI "K(" name ", " value ")"
.br
.BI "K_VALIST(" ap ")"
.br
.BI "K_TAB(" v ", " n ")"
.br
.RB ... );
.t)
.br
.I val
.B =
.IB func "(" arg1 ,
.RB ... ,
.IB argn ,
.B "NO_KWARGS);"
.PP
.B unsigned
.BI "KW_COUNT(" set ");"
.br
.B void
.BI "KW_COPY(" \c
.t(
.IB fromset ", " toset ","
.br
.BI "const struct " fromset "_kwset *" kw ","
.br
.BI "struct kwval *" v ", size_t " n ");"
.t)
.PP
.BI "void kw_unknown(const char *" set ", const char *" kw );
.br
.BI "void kw_parseempty(\fP" \c
.t(
.BI   "const char *" set ,
.BI   "const char *" kwfirst ,
.BI   "va_list *" ap ,
.br
.BI   "const struct kwval *" v ,
.BI   "size_t " n );
.t)
.PP
.B "kw_unkhookfn *kw_unkhook;"
.br
.B "kw_unkhookfn kw_defunknown;"
.
.\"--------------------------------------------------------------------------
.SH DESCRIPTION
.
.SS Theory
In standard C,
the actual arguments provided to a function
are matched up with the formal arguments
given in the function definition
according to their ordering in a list.
Unless the (rather cumbersome) machinery for dealing with
variable-length argument tails
.RB ( <stdarg.h> )
is used,
exactly the correct number of arguments must be supplied,
and in the correct order.
.PP
A
.I keyword argument
is matched by its distinctive
.IR name ,
rather than by its position in a list.
Keyword arguments may be
.IR omitted ,
causing some default behaviour by the function.
A function can detect whether
a particular keyword argument was supplied:
so the default behaviour need not be the same as
that caused by any specific value of the argument.
.PP
Keyword arguments can be provided in three ways.
.hP 1.
Directly, as a variable-length argument tail,
consisting (for the most part \(en see below) of alternating
keyword names, as pointers to null-terminated strings, and
argument values, and
terminated by a null pointer.
This is somewhat error-prone,
and the support library defines some macros
which help ensure that keyword argument lists are well formed.
.hP 2.
Indirectly, through a
.B va_list
object capturing a variable-length argument tail
passed to some other function.
Such indirect argument tails have the same structure as
the direct argument tails described above.
Because
.B va_list
objects are hard to copy,
the keyword-argument support library consistently passes
.B va_list
objects
.I by reference
throughout its programming interface.
.hP 3.
Indirectly, through a vector of
.B struct kwval
objects,
each of which contains
a keyword name, as a pointer to a null-terminated string, and
the
.I address
of a corresponding argument value.
(This indirection is necessary so that
the items in the vector can be of uniform size.)
Argument vectors are rather inconvenient to use,
but are the only practical way in which a caller can decide at runtime
which arguments to include in a call,
which is useful when writing wrapper functions.
.PP
Perhaps surprisingly,
keyword arguments have a relatively small performance impact.
On the author's aging laptop,
a call to a simple function,
passing two out of three keyword arguments,
takes about 30 cycles longer than
calling a standard function which just takes integer arguments.
On the other hand,
quite a lot of code is involved in decoding keyword arguments,
so code size will naturally suffer.
.
.SS Type definitions
The header file defines two simple structure types.
.PP
.IP
.nf
.ft B
struct kwval {
  const char *kw;
  const void *val;
};
.fi
.PP
The
.B kwval
structure describes a keyword argument name/value pair.
The
.B kw
member points to the name,
as a null-terminated string.
The
.B val
member always contains the
.I address
of the value.
(This somewhat inconvenient arrangement
makes the size of a
.B kwval
object independent of the actual argument type.)
.PP
.IP
.nf
.ft B
struct kwtab {
  const struct kwval *v;
  size_t n;
};
.fi
.PP
The
.B kwtab
structure describes a list of keyword arguments,
represented as a vector of
.B kwval
structures.
The
.B v
member points to the start of the vector;
the
.B n
member contains the number of elements in the vector.
.PP
The
.B kw_unkhookfn
type is the type of
unknown-keyword handler functions.
See the descriptions of
.B kw_unknown
and
.B kw_unkhook
below.
.
.SS Calling functions with keyword arguments
Functions which accept keyword arguments are ordinary C functions
with variable-length argument tails.
Hence, they can be called using ordinary C (of the right kind)
and all will be well.
However, argument lists must follow certain rules
(which will be described in full below);
failure to do this will result in
.IR "undefined behaviour" .
The header file provides integration with some C compilers
in the form of macros which can be used to help the compiler diagnose
errors in calls to keyword-accepting functions;
but such support is rather limited at the moment.
Some additional macros are provided for use in calls to such functions,
and it is recommended that, where possible, these are used.
In particular, it's all too easy to forget the trailing null terminator
which marks the end of a list of keyword arguments.
.PP
That said, the underlying machinery is presented first,
and the convenience macros are described later.
.PP
The argument tail,
following the mandatory arguments,
consists of a sequence of zero or more alternating
keyword names,
as pointers to null-terminated strings
(with type
.BR "const char *" ),
and their argument values.
This sequence is finally terminated by a null pointer
(again with type
.BR "const char *" )
in place of a keyword name.
.PP
Each function may define for itself which keyword names it accepts,
and what types the corresponding argument values should have.
There are also (currently) three special keyword names.
.TP
.B kw.valist
This special keyword is followed by a pointer to
a variable-length argument tail cursor object, of type
.BR "va_list *" .
This cursor object will be modified as the function extracts
successive arguments from the tail.
The argument tail should consist of alternating
keyword names and argument values, as described above,
including the first keyword name.
(This is therefore different from the convention used when calling
keyword argument parser functions:
see the description of the
.B KW_PARSEFN
macro below for more details about these.)
The argument tail may itself contain the special keywords.
.TP
.B kw.tab
This special keyword is followed by
.I two
argument values:
a pointer to the base of a vector of
.B kwval
structures,
and the number of elements in this vector
(as a
.BR size_t ).
Each element of the vector describes a single keyword argument:
the
.B kw
member points to the keyword's name, and
the
.B val
member points to the value.
The vector may contain special keywords.
The
.B val
pointer for a
.B kw.valist
argument should contain the address of an object of type
.B "va_list *"
(and not point directly to the cursor object,
since
.B val
is has type
.B "const void *"
but the cursor will be modified as its argument tail is traversed).
The
.B val
pointer for a
.B kw.tab
argument should contain the address of a
.B kwtab
structure which itself contains the base address and length of
the argument vector to be processed.
.TP
.B kw.unknown
This keyword is never accepted by any function.
If it is encountered,
the
.B kw_unknown
function is called to report the situation as an error;
see below.
.PP
It is possible to construct a circular structure
of indirect argument lists
(in a number of ways).
Don't try to pass such a structure to a function:
the result will be unbounded recursion
or some other bad outcome.
.PP
The macro
.BI "KWARGS(" body ")"
wraps up a sequence of keyword arguments.
The single
.I body
argument consists of a sequence of calls to
the keyword-argument macros described below,
one after another without any separation.
.PP
If there are no keyword arguments,
use the argument-less macro
.B NO_KWARGS
instead.
There are two reasons for this.
.hP \*o
C89 doesn't permit empty macro arguments for some reason,
so
.B NO_KWARGS
is necessary when using a C89 compiler.
.hP \*o
Omitting the null terminator is a common mistake,
so
.B <keyword.h>
tries to get the compiler to warn if you miss it.
However, the
.B KWTAIL
macro introduces an extra real argument
.BR kwfirst_ ,
because it's not possible to scan a variable-length argument tail
if there are no mandatory arguments.
If you use
.BR KWARGS() ,
with an empty argument list,
then the null terminator is passed as
.B kwfirst_
and the variable-length tail ends up empty,
which might trigger a compiler warning
about the missing terminator.
.B NO_KWARGS
passes
.I two
null terminators:
a real one to indicate that there are no keyword arguments,
and a dummy one to placate the compiler.
.PP
The following keyword-argument macros can be used
within
.BR KWARGS 's
.I body
argument.
.TP
.BI "K(" name ", " value ")"
Passes a keyword name and its corresponding value,
as a pair of arguments.
The
.I name
should be a single identifier
(not a quoted string).
The
.I value
may be any C expression
of the appropriate type.
.TP
.BI "K_VALIST(" ap ")"
Passes an indirect variable-length argument tail.
The argument
.I ap
should be an lvalue of type
.B va_list
which will be passed by reference.
.TP
.BI "K_TAB(" v ", " n ")"
Passes a vector of keyword arguments.
The argument
.I v
should be the base address of the vector, and
.I n
should be the number of elements in the vector.
.
.SS Defining functions with keyword arguments
A
.I "keyword set"
defines the collection of keyword arguments
accepted by a particular function.
The same keyword set may be used by several functions.
(If your function currently accepts no keyword arguments,
but you plan to add some later,
do not define a keyword set,
and use the
.B KWPARSE_EMPTY
macro described below.)
.PP
Each keyword set has a name,
which is a C identifier.
It's good to choose meaningful and distinctive names for keyword sets.
Keyword set names are meaningful at runtime:
they are used as part of the
.B kw_unknown
protocol (described below),
and may be examined by handler functions,
or reported to a user in error messages.
For a keyword set which is used only by a single function,
it is recommended that the set be given the same name as the function.
.PP
The keyword arguments for a keyword set named
.I set
are described by a `list macro' named
.IB set _KWSET \fR.
This macro takes a single argument,
conventionally named
.RB ` _ '.
It should expand to a sequence of one or more list items of the form
.IP
.BI "_(" type ", " name ", " default ")"
.PP
with no separation between them.
.PP
For example:
.IP
.nf
.ft B
#define example_KWSET(_) \e
.in +4m
_(int, x, 0) \e
_(const char *, y, NULL)
.fi
.ft P
.PP
Each
.I name
should be a distinct C identifier;
they will be used to name structure members.
An argument
.I name
should not end with the suffix
.RB ` _suppliedp '
(for reasons which will soon become apparent).
.PP
Each
.I type
should be a C
.I type-name
such that
.IP
.IB type " " name ;
.PP
is a valid declaration:
so it may consist of declaration specifiers and
(possibly qualified) pointer declarator markers,
but not array or function markers
(since they must be placed after the
.IR name ).
This is the same requirement made by the standard
.BR va_arg (3)
macro.
.PP
Each
.I default
should be an initializer expression
or brace-enclosed list,
suitable for use in an aggregate initializer
for a variable with automatic storage duration.
(In C89, aggregate initializers may contain only constant expressions;
this restriction was lifted in C99.)
.PP
The macro
.B KWTAIL
is expected to be used at the end of function parameter type list
to indicate that the function accepts keyword arguments;
if there are preceding mandatory arguments
then the
.B KWTAIL
marker should be separated from them with a comma
.RB ` , '.
(It is permitted for a function parameter type list to contain
only a
.B KWTAIL
marker.)
.PP
Specifically,
the macro declares a mandatory argument
.B const char *kwfirst_
(to collect the first keyword name),
and a variable-length argument tail.
.PP
The macro
.B KWPARSE
(described below)
assumes that the enclosing function's argument list ends with a
.B KWTAIL
marker.
The marker should be included both in the function's definition and
in any declarations, e.g., in the corresponding header file.
.PP
The
.B KWCALL
macro acts as a declaration specifier for
functions which accept keyword arguments.
Its effect is to arrange for the compiler to check,
as far as is possible,
that calls to the function are well-formed
according to the keyword-argument rules.
The exact checking performed depends on the compiler's abilities
(and how well supported the compiler is):
it may check that every other argument is a string;
it may check that the list is terminated with a null pointer;
it may not do anything at all.
Again, this marker should be included in a function's definition and
in any declarations.
.PP
The
.B KWSET_STRUCT
macro defines a
.IR "keyword structure" .
If
.I set
is a keyword-set name then
.IP
.BI "KWSET_STRUCT(" set ");"
.PP
declares a structure
.B struct
.IB set _kwargs \fR.
For each argument defined in the keyword set,
this structure contains two members:
one has exactly the
.I name
and
.I type
listed in the keyword set definition;
the other is a 1-bit-wide bitfield of type
.B "unsigned int"
named
.IB name _suppliedp \fR.
.PP
The macro
.B KWDECL
declares and initializes a keyword argument structure variable.
If
.I set
is a keyword-set name then
.IP
.I declaration-specifiers
.BI "KWDECL(" set ", " kw ");"
.PP
declares a variable of type
.B struct
.IB set _kwargs
named
.IR kw .
The optional
.I declaration-specifiers
may provide additional storage-class,
qualifiers,
or other declaration specifiers.
The
.RB ` _suppliedp '
flags are initialized to zero;
the other members are initialized with the corresponding defaults
from the keyword-set definition.
.PP
The macro
.B KWSET_PARSEFN
defines a keyword argument
.IR "parser function" .
If
.I set
is a keyword-set name then
.IP
.I declaration-specifiers
.BI "KWSET_PARSEFN(" set ")"
.PP
(no trailing semicolon!)
defines a function
.IP
.B void
.IB set _kwparse( \c
.t(
.BI "struct " set "_kwargs *" kw ","
.br
.BI "const char *" kwfirst ", va_list *" ap ","
.br
.BI "const struct kwval *" v ", size_t " n ");"
.t)
.PP
The macro call can
(and usually will)
be preceded by storage class specifiers such as
.BR static ,
for example to adjust the linkage of the name.
(I don't recommend declaring parser functions
.BR inline :
parser functions are somewhat large, and
modern compilers are pretty good at
figuring out whether to inline static functions.)
.PP
The function's behaviour is as follows.
It parses keyword arguments from
a variable-length argument tail, and/or
a vector of
.B kwval
structures.
When a keyword argument is recognized,
for some keyword
.IR name ,
the keyword argument structure pointed to by
.I kw
is updated:
the flag
.IB name _suppliedp
is set to 1;
and the argument value is stored (by simple assignment) in the
.I name
member.
Hence, if the
.RB ` _suppliedp '
members are initialized to zero,
the caller can determine which keyword arguments were supplied.
It is not possible to discover whether two or more arguments
have the same keyword:
in this case,
the value from the last such argument is left
in the keyword argument structure,
and any values from earlier arguments are lost.
(For this purpose,
the argument vector
.I v
is scanned
.I after
the variable-length argument tail captured in
.IR ap .)
.PP
The variable-argument tail is read from the list described by
.BI * ap \fR.
The argument tail is expected to consist of alternating
keyword strings (as ordinary null-terminated strings)
and the corresponding values,
terminated by a null pointer of type
.B "const char *"
in place of a keyword;
except that the first keyword
(or terminating null pointer, if no arguments are provided)
is expected to have been extracted already
and provided as the
.I kwfirst
argument;
the first argument retrieved using the
.B va_list
cursor object should then be the value
corresponding to the keyword named by
.IR kwfirst .
(This slightly unusual convention makes it possible for a function to
collect the first keyword as a separate mandatory argument,
which is essential if there are no other mandatory arguments.
It also means that the compiler will emit a diagnostic
if you attempt to call a function which expects keyword arguments,
but don't supply any and
forget the null pointer which terminates the (empty) list.)
If
.I kwfirst
is a null pointer,
then
.I ap
need not be a valid pointer;
otherwise, the cursor object
.BI * ap
will be modified as the function extracts
successive arguments from the tail.
.PP
The keyword vector is read from the vector of
.B kwval
structures starting at address
.I v
and containing the following
.I n
items.
If
.I n
is zero then
.I v
need not be a valid pointer.
.PP
The function also handles the special
.B kw.valist
and
.B kw.tab
arguments described above.
If an unrecognized keyword argument is encountered,
then
.B kw_unknown
is called:
see below for details.
.PP
The
.B KW_PARSE
macro invokes a keyword argument parsing function.
If
.I set
is a keyword-set name,
.I kw
names a keyword argument structure variable of type
.B struct
.IB set _kwargs \fR,
and
.I kwfirst
is the name of the enclosing function's last mandatory argument,
which must have type
.BR "const char *" ,
then
.IP
.BI "KW_PARSE(" set ", " kw ", " kwfirst ");"
.PP
calls the function
.IB set _kwparse
with five arguments:
the address of the keyword argument structure
.IR kw ;
the string pointer
.IR kwfirst ;
the address of a temporary argument-tail cursor object of type
.BR va_list ,
constructed on the assumption that
.I kwfirst
is the enclosing function's final keyword argument;
a null pointer; and
the value zero (signifying an empty keyword-argument vector).
If the variable
.I kw
was declared using
.B KWDECL
and the function
.IB set _kwparse
has been defined using
.B KWSET_PARSEFN
then the effect is to parse the keyword arguments passed to the function
and set the members of
.I kw
appropriately.
.PP
The macro
.B KWPARSE
(note the lack of underscore)
combines
.B KWDECL
and
.BR KW_PARSE .
If
.I set
is a keyword-set name then
.IP
.BI "KWPARSE(" set ");"
.PP
declares and initializes a keyword argument structure variable
with the fixed name
.BR kw ,
and parses the keyword arguments provided to the enclosing function,
storing the results in
.BR kw .
It assumes that the first keyword name
is in an argument named
.BR kwfirst_ ,
as set up by the
.B KWTAIL
marker described above.
.PP
The macro expands both to a variable declaration and a statement:
in C89, declarations must precede statements,
so under C89 rules this macro must appear exactly between
the declarations at the head of a brace-enclosed block
(typically the function body)
and the statements at the end.
This restriction was lifted in C99,
so the macro may appear anywhere in the function body.
However, it is recommended that callers avoid taking actions
which might require cleanup
before attempting to parse their keyword arguments,
since keyword argument parsing functions invoke the
.B kw_unknown
handler if they encounter an unknown keyword,
and the calling function will not get a chance
to tidy up after itself if this happens.
.PP
As mentioned above,
it is not permitted to define an empty keyword set.
(Specifically, invoking
.B KWSET_STRUCT
for an empty keyword set would result in attempting to define
a structure with no members, which C doesn't allow.)
On the other hand, keyword arguments are a useful extension mechanism,
and it's useful to be able to define a function which doesn't
currently accept any keywords,
but which might in the future be extended to allow keyword arguments.
The macros
.B KW_PARSE_EMPTY
and
.B KWPARSE_EMPTY
are analogues of
.B KW_PARSE
and
.B KWPARSE
respectively,
and handle this case.
These macros take a keyword-set name as an argument,
but this name is used only in diagnostic messages
(e.g., if an unknown keyword name is encountered)
and need not
(and probably should not)
correspond to a defined keyword set.
.PP
If
.I set
is an identifier then
.IP
.BI "KW_PARSE_EMPTY(" set ", " kwfirst ");"
.PP
calls the function
.B kw_parseempty
with five arguments:
the
.I set
name, as a string;
the string pointer
.IR kwfirst ;
the address of a temporary argument-tail cursor object of type
.BR va_list ,
constructed on the assumption that
.I kwfirst
is the enclosing function's final keyword argument;
a null pointer; and
the value zero (signifying an empty keyword-argument vector).
The effect is to check that the argument tail contains
no keyword arguments other than the special predefined ones.
.PP
If
.I set
is an identifier then
.IP
.BI "KWPARSE_EMPTY(" set ");"
.PP
(note the lack of underscore)
checks that the enclosing function has been passed
no keyword arguments other than the special predefined ones.
It assumes that the function's parameter type list ends with the
.B KWTAIL
marker described above.
.PP
The
.B kw_parseempty
function checks an keyword argument list
to make sure that contains no keyword arguments
(other than the special ones described above).
.PP
The
.I set
argument should point to a null-terminated string:
this will be reported as the keyword set name to
.BR kw_unknown ,
though it need not
(and likely will not)
refer to any defined keyword set.
The remaining arguments are as for
the keyword parsing functions
defined by the
.B KWSET_PARSEFN
macro.
.
.SS "Wrapper functions"
Most users will not need the hairy machinery involving argument vectors.
Their main use is in defining
.IR "wrapper functions" .
Suppose there is a function
.I f
which accepts some keyword arguments,
and we want to write a function
.I g
which accepts the same keywords recognized by
.I f
and some additional ones.
Unfortunately
.I f
may behave differently depending on whether or not
a particular keyword argument is supplied at all, but
it's not possible to synthesize a valid
.B va_list
other than by simply capturing a live argument tail,
and it's not possible to decide at runtime
whether or not to include some arguments in a function call.
It's still possible to write
.IR g ,
by building a vector of keyword arguments,
collected one-by-one depending on the corresponding
.RB ` _suppliedp '
flags (see below).
A few macros are provided to make this task easier.
.PP
The macro
.B KW_COUNT
returns the number of keywords defined in a keyword set.
If
.I set
is a keyword-set name, then
.IP
.BI "KW_COUNT(" set ")"
.PP
returns the number of keywords defined by
.IR set ,
as a constant expression of type
.BR "unsigned int" .
.PP
The macro
.B KW_COPY
populates a vector of
.B kwval
structures from a keyword-argument structure.
If
.I fromset
and
.I toset
are two keyword-set names then
.IP
.BI "KW_COPY(" fromset ", " toset ", " kw ", " v ", " n ");"
.PP
will populate the vector
.IR v ,
taking argument values from
.IR kw .
The
.I toset
must be a subset of
.IR fromset :
i.e., for every keyword defined in
.I toset
there is a keyword defined in
.I fromset
with the same name and type.
The remaining arguments are as follows:
.I kw
is a pointer to a
.BI "struct " fromset "_kwset"
keyword-argument structure which has been filled in,
e.g., by the keyword-argument parsing function
.IB fromset _kwparse \fR;
.I v
is a pointer to a sufficiently large vector of
.B "struct kwval"
objects;
and
.I n
is an lvalue designating an object of integer type.
Successive elements of
.IR v ,
starting at index
.IR n ,
are filled in to refer to
the keyword arguments defined in
.I toset
whose
.RB ` _suppliedp '
flag is set in the argument structure pointed to by
.IR kw ;
for each such argument,
a pointer to the keyword name is stored in
the corresponding vector element's
.B kw
member, and
a pointer to the argument value,
held in the keyword argument structure,
is stored in the vector element's
.B val
member.
At the end of this,
the index
.I n
is advanced so as to contain the index of the first unused element of
.IR v .
Hence, at most
.BI KW_COUNT( toset )
elements of
.I v
will be used.
.
.SS Handling unknown-keyword errors
When parsing a variable-length argument tail,
it is not possible to continue after
encountering an unknown keyword name.
This is because it is necessary
to know the (promoted) type of the following argument value
in order to skip past it;
but the only clue provided as to the type is the keyword name,
which in this case is meaningless.
.PP
In this situation,
the parser functions generated by
.B KW_PARSEFN
(and the
.B kw_parseempty
function)
call
.BR kw_unknown .
This is a function of two arguments:
.I set
points to the name of the keyword set expected by the caller,
as a null-terminated string; and
.I kw
is the unknown keyword which was encountered.
All that
.B kw_unknown
does is invoke the function whose address is stored in
the global variable
.B kw_unkhook
with the same arguments.
The
.B kw_unknown
function never returns to its caller:
if the
.B kw_unkhook
function returns (which it shouldn't)
then
.B kw_unknown
writes a fatal error message to standard error
and calls
.BR abort (3).
.PP
By default
.B kw_unkhook
points to the function
.BR kw_defunknown ,
which just writes an error message
quoting the keyword set name
and offending keyword
to standard error
and calls
.BR abort (3).
.PP
(In freestanding environments,
the behaviour may be somewhat different:
porting the library to such environments involves
choosing appropriate behaviour for the target platform.)
.PP
As an example of the kind of special effect
which can be achieved using this hook,
the following hacking answers whether
a function recognizes a particular keyword argument.
.IP
.nf
.ft B
#define KWARGS_TEST(k, val) KWARGS(K(k, val) K(kw.unknown, 0))

static jmp_buf kw_test_jmp;

static void kw_test_unknown(const char *set, const char *kw)
{
  if (strcmp(kw, "kw.unknown")) longjmp(kw_test_jmp, 1);
  else longjmp(kw_test_jmp, 2);
}

#define KW_TEST(flag, set, call) do { \e
  kw_unkhookfn *oldunk = kw_unkhook; \e
  kw_unkhook = kw_test_unknown; \e
  switch (setjmp(kw_test_jmp)) { \e
    case 0: call; abort(); \e
    case 1: flag = 1; break; \e
    case 2: flag = 0; break; \e
    default: abort(); \e
  } \e
  kw_unkhook = oldunk; \e
} while (0)

/* Example of use */
int f;
KW_TEST(f, somefunc(1, "two", 3, KWARGS_TEST("shiny", 68.7)));
/* now f is nonzero if `somefunc' accepts the `shiny' keyword
 * (which we hope wants a double argument)
 */
.ft P
.fi
.
.\"--------------------------------------------------------------------------
.SH BUGS
.
The unknown-keyword hook is inadequate for a modern library,
but dealing with multiple threads isn't currently possible
without writing (moderately complex) system-specific code.
The author's intention is that the hook variable
.B kw_unkhook
be `owned' by some external library
which can make its functionality available to client programs
in a safer and more convenient way.
On Unix-like platforms
(including Cygwin)
that library will be (a later version) of
.BR mLib ;
other platforms will likely need different arrangements.
The author is willing to coordinate any such efforts.
.PP
The whole interface is rather clunky.
Working with keyword-argument vectors is especially unpleasant.
The remarkable thing is not that it's done well,
but that it can be done at all.
.
.\"--------------------------------------------------------------------------
.SH SEE ALSO
.
.BR va_start (3),
.BR va_arg (3),
.BR va_end (3).
.
.\"--------------------------------------------------------------------------
.SH AUTHOR
.
Mark Wooding,
<mdw@distorted.org.uk>
.
.\"----- That's all, folks --------------------------------------------------