NAME
avcall - build a C argument list incrementally and call a C function on
it.
SYNOPSIS
#include <avcall.h>
av_alist alist;
av_start_type(alist, &func [[, return_type], &return_value ]);
av_type(alist, [arg_type,] value);
av_call(alist);
DESCRIPTION
This set of macros builds an argument list for a C function and calls
the function on it. It significantly reduces the amount of ‘glue’ code
required for parsers, debuggers, imbedded interpreters, C extensions to
application programs and other situations where collections of
functions need to be called on lists of externally-supplied arguments.
Function calling conventions differ considerably on different machines
and avcall attempts to provide some degree of isolation from such
architecture dependencies.
The interface is like stdarg(3) in reverse. All of the macros return 0
for success, < 0 for failure (e.g., argument list overflow or type-not-
supported).
(1) #include <avcall.h>
and declare the argument list structure
av_alist alist;
(2) Set any special flags. This is architecture and compiler
dependent. Compiler options that affect passing conventions may
need to be flagged by #defines before the #include <avcall.h>
statement. However, the configure script should have determined
which #defines are needed and put them at the top of avcall.h.
(3) Initialize the alist with the function address and return value
pointer (if any). There is a separate macro for each simple
return type ([u]char, [u]short, [u]int, [u]long, [u]longlong,
float, double, where ‘u’ indicates ‘unsigned’). The macros for
functions returning structures or pointers require an explicit
type argument.
E.g.,
av_start_int (alist, &func, &int_return);
av_start_double (alist, &func, &double_return);
av_start_void (alist, &func);
av_start_struct (alist, &func, struct_type, splittable,
&struct_return);
av_start_ptr (alist, &func, pointer_type,
&pointer_return);
The splittable flag specifies whether the struct_type can be returned
in registers such that every struct field fits entirely in a single
register. This needs to be specified for structs of size
2*sizeof(long). For structs of size <= sizeof(long), splittable is
ignored and assumed to be 1. For structs of size > 2*sizeof(long),
splittable is ignored and assumed to be 0. There are some handy macros
for this:
av_word_splittable_1 (type1)
av_word_splittable_2 (type1, type2)
av_word_splittable_3 (type1, type2, type3)
av_word_splittable_4 (type1, type2, type3, type4)
For a struct with three slots
struct { type1 id1; type2 id2; type3 id3; }
you can specify splittable as av_word_splittable_3 (type1, type2,
type3) .
(4) Push the arguments on to the list in order. Again there is a
macro for each simple built-in type, and the macros for
structure and pointer arguments require an extra type argument:
av_int (alist, int_value);
av_double (alist, double_value);
av_struct (alist, struct_or_union_type, struct_value);
av_ptr (alist, pointer_type, pointer_value);
(5) Call the function, set the return value, and tidy up:
av_call (alist);
NOTES
(1) Functions whose first declaration is in Kernighan & Ritchie style
(i.e., without a typed argument list) MUST use default K&R C expression
promotions (char and short to int, float to double) whether they are
compiled by a K&R or an ANSI compiler, because the true argument types
may not be known at the call point. Such functions typically back-
convert their arguments to the declared types on function entry. (In
fact, the only way to pass a true char, short or float in K&R C is by
an explicit cast: func((char)c,(float)f) ). Similarly, some K&R
compilers (such as Sun cc on the sparc) actually return a float as a
double.
Hence, for arguments of functions declared in K&R style you should use
av_int() and av_double() rather than av_char(), av_short() or
av_float(). If you use a K&R compiler, the avcall header files may be
able to detect this and define av_float(), etc, appropriately, but with
an ANSI compiler there is no way avcall can know how a function was
declared, so you have to correct the argument types yourself.
(2) The explicit type arguments of the av_struct() and av_ptr() macros
are typically used to calculate size, alignment, and passing
conventions. This may not be sufficient for some machines with unusual
structure and pointer handling: in this case additional av_start_type()
and av_type() macros may be defined.
(3) The macros av_start_longlong(), av_start_ulonglong(), av_longlong()
and av_ulonglong() work only if the C compiler has a working long long
64-bit integer type.
(4) The struct types used in av_start_struct() and av_struct() must
only contain (signed or unsigned) int, long, long long or pointer
fields. Struct types containing (signed or unsigned) char, short,
float, double or other structs are not supported.
SEE ALSO
stdarg(3), varargs(3).
BUGS
The current implementations have been tested on a selection of common
cases but there are probably still many bugs.
There are typically built-in limits on the size of the argument-list,
which may also include the size of any structure arguments.
The decision whether a struct is to be returned in registers or in
memory considers only the struct’s size and alignment. This is
inaccurate: for example, gcc on m68k-next returns struct { char a,b,c;
} in registers and struct { char a[3]; } in memory, although both types
have the same size and the same alignment.
NON-BUGS
All information is passed in CPU registers and the stack. The avcall
package is therefore multithread-safe.
PORTING AVCALL
Ports, bug-fixes, and suggestions are most welcome. The macros required
for argument pushing are pretty grungy, but it does seem to be possible
to port avcall to a range of machines. Ports to non-standard or
non-32-bit machines are especially welcome so we can sort the interface
out before it’s too late.
Knowledge about argument passing conventions can be found in the gcc
source, file gcc-2.6.3/config/cpu/cpu.h, section "Stack layout;
function entry, exit and calling."
Some of the grunge is usually handled by a C or assembly level glue
routine that actually pushes the arguments, calls the function and
unpacks any return value. This is called __builtin_avcall(). A
precompiled assembler version for people without gcc is also made
available. The routine should ideally have flags for the passing
conventions of other compilers.
Many of the current routines waste a lot of stack space and generally
do hairy things to stack frames - a bit more assembly code would
probably help things along quite a bit here.
AUTHOR
Bill Triggs <Bill.Triggs@inrialpes.fr>.
ACKNOWLEDGEMENTS
Some initial ideas were stolen from the C interface to the Zelk
extensions to Oliver Laumann’s Elk scheme interpreter by J.P.Lewis, NEC
C&C Research, <zilla@ccrl.nj.nec.com> (for Sun4 & SGI), and Roy
Featherstone’s <roy@robots.oxford.ac.uk> personal C interface library
for Sun[34] & SGI. I also looked at the machine-dependent parts of the
GCC and GDB distributions, and put the gcc asm() extensions to good
use. Thanks guys!
This work was partly supported by EC-ESPRIT Basic Research Action
SECOND.
14 January 2001