VACALL(3)VACALL(3)NAMEvacall - C functions called with variable arguments
SYNOPSIS
#include <vacall.h>
extern void* vacall_function;
void function (alist)
va_alist alist;
{
va_start_type(alist[, return_type]);
arg = va_arg_type(alist[, arg_type]);
va_return_type(alist[[, return_type], return_value]);
}
vacall_function = &function;
val = ((return_type (*) ()) vacall) (arg1,arg2,...);
DESCRIPTION
This set of macros permit a C function function to be called with vari‐
able arguments and to return variable return values. This is much like
the varargs(3) facility, but also allows the return value to be speci‐
fied at run time.
Function calling conventions differ considerably on different machines,
and vacall attempts to provide some degree of isolation from such
architecture dependencies.
The function that can be called with any number and type of arguments
and which will return any type of return value is vacall. It will do
some magic and call the function stored in the variable vacall_func‐
tion. If you want to make more than one use of vacall, use the trampo‐
line(3) facility to store &function into vacall_function just before
calling vacall.
Within function, the following macros can be used to walk through the
argument list and specify a return value:
va_start_type(alist[, return_type]);
starts the walk through the argument list and specifies the
return type.
arg = va_arg_type(alist[, arg_type]);
fetches the next argument from the argument list.
va_return_type(alist[[, return_type], return_value]);
ends the walk through the argument list and specifies the return
value.
The type in va_start_type and va_return_type shall be one of void, int,
uint, long, ulong, longlong, ulonglong, double, struct, ptr or (for
ANSI C calling conventions only) char, schar, uchar, short, ushort,
float, depending on the class of return_type.
The type specifiers in va_start_type and va_return_type must be the
same. The return_type specifiers passed to va_start_type and
va_return_type must be the same.
The type in va_arg_type shall be one of int, uint, long, ulong, long‐
long, ulonglong, double, struct, ptr or (for ANSI C calling conventions
only) char, schar, uchar, short, ushort, float, depending on the class
of arg_type.
In va_start_struct(alist, return_type, splittable); the splittable flag
specifies whether the struct return_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:
va_word_splittable_1 (type1)
va_word_splittable_2 (type1, type2)
va_word_splittable_3 (type1, type2, type3)
va_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 va_word_splittable_3 (type1, type2,
type3) .
NOTES
Functions which want to emulate Kernighan & Ritchie style functions
(i.e., in ANSI C, functions without a typed argument list) cannot use
the type values char, schar, uchar, short, ushort, float. As pre‐
scribed by the default K&R C expression promotions, they have to use
int instead of char, schar, uchar, short, ushort and double instead of
float.
The macros va_start_longlong(), va_start_ulonglong(), va_return_long‐
long(), va_return_ulonglong(), va_arg_longlong() and va_arg_ulonglong()
work only if the C compiler has a working long long 64-bit integer
type.
The struct types used in va_start_struct() and va_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.
EXAMPLE
This example, a possible implementation of execl(3) on top of execv(2)
using varargs(3),
#include <varargs.h>
#define MAXARGS 100
/* execl is called by execl(file, arg1, arg2, ..., (char *)0); */
int execl (va_alist)
va_dcl
{
va_list ap;
char* file;
char* args[MAXARGS];
int argno = 0;
va_start (ap);
file = va_arg(ap, char*);
while ((args[argno] = va_arg(ap, char*)) != (char *)0)
argno++;
va_end (ap);
return execv(file, args);
}
looks like this using vacall(3):
#include <vacall.h>
#define MAXARGS 100
/* execl is called by vacall(file, arg1, arg2, ..., (char *)0); */
void execl (ap)
va_alist ap;
{
char* file;
char* args[MAXARGS];
int argno = 0;
int retval;
va_start_int (ap);
file = va_arg_ptr(ap, char*);
while ((args[argno] = va_arg_ptr(ap, char*)) != (char *)0)
argno++;
retval = execv(file, args);
va_return_int (ap, retval);
}
vacall_function = &execl;
SEE ALSOvarargs(3), trampoline(3), callback(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 mem‐
ory considers only the struct's size and alignment. This is inaccurate:
for example, gcc on m68k-next returns struct { char a,b,c; } in regis‐
ters and struct { char a[3]; } in memory, although both types have the
same size and the same alignment.
<vacall.h> cannot be included when <varargs.h> or <stdarg.h> is
included. (Name clash for va_alist.)
The argument list can only be walked once.
The use of the global variable vacall_function is not reentrant. This
is fixed in the callback(3) package.
PORTING
Knowledge about argument passing conventions can be found in the gcc
source, file gcc-2.6.3/config/cpu/cpu.h, section "Stack layout; func‐
tion entry, exit and calling."
The implementation of varargs for gcc can be found in the gcc source,
files gcc-2.6.3/ginclude/va*.h.
gcc's __builtin_saveregs() function is defined in the gcc source, file
gcc-2.6.3/libgcc2.c.
AUTHOR
Bruno Haible <bruno@clisp.org>
ACKNOWLEDGEMENTS
Many ideas and a lot of code were cribbed from the gcc source.
14 January 2001 VACALL(3)
