vacall - C functions called with variable arguments

#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,...);

This set of macros permit a C function function to be called with variable arguments and to return variable return values. This is much like the varargs(3) facility, but also allows the return value to be specified 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_function. If you want to make more than one use of vacall, use the trampoline(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, longlong, 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)

struct { type1 id1; type2 id2; type3 id3; }

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 prescribed 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_longlong(), 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.

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;

varargs(3), trampoline(3), callback(3).

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.

<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.

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."

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.

Bruno Haible <bruno@clisp.org>

Many ideas and a lot of code were cribbed from the gcc source.