.\" Copyright (C) 1995-2017 Bruno Haible
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.TH VACALL 3 "1 January 2017"
.SH NAME
vacall \- C functions called with variable arguments
.SH SYNOPSIS
.B #include <vacall.h>
.LP
.B extern void* vacall_function;
.LP
.nf
.BI "void " function " (va_alist" alist ")"
.BI "{"
.BI "  va_start_" type "(" alist "[, " return_type "]);"
.BI "  " arg " = va_arg_" type "(" alist "[, " arg_type "]);"
.BI "  va_return_" type "(" alist "[[, " return_type "], " return_value "]);"
.BI "}"
.fi
.LP
.BI "vacall_function = " "&function" ";"
.LP
.IB "val" " = ((" return_type " (*) ()) vacall) (" arg1 , arg2 , ... ");"
.SH DESCRIPTION
This set of macros permit a C function
.I function
to be called with variable arguments and to return variable return values.
This is much like the
.BR stdarg (3)
facility, but also allows the return value to be specified at run time.

Function calling conventions differ considerably on different machines, and
.I 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
.BR vacall .
It will do some magic and call the function stored in the variable
.BR vacall_function .
If you want to make more than one use of
.IR vacall ,
use the
.IR trampoline (3)
facility to store
.I &function
into
.B vacall_function
just before calling
.BR vacall .

Within
.IR function ,
the following macros can be used to walk through the argument list and
specify a return value:
.RS 0
.TP
.BI "va_start_" type "(" alist "[, " return_type "]);"
starts the walk through the argument list and specifies the return type.
.TP
.IB arg " = va_arg_" type "(" alist "[, " arg_type "]);"
fetches the next argument from the argument list.
.TP
.BI "va_return_" type "(" alist "[[, " return_type "], " return_value "]);"
ends the walk through the argument list and specifies the return value.
.RE

The
.I type
in
.BI va_start_ type
and
.BI va_return_ type
shall be one of
.BR void ", " int ", " uint ", " long ", " ulong ", " longlong ", " ulonglong ", " double ", " struct ", " ptr
or (for ANSI C calling conventions only)
.BR char ", " schar ", " uchar ", " short ", " ushort ", " float ,
depending on the class of
.IR return_type .

The
.I type
specifiers in
.BI va_start_ type
and
.BI va_return_ type
must be the same.
The
.I return_type
specifiers passed to
.BI va_start_ type
and
.BI va_return_ type
must be the same.

The
.I type
in
.BI va_arg_ type
shall be one of
.BR int ", " uint ", " long ", " ulong ", " longlong ", " ulonglong ", " double ", " struct ", " ptr
or (for ANSI C calling conventions only)
.BR char ", " schar ", " uchar ", " short ", " ushort ", " float ,
depending on the class of
.IR arg_type .

In
.BI "va_start_struct(" alist ", " return_type ", " splittable );
the
.I splittable
flag specifies whether the struct
.I 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),
.I splittable
is ignored and assumed to be 1. For structs of size > 2*sizeof(long),
.I splittable
is ignored and assumed to be 0. There are some handy macros for this:
.nf
.BI "va_word_splittable_1 (" type1 )
.BI "va_word_splittable_2 (" type1 ", " type2 )
.BI "va_word_splittable_3 (" type1 ", " type2 ", " type3 )
.BI "va_word_splittable_4 (" type1 ", " type2 ", " type3 ", " type4 )
.fi
For a struct with three slots
.nf
.BI "struct { " "type1 id1" "; " "type2 id2" "; " "type3 id3" "; }"
.fi
you can specify
.I splittable
as
.BI "va_word_splittable_3 (" type1 ", " type2 ", " type3 )
.RB .

.SH NOTES

Functions which want to emulate Kernighan & Ritchie style functions (i.e.,
in ANSI C, functions without a typed argument list) cannot use the
.I type
values
.BR char ", " schar ", " uchar ", " short ", " ushort ", " float .
As prescribed by the default K&R C expression promotions, they have
to use
.B int
instead of
.BR char ", " schar ", " uchar ", " short ", " ushort
and
.B double
instead of
.BR float .

The macros
.BR va_start_longlong(\|) ,
.BR va_start_ulonglong(\|) ,
.BR va_return_longlong(\|) ,
.BR va_return_ulonglong(\|) ,
.B va_arg_longlong(\|)
and
.B va_arg_ulonglong(\|)
work only if the C compiler has a working
.B long long
64-bit integer type.

The struct types used in
.B va_start_struct(\|)
and
.B 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.

.SH EXAMPLE

This example, a possible implementation of
.BR execl (3)
on top of
.BR execv (2)
using
.BR stdarg (3),

.nf
.ft B
#include <stdarg.h>
#define MAXARGS 100
/* execl is called by execl(file, arg1, arg2, ..., (char *)0); */
int execl (...)
{
  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);
}
.ft
.fi

looks like this using
.BR vacall (3):

.nf
.ft B
#include <vacall.h>
#define MAXARGS 100
/* execl is called by vacall(file, arg1, arg2, ..., (char *)0); */
void execl (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;
.ft
.fi

.SH SEE ALSO
.BR stdarg (3),
.BR trampoline (3),
.BR callback (3).

.SH 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
.B "struct { char a,b,c; }"
in registers and
.B "struct { char a[3]; }"
in memory, although both types have the same size and the same alignment.

The argument list can only be walked once.

The use of the global variable
.B vacall_function
is not reentrant. This is fixed in the
.BR callback (3)
package.

.SH PORTING

Knowledge about argument passing conventions can be found in the gcc
source, file
.RI 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.

.SH AUTHOR

Bruno Haible <bruno@clisp.org>

.SH ACKNOWLEDGEMENTS

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