Variable function arguments and the stack

Variable function arguments and the stack

[Robert Kiesling]

I trying to write a function that calls libc functions with
variable arguments - for example, scanf.  I would like to do
this by pushing the arguments onto the stack and then calling 
the function, something like this.  (This code is for an x86
machine.)

static long long int scalar_args[512];
static char *ptr_args[512][BUFSIZE];

Re: Variable function arguments and the stack

[David Daney]

Robert Kiesling wrote:
> I trying to write a function that calls libc functions with
> variable arguments - for example, scanf.  I would like to do
> this by pushing the arguments onto the stack and then calling 
> the function, something like this.  (This code is for an x86
> machine.)
> 
> static long long int scalar_args[512];
> static char *ptr_args[512][BUFSIZE];

Use libffi which ships with GCC.  Doing things like this is what it was 
designed for.

David Daney

Re: Variable function arguments and the stack

[Robert Kiesling]

[ Charset ISO-8859-1 unsupported, converting... ]
> Robert Kiesling wrote:
> > I trying to write a function that calls libc functions with
> > variable arguments - for example, scanf.  I would like to do
> > this by pushing the arguments onto the stack and then calling 
> > the function, something like this.  (This code is for an x86
> > machine.)
> > 
> > static long long int scalar_args[512];
> > static char *ptr_args[512][BUFSIZE];
> 
> Use libffi which ships with GCC.  Doing things like this is what it was 
> designed for.

Thanks - the library looks like it should work very nicely.  

Robert Kiesling

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Re: Variable function arguments and the stack

[Robert Kiesling]

> David Daney wrote:
> > Robert Kiesling wrote:
> > > I trying to write a function that calls libc functions with
> > > variable arguments - for example, scanf.  I would like to do
> > > this by pushing the arguments onto the stack and then calling 
> > > the function, something like this.  (This code is for an x86
> > > machine.)
> > > 
> > > static long long int scalar_args[512];
> > > static char *ptr_args[512][BUFSIZE];
> > 
> > Use libffi which ships with GCC.  Doing things like this is what it was 
> > designed for.

Libffi clobbers either the arguments or the stream argument (the 
first argument before the format string in functions like fprintf
or sprintf).  It should make a distinction whether the args or 
the stream are writeable or readable, and that, of course is the 
idea.

Regards,

Robert

-- 
Ctalk Home Page: http://www.ctalklang.org/

Re: Variable function arguments and the stack

[Andrew Haley]

Robert Kiesling writes:
 > > David Daney wrote:
 > > > Robert Kiesling wrote:
 > > > > I trying to write a function that calls libc functions with
 > > > > variable arguments - for example, scanf.  I would like to do
 > > > > this by pushing the arguments onto the stack and then calling 
 > > > > the function, something like this.  (This code is for an x86
 > > > > machine.)
 > > > > 
 > > > > static long long int scalar_args[512];
 > > > > static char *ptr_args[512][BUFSIZE];
 > > > 
 > > > Use libffi which ships with GCC.  Doing things like this is what it was 
 > > > designed for.
 > 
 > Libffi clobbers either the arguments or the stream argument (the 
 > first argument before the format string in functions like fprintf
 > or sprintf).  It should make a distinction whether the args or 
 > the stream are writeable or readable, and that, of course is the 
 > idea.

I'm a bit mystified by this comment.  All arguments are passed by
value, and are destroyed after the call.  What does libffi do that it
should not do?

Andrew.

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