Constructing/modifying va_lists

[Ross]

Hi,

I have a question about using va_lists. I have a pair of text
formatting functions defined as follows:

void fmt1(char *format,va_list l) {
// ...
}

void fmt2(char *format,...) {
va_list l;
va_start(l,format);
// ...
fmt1(format,?);
// ...
va_end(l);
}

I'd like fmt2() to examine the va_list, replace some of the arguments
(based on the format string), then call fmt1() with either the modified
va_list or a newly constructed one. Is this possible? I'm sure I've
seen this being done somwhere...

Thanks in advance,
Ross

[Eric Sosman]

Ross wrote:

Hi,

I have a question about using va_lists. I have a pair of text
formatting functions defined as follows:

void fmt1(char *format,va_list l) {
// ...
}

void fmt2(char *format,...) {
va_list l;
va_start(l,format);
// ...
fmt1(format,?);
// ...
va_end(l);
}

I'd like fmt2() to examine the va_list, replace some of the arguments
(based on the format string), then call fmt1() with either the modified
va_list or a newly constructed one. Is this possible? I'm sure I've
seen this being done somwhere...

The machinery of C can't do it: The only means C has for
building an argument list is to write a list of arguments in
a function call.

If you've seen it done somewhere, the doing involved either
some kind of extensions to the C language or some extra-linguistic
knowledge about how a particular implementation handles function
arguments, possibly both.

--
Eric Sosman
es*****@acm-dot-org.invalid

[shaanxxx]

Just to add here , when you say va_list, you are refering to pointer
which runs on stack of fuction. To understand that, we need to know how
variables arguments are implemented.

Thanks
Shaan
Eric Sosman wrote:

Ross wrote:

Hi,

I have a question about using va_lists. I have a pair of text
formatting functions defined as follows:

void fmt1(char *format,va_list l) {
// ...
}

void fmt2(char *format,...) {
va_list l;
va_start(l,format);
// ...
fmt1(format,?);
// ...
va_end(l);
}

I'd like fmt2() to examine the va_list, replace some of the arguments
(based on the format string), then call fmt1() with either the modified
va_list or a newly constructed one. Is this possible? I'm sure I've
seen this being done somwhere...

The machinery of C can't do it: The only means C has for
building an argument list is to write a list of arguments in
a function call.

If you've seen it done somewhere, the doing involved either
some kind of extensions to the C language or some extra-linguistic
knowledge about how a particular implementation handles function
arguments, possibly both.

--
Eric Sosman
es*****@acm-dot-org.invalid

[goose]

Ross wrote:

Hi,

I have a question about using va_lists. I have a pair of text
formatting functions defined as follows:

void fmt1(char *format,va_list l) {
// ...
}

void fmt2(char *format,...) {
va_list l;
va_start(l,format);
// ...
fmt1(format,?);
// ...
va_end(l);
}

I'd like fmt2() to examine the va_list, replace some of the arguments
(based on the format string), then call fmt1() with either the modified
va_list or a newly constructed one. Is this possible? I'm sure I've
seen this being done somwhere...

Thanks in advance,

I have to ask, what is wrong with:
void fmt1 (char *format, ...)
{
...
}

void fmt2 (char *format, ...)
{
va_list all_args;
va_start (all_args, format);
/* Extract and modify all the args and
* store them in variables T1, T2, T3 ... Tn.
*/
if (case1) {
fmt1 (format, T1, T2, T3); /* up to Tn */
} else if (case2) {
fmt1 (format, T2, T3, T1);
} else if (case3) {
fmt1 (format, T3, T1, T2);
} /* carry on in this form for all the cases you need */
va_end (all_args);
}

Admittedly, fmt2 turns out a little verbose and
cannot easily handle all different cases that may
result.

The /other/ way is to design your own parameter
passing method and use that instead (storing
values in filescope variables, carrying type information,
etc).

goose,

[Keith Thompson]

"shaanxxx" <sh******@yahoo.comwrites:

Just to add here , when you say va_list, you are refering to pointer
which runs on stack of fuction. To understand that, we need to know how
variables arguments are implemented.

Please don't top-post. Read <http://www.caliburn.nl/topposting.html>.

When you say va_list, you are referring to a va_list, which behaves as
the standard specifies. It is simply "an object type suitable for
holding information needed by the macros va_start, va_arg, va_end, and
va_copy". The standard doesn't even use the word "stack".

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.