altering an av_list variable

[Punkie]

When having optional function arguments, the extra arguments are accessed
via an av_list structure. While traversing is trivial, changing the value
isnt.

<code>
void my_sprintf (char *dest, size_t size, const char *format, ...)
{
<snip>
va_list argptr;
va_start(argptr, format);
while((pos = strcspn(format,"%")) != max) {
switch(format[pos+1]) {
case 'm':
format[pos+1] = 's';
argval = va_arg(cpy,int);
/* change the argval to a char* */
break;
default:
va_arg(cpy,int);
}
}
sprintf(*dest, const char *format, argptr); /* call the std function */
va_end(argptr);
}
</code>

In this example you see an attempt to make the sprintf function also
recognise the "%m" format. It should retrieve the (int) argument and change
its value.

Is it possible to alter the arguments in the va_list at all? Are there other
tricks to do this without rewriting sprintf completely?

[Tor Rustad]

Punkie wrote:

[...]

Are there other tricks to do this without rewriting sprintf completely?

See

http://www.ijs.si/software/snprintf/
A quick and non-portable way, is getting the length via:

null = fopen("/dev/null", "w"); /* UNIX/Linux*/
null = fopen("NUL", "w"); /* Windows */

and then calling

len = vfprintf(null, format, ap);
Note, vfprintf() return int, not size_t.

--
Tor <torust [at] online [dot] no>

[Punkie]

Indeed, a good way to do this without rewriting sprintf is to let *others*
rewrite it. However something i didnt find among them is typechecking.

Any sprintf with the same std formatting can use the attributes to defer
typechecking too. Like:
void my_sprintf (char *dest, size_t size, const char *format, ...)
__attribute__((format(printf, 3, 4)));

Altering the types accepted creates a problem as we cant use the attribute
anymore.
2routes to solve this:
*own sprintf implementation, that also does this checking somehow. hmmm but
how?
*filter the format and va_list. Like in the code I presented earlier,
changing the type or eliminaing the arg from the va_list.

Tor Rustad wrote:

Punkie wrote:

[...]

>Are there other tricks to do this without rewriting sprintf completely?

See

http://www.ijs.si/software/snprintf/
A quick and non-portable way, is getting the length via:

null = fopen("/dev/null", "w"); /* UNIX/Linux*/
null = fopen("NUL", "w"); /* Windows */

and then calling

len = vfprintf(null, format, ap);
Note, vfprintf() return int, not size_t.

[Kenny McCormack]

In article <QE**********************@phobos.telenet-ops.be>,
Punkie <pu****@hotmail.comwrote:

>When having optional function arguments, the extra arguments are accessed
via an av_list structure. While traversing is trivial, changing the value
isnt.

"av_list" and "va_list" are very different things. The people here will
not talk to you about the former.

[Ian Collins]

Punkie wrote:

Please don't top-post.

Tor Rustad wrote:

>Punkie wrote:

[...]

>>Are there other tricks to do this without rewriting sprintf completely?

See

http://www.ijs.si/software/snprintf/
A quick and non-portable way, is getting the length via:

null = fopen("/dev/null", "w"); /* UNIX/Linux*/
null = fopen("NUL", "w"); /* Windows */

and then calling

len = vfprintf(null, format, ap);
Note, vfprintf() return int, not size_t.

Indeed, a good way to do this without rewriting sprintf is to let *others*
rewrite it. However something i didnt find among them is typechecking.

Any sprintf with the same std formatting can use the attributes to defer
typechecking too. Like:
void my_sprintf (char *dest, size_t size, const char *format, ...)
__attribute__((format(printf, 3, 4)));

It should be noted that __attribute__ is a (very handy) gcc extension.

--
Ian Collins.