Hi folks!
I have a function that gets a 'va_list'. I am passing the
'va_list' two times to a function like 'vprintf' to print
it out.
I thought that this was portable until I came across a
platform (AS/400) where it fails: The 'va_list' is actually
defined as some some kind of an array, so passing it happens
by *reference*: The first user will destroy it and the second
time it does not work.
My question is: Does the standard allow this and AS/400 is
broken or am I invoking undefined behaviour?
Below is a test program
---------------- snip --------------------------
/*
* Test if a va_list argument can be used to traverse the
* variable arguments more than once.
*/
#include <stdio.h>
#include <stdarg.h>
static void scanargs(int dest[2],va_list ap)
{
dest[0] = va_arg(ap,int);
dest[1] = va_arg(ap,int);
}
static void teststdarg(int dest[4], ...)
{
va_list ap;
va_start(ap,des t);
scanargs(dest,a p);
scanargs(dest + 2,ap);
va_end(ap);
}
int main(int argc,char *argv[])
{
int a[4];
teststdarg(a,1, 2,3,4);
if((a[0] == 1 && a[1] == 2) &&
(a[2] == 1 && a[3] == 2) || (a[2] == 3 && a[3] == 4)) {
if(a[2] == 1)
printf("We can use a 'va_list' at least twice.\n");
else
printf("WARNING : Reusing 'va_list' gives different results!\n");
} else {
fprintf(stderr, "ERROR: Something is very bad here!\n");
exit(20);
}
return 0;
}
---------------- snap -------------------------- 2 3346
Joerg Schoen wrote: I have a function that gets a 'va_list'. I am passing the 'va_list' two times to a function like 'vprintf' to print it out.
I thought that this was portable until I came across a platform (AS/400) where it fails: The 'va_list' is actually defined as some some kind of an array, so passing it happens by *reference*: The first user will destroy it and the second time it does not work.
That's permissible behaviour. You should re-initialize the list
with va_start before attempting to use it again.
static void teststdarg(int dest[4], ...) { va_list ap;
va_start(ap,des t);
scanargs(dest,a p); scanargs(dest + 2,ap);
va_end(ap); }
This should be written as
static void teststdarg(int dest[4], ...)
{
va_list ap;
va_start(ap,des t);
scanargs(dest,a p);
va_end(ap);
va_start(ap, dest);
scanargs(dest + 2,ap);
va_end(ap);
}
Jeremy.
Jeremy Yallop <je****@jdyallo p.freeserve.co. uk> wrote: That's permissible behaviour. You should re-initialize the list with va_start before attempting to use it again.
Which is fine if you're creating the va_arg in the first place, but is a
bit difficult if it's comming from somewhere else (e.g., being passed to
you as a parameter). That's why C99 has va_copy().
-Larry Jones
Talk about someone easy to exploit! -- Calvin This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
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