Variable length argument list - 32 / 64 bit problem?

[Joakim Hove]

Hello,

I have written the following (stripped down) code:

/********************************/
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
char * path_fmt_alloc_path(const char * fmt , ...) {
char * new_path;
va_list ap;
va_start(ap , fmt);
{
int path_length = vsnprintf(new_path , 0 , fmt , ap);
new_path = malloc(path_length + 1);
vsnprintf(new_path , path_length + 1 , fmt , ap);
}
va_end(ap);
return new_path;
}
int test(const char * fmt , int id) {
char * s = path_fmt_alloc_path(fmt , id);
printf("%d -%s \n",id ,s);
free(s);
}

int main(int argc, char **argv) {
const char * fmt = "%d";

test(fmt , 1 );
test(fmt , 2 );
test(fmt , 3 );
}
/*********************************/

When compiling with:
bash% gcc -m32 va_test.c
I get the result:

1 -1
2 -2
3 -3

i.e. what I expected. However it fails when compiling in 64 bit mode:

bash% gcc va_test.c
1 --
2 -0
3 -0

This is done with gcc version 3.4.6...
/***************************/

I am in some invoking undefined behaviour, or is this a bug somewhere
in my 64 bit toolchain?
Best Regards

Joakim Hove

[David Resnick]

On Mar 14, 9:45 am, Joakim Hove <[e-mail]:

Hello,

I have written the following (stripped down) code:

/********************************/
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

char * path_fmt_alloc_path(const char * fmt , ...) {
char * new_path;
va_list ap;
va_start(ap , fmt);
{
int path_length = vsnprintf(new_path , 0 , fmt , ap);

new_path is not initialized. You mean to be printing to NULL here to
get the length, but... Maybe whether you get it set to 0/NULL depends
on which compiler you get. I'd suggest (vsnprintf(NULL,...) to make
your meaning clear. If new_path is not 0, you are writing to a wild
pointer.

-David

[Eric Sosman]

Joakim Hove wrote:

Hello,

I have written the following (stripped down) code:

/********************************/
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
char * path_fmt_alloc_path(const char * fmt , ...) {
char * new_path;
va_list ap;
va_start(ap , fmt);
{
int path_length = vsnprintf(new_path , 0 , fmt , ap);

va_end(ap);
va_start(ap, fmt);

new_path = malloc(path_length + 1);

if (new_path != NULL)

vsnprintf(new_path , path_length + 1 , fmt , ap);
}
va_end(ap);
return new_path;
}
int test(const char * fmt , int id) {
char * s = path_fmt_alloc_path(fmt , id);

if (s != NULL)

printf("%d -%s \n",id ,s);
free(s);
}

int main(int argc, char **argv) {
const char * fmt = "%d";

test(fmt , 1 );
test(fmt , 2 );
test(fmt , 3 );
}
[...]
I am in some invoking undefined behaviour, or is this a bug somewhere
in my 64 bit toolchain?

Try "refreshing" the va_list and see it that helps.

--
Er*********@sun.com

[Joakim Hove]

Hello,

thank you both for answering. I have tried to explicitly initialize
the new_path variable to NULL; but that did unfortunately not help.

* * *Try "refreshing" the va_list and see it that helps.

Ehhh - what do you mean be refreshing?

Best Regards

Joakim

[Eric Sosman]

Joakim Hove wrote:

Hello,

thank you both for answering. I have tried to explicitly initialize
the new_path variable to NULL; but that did unfortunately not help.

> Try "refreshing" the va_list and see it that helps.

Ehhh - what do you mean be refreshing?

Go back and look at the lines I added to your code.

--
Er*********@sun.com

[Joakim Hove]

On 14 Mar, 17:55, Eric Sosman <[e-mail]:

Ehhh - what do you mean be refreshing?

* * *Go back and look at the lines I added to your code.

Got it - thanks a lot :-) Now it is working.

Joakim