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No format string passed to variable argument list function

Hi,

I have a simple printf-like function:

int print(const char *format, ...)
{
char buffer[1024];
va_list argptr;
int i;

va_start(argptr , format);
i = vsnprintf(buffe r, sizeof(buffer), format, argptr);
va_end(argptr);

buffer[sizeof(buffer)-1] = '\0';

printf("%s\n",b uffer); /* this bit just for the sake of testing */

return i;
}

If I call the function using something like:
char message[50];
strcpy(message, "hi there");
print("%s",mess age);

everything works, but if I do:
print(message);

it doesn't (program crashes with an abort).

Is there something wrong with what I'm doing, or should I be looking
elsewhere to work out the cause of my crash?

Thanks a lot,
Adam
Oct 15 '08 #1
30 2766
Adam <ne**@snowstone .org.ukwrites:
Hi,

I have a simple printf-like function:

int print(const char *format, ...)
{
char buffer[1024];
va_list argptr;
int i;

va_start(argptr , format);
i = vsnprintf(buffe r, sizeof(buffer), format, argptr);
va_end(argptr);

buffer[sizeof(buffer)-1] = '\0';

printf("%s\n",b uffer); /* this bit just for the sake of testing */

return i;
}

If I call the function using something like:
char message[50];
strcpy(message, "hi there");
print("%s",mess age);

everything works, but if I do:
print(message);

it doesn't (program crashes with an abort).
I don't see a problem myself. I took your function and added a main function:

#include <stdarg.h>
#include <stdio.h>

/* your function */

int main(void) {
print("%s\n", "Hello world!");
print("Goodbye world!\n");
return 0;
}

and it compiles and runs without problems on three different systems.

There could be something that I'm missing, though, which doesn't show up
on those systems. Can you post a complete program that aborts, and tell
us the system and compiler you're using?

It certainly seems like v*printf should be able to handle a va_list from
a function with no extra arguments, though the standard doesn't
explicitly seem to say so. It's conceivable that there's a bug in your
standard library, but hard to say just yet.
Oct 15 '08 #2
Adam <ne**@snowstone .org.ukwrites:
I have a simple printf-like function:

int print(const char *format, ...)
{
char buffer[1024];
va_list argptr;
int i;

va_start(argptr , format);
i = vsnprintf(buffe r, sizeof(buffer), format, argptr);
va_end(argptr);

buffer[sizeof(buffer)-1] = '\0';
This isn't your problem, but what's the purpose of the above?
vsnprintf() should already write a '\0'-terminated string to buffer;
you're just setting character 1023 (which is likely far beyond the end
of the string) to '\0'.
printf("%s\n",b uffer); /* this bit just for the sake of testing */

return i;
}

If I call the function using something like:
char message[50];
strcpy(message, "hi there");
print("%s",mess age);

everything works, but if I do:
print(message);

it doesn't (program crashes with an abort).

Is there something wrong with what I'm doing, or should I be looking
elsewhere to work out the cause of my crash?
It would help if you could post a small, complete, compilable program
that demonstrates the problem. I don't see anything in what you
posted that explains the symptom you're seeing.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Oct 16 '08 #3
Adam wrote:
Hi,

I have a simple printf-like function:
Did you include <stdarg.h>?

--
Ian Collins
Oct 16 '08 #4
Adam <n...@snowstone .org.ukwrote:
I have a simple printf-like function:

int print(const char *format, ...)
{
* char buffer[1024];
Maybe make this static. There's no upper or lower limit
on automatic storage, but I prefer not to allocate more
than 256 bytes in a single function.
* va_list argptr;
* int i;

* va_start(argptr , format);
* i = vsnprintf(buffe r, sizeof(buffer), format, argptr);
Note that [v]snprintf is new in C99. Although many C90
implementations have it as an extension, they may
differ in semantics and prototype.

Did you include the correct header or prototype?
Is print defined before or after use? If after,
did you prototype it correctly before use?
* va_end(argptr);

* buffer[sizeof(buffer)-1] = '\0';
This shouldn't be necessary.
* printf("%s\n",b uffer); /* this bit just for the sake of testing */
puts(buffer) is better since the result string could have
% characters in it that will be subject to conversion.
* return i;
}

If I call the function using something like:
char message[50];
strcpy(message, "hi there");
char message[50] = "hi there";
print("%s",mess age);

everything works, but if I do:
print(message);

it doesn't (program crashes with an abort).
Post a compilable program that exhibits the problem.

Also try print("%s\n", message);
Is there something wrong with what I'm doing,
Nothing obvious.
or should I be looking elsewhere to work out the
cause of my crash?
If you're using a C90 extension, you should check
the manual on what vsnprintf does.

--
Peter
Oct 16 '08 #5
Ian Collins <ia******@hotma il.comwrites:
Adam wrote:
>I have a simple printf-like function:
Did you include <stdarg.h>?
He must have; otherwise the identifier "va_arg" wouldn't be visible.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Oct 16 '08 #6
Keith Thompson wrote:
Ian Collins <ia******@hotma il.comwrites:
>Adam wrote:
>>I have a simple printf-like function:
Did you include <stdarg.h>?

He must have; otherwise the identifier "va_arg" wouldn't be visible.
Or va_list. Yes it was a silly question!

--
Ian Collins
Oct 16 '08 #7
Ian Collins <ian-n...@hotmail.co mwrote:
Keith Thompson wrote:
Ian Collins <ian-n...@hotmail.co mwrites:
Adam wrote:
I have a simple printf-like function:
>
Did you include <stdarg.h>?
He must have; otherwise the identifier "va_arg"
wouldn't be visible.

Or va_list. *Yes it was a silly question!
Not necessarily. How do you know Adam isn't using
<varargs.h>?

--
Peter
Oct 16 '08 #8
Peter Nilsson <ai***@acay.com .auwrites:
Adam <n...@snowstone .org.ukwrote:
<snip>
>Â* printf("%s\n",b uffer); /* this bit just for the sake of testing */

puts(buffer) is better since the result string could have
% characters in it that will be subject to conversion.
Did you misread the printf? It looks fine to me (though puts probably
has the edge).

--
Ben.
Oct 16 '08 #9
Ian Collins <ia******@hotma il.comwrites:
Keith Thompson wrote:
>Ian Collins <ia******@hotma il.comwrites:
>>Adam wrote:
I have a simple printf-like function:

Did you include <stdarg.h>?

He must have; otherwise the identifier "va_arg" wouldn't be visible.
Or va_list. Yes it was a silly question!
Yes, I meant va_list.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Oct 16 '08 #10

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