vsnprintf is broken

The_Nerd · 03-14-2005, 12:13 AM

Alright, here is my code:

Code:

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

void Message(char *cpStr, ...)
{
	va_list args;
	va_start(args, cpStr);
	long lLength;
	char *cpTemp;
	lLength=vsnprintf(NULL, 0, cpStr, args);
	cpTemp=(char *)malloc((lLength+1)*sizeof(char));
	if (!cpTemp)
	{
		fprintf(stderr, "Error: Unable to allocate memory for opperation");
	}
	if (vsnprintf(cpTemp, lLength, cpStr, args)<0)
	{
		fprintf(stderr, "Error: Unable to complete opperation");
	}
	va_end(args);
	fprintf(stderr, "%s\n", cpTemp);
	free(cpTemp);
}

int main(int argc, char **argv)
{
	Message("Initializing SDL. %s\n", "Hello world!");
}

I compile this with g++, and the programs runs. However, the string it prints is:

Code:

Initializing SDL. g

When it should be:

Code:

Initializing SDL. Hello World!

I've already tested lLength, it is holding the correct length after the call to the first vsnprintf(NULL, 0, etc, etc...);

Help would be good!

My system is a 64bit system running Fedora Core 3.

Hivemind · 03-14-2005, 05:17 AM

The following code works for me. Note that I am not including malloc.h. It's a non-standard header that's not needed in your program. vsnprintf() is also non-standard btw.

Code:

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

static void message(const char *, ...);

int
main(void)
{
   message("Hello, %s\n", "World!");

   return 0;
}

static void
message(const char *format, ...)
{
   va_list args;
   int length = 0;
   char *buffer = NULL;

   va_start(args, format);

   length = vsnprintf(NULL, 0, format, args);

   assert(length > 0);

   ++length; /* Make sure we have room for \0 */

   buffer = malloc(length); /* sizeof(char) not needed */

   vsnprintf(buffer, length, format, args);

   va_end(args);

   printf("%s", buffer);

   free(buffer);
}

Hivemind · 03-14-2005, 05:56 AM

One more thing: If the string passed to the message() function doesn't end with a newline you might want to flush the output buffer so you make sure the string is displayed right away.

itsme86 · 03-14-2005, 05:00 PM

Tell me where you found that it was okay to pass a NULL pointer as the first argument to vsnprintf() like you did here:

Code:

lLength=vsnprintf(NULL, 0, cpStr, args);

This is most likely evil. And trust me, the last place you want to declare as the broken part of your code is within a libc function. Guaranteed you're just using it incorrectly.

Hivemind · 03-14-2005, 05:11 PM

Works fine with newlib's vsnprintf() too.
My manual says:
The number of characters that would have been written into the array, not counting the terminating null character, had count been large enough. It does this even if count is zero; in this case buf can be NULL.

keefaz · 03-14-2005, 07:00 PM

There is a working sample code for vsnprintf() in the corresponding man page, just use it

itsme86 · 03-14-2005, 07:33 PM

Quote:

Works fine with newlib's vsnprintf() too.
My manual says:
The number of characters that would have been written into the array, not counting the terminating null character, had count been large enough. It does this even if count is zero; in this case buf can be NULL.

I stand corrected. Apologies.

The_Nerd · 03-17-2005, 12:21 PM

I tried man pages, webpages, and it was still broken. Not to mention that it works on another computer! Anyhow, I just wrote my own, and it works fine. Thanks guys.

akulo · 02-10-2006, 03:46 PM

Like the original poster, I've been having problems with vsnprintf() producing garbage on a 64--bit machine running Linux (SuSE, not Fedora). I've found a workaround, at least on my system:

Code:

	va_list args;

	va_start(args, cpStr);

	long lLength;
	char *cpTemp;
	lLength=vsnprintf(NULL, 0, cpStr, args);
	cpTemp=(char *)malloc((lLength+1)*sizeof(char));
	if (!cpTemp)
	{
		fprintf(stderr, "Error: Unable to allocate memory for opperation");
	}

	va_end(args);

	va_start(args, cpStr);

	if (vsnprintf(cpTemp, lLength, cpStr, args)<0)
	{
		fprintf(stderr, "Error: Unable to complete opperation");
	}

	va_end(args);

	fprintf(stderr, "%s\n", cpTemp);
	free(cpTemp);

.