From: eyal DOT ben-david AT aks DOT com
To: blizzar AT hem1 DOT passagen DOT se
cc: djgpp AT delorie DOT com
Message-ID: <42256535.004F387B.00@aks.com>
Date: Sun, 19 Oct 1997 16:33:07 +0200
Subject: Re: sprintf() string length?
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>>I once read a solution to this (can't remember where though!):
>>
>>The "fprintf" command will return the number of characters output,
>>and you should fopen DOS's "NULL" file to send the output to.
>>"NULL" is a "black hole" which will swallow and ignore anything you
>>send to it.  Although "NULL" never appears in a directory, it
>>exists everywhere.  (I think the UNIX equivalent is "/dev/nul",
>>but I've never used UNIX.)
This  is a fine idea. It works with DJGPP (see below)
but I don't think that it is portable.

>Does anyone know if this works, and how compatible it is?
>Writing to a NULL pointer doesn't sound too good to me!
You don't write to a NULL pointer but to the NUL (one L) device.

Here is a test program that demonstrate the method

/*************   cut  here **************/

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

int get_sprintf_len(const char* format, ...)
{
   va_list arg_ptr;
   FILE*   NulDevice;
   int     len;

   va_start(arg_ptr, format);

   NulDevice = fopen("NUL", "w");
   len       = vfprintf(NulDevice, format, arg_ptr);
   fclose(NulDevice);

   va_end(arg_ptr);
   return len;
}


int main()
{
   const char* message;
   int         len;

   message = "Hello get_sprintf_len !\n";
   len = get_sprintf_len("%s",  message);
   printf("len  =  %d\n", len);

   if (len > 0)
   {
      char* str;

      str = (char*) malloc(len + 1);
      sprintf(str, "%s", message);
      printf("Message copied is: %s\n", str);
      free(str);
   }

   return 0;
}

Eyal