Re: Using vsnprintf or vasprintf in sid

[Scott Dattalo <scott at dattalo dot com>]

On 26 May 2003, Ben Elliston wrote:

> Hi Dave!
> 
> Dave Brolley <brolley@redhat.com> writes:
> 
> > I have a need for printf-like printing into a buffer, however I have
> > no control over the input and so I don't know the size of the buffer
> > which will be required. It looks like vsnprintf (_ISOC99_SOURCE) or
> > vasprintf (_GNU_SOURCE) are what I need. Is it ok to use these
> > function in sid and, if so, how do I get configure to add the
> > necessary -Dxxxx to the sid compilations and (if necessary) specify
> > the library to look in?
> 
> I'm not aware of any policy (the problem with SID is that it lacks the
> strict policies and exciting politics of other free software
> projects!) that would prevent you from using either of these functions
> in SID, provided that you use Autoconf tests as you suggest.  I think
> it's important to have a fall-back implementation, though, in the case
> that neither is available.
> 
> Take a look at using AC_CHECK_FUNC to do the test and set a #define in
> config.h to indicate the function(s) presence.  If you can, try and
> isolate the test to the configure script used in the part of the
> source tree you're working in rather than running the test in the
> top-level configure script.

IIRC, when I used vnsprintf in SDCC the Solaris users were affected. 
Here's the code in question:


#ifdef USE_VSNPRINTF
  // Alas, vsnprintf is not ANSI standard, and does not exist
  // on Solaris (and probably other non-Gnu flavored Unixes).

/*-----------------------------------------------------------------*/
/* SAFE_snprintf - like snprintf except the string pointer is      */
/*                 after the string has been printed to. This is   */
/*                 useful for printing to string as though if it   */
/*                 were a stream.                                  */
/*-----------------------------------------------------------------*/
void SAFE_snprintf(char **str, size_t *size, const  char  *format, ...)
{
  va_list val;
  int len;

  if(!str || !*str)
    return;

  va_start(val, format);

  vsnprintf(*str, *size, format, val);

  va_end (val);

  len = strlen(*str);
  if(len > *size) {
    fprintf(stderr,"WARNING, it looks like %s has 
overflowed\n",__FUNCTION__);
    fprintf(stderr,"len = %d is > str size %d\n",len,*size);
  }

  *str += len;
  *size -= len;

}

#else  //  USE_VSNPRINTF

// This version is *not* safe, despite the name.

void SAFE_snprintf(char **str, size_t *size, const  char  *format, ...)
{
  va_list val;
  int len;
  static char buffer[1024]; /* grossly conservative, but still not 
inherently safe */

  if(!str || !*str)
    return;

  va_start(val, format);

  vsprintf(buffer, format, val);
  va_end (val);

  len = strlen(buffer);
  if(len > *size) {
    fprintf(stderr,"WARNING, it looks like %s has 
overflowed\n",__FUNCTION__);
    fprintf(stderr,"len = %d is > str size %d\n",len,*size);
  }

  strcpy(*str, buffer);
  *str += len;
  *size -= len;

}

#endif    //  USE_VSNPRINTF


So the fix is a hack that really isn't fool-proof. 

Scott