(doprnt): Use a fixed buffer to store the format

specification, but only if we are sure it will fit.  Otherwise,
use alloca () to get enouhg space.  Don't allow negative size
specifications to core dump; instead, treat them as positive
("%-20d" == "%20d").

src/doprnt.c

@@ -42,26 +42,38 @@ doprnt (buffer, bufsize, format, format_end, nargs, args)
   int cnt = 0;			/* Number of arg to gobble next */
   register char *fmt = format;	/* Pointer into format string */
   register char *bufptr = buffer; /* Pointer into output buffer.. */
+
   /* Use this for sprintf unless we need something really big.  */
   char tembuf[100];
+
   /* Size of sprintf_buffer.  */
   int size_allocated = 100;
+
   /* Buffer to use for sprintf.  Either tembuf or same as BIG_BUFFER.  */
   char *sprintf_buffer = tembuf;
+
   /* Buffer we have got with malloc.  */
   char *big_buffer = 0;
+
   register int tem;
   char *string;
-  char fmtcpy[20];
+  char fixed_buffer[20];	/* Default buffer for small formatting. */
+  char *fmtcpy;
   int minlen;
   int size;			/* Field width factor; e.g., %90d */
 
   if (format_end == 0)
     format_end = format + strlen (format);
 
+  if ((format_end - format + 1) < sizeof (fixed_buffer))
+    fmtcpy = fixed_buffer;
+  else
+    fmtcpy = alloca (format_end - format + 1);
+
   bufsize--;
-  while (fmt != format_end && bufsize > 0)	/* Loop until end of format
-						   string or buffer full */
+
+  /* Loop until end of format string or buffer full. */
+  while (fmt != format_end && bufsize > 0)
     {
       if (*fmt == '%')	/* Check for a '%' character */
 	{
@@ -71,16 +83,22 @@ doprnt (buffer, bufsize, format, format_end, nargs, args)
 	  /* Copy this one %-spec into fmtcpy.  */
 	  string = fmtcpy;
 	  *string++ = '%';
-	  while (string < fmtcpy + sizeof fmtcpy - 1)
+	  while (1)
 	    {
 	      *string++ = *fmt;
-	      if (! (*fmt >= '0' && *fmt <= '9') && *fmt != '-' && *fmt != ' ')
+	      if (! (*fmt >= '0' && *fmt <= '9')
+		  && *fmt != '-' && *fmt != ' '&& *fmt != '.')
 		break;
 	      fmt++;
 	    }
 	  *string = 0;
 	  /* Get an idea of how much space we might need.  */
 	  size_bound = atoi (&fmtcpy[1]) + 50;
+
+	  /* Avoid pitfall of negative "size" parameter ("%-200d"). */
+	  if (size_bound < 0)
+	    size_bound = -size_bound;
+
 	  /* Make sure we have that much.  */
 	  if (size_bound > size_allocated)
 	    {