Source Code 1992 March/Source_Code_CD-ROM_Walnut_Creek_March_1992.iso/usenet/compsrcs/misc/volume06/u16.pc1

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From decwrl!labrea!eos!ames!mailrus!csd4.milw.wisc.edu!leah!itsgw!steinmetz!uunet!allbery Sun Jan 29 16:45:48 PST 1989
Article 788 of comp.sources.misc:
Path: granite!decwrl!labrea!eos!ames!mailrus!csd4.milw.wisc.edu!leah!itsgw!steinmetz!uunet!allbery
From: [e-mail] (Brandon S. Allbery - comp.sources.misc)
Newsgroups: comp.sources.misc
Subject: v06i013: u16 patch 1
Message-ID: <[e-mail]>
Date: 24 Jan 89 03:07:23 GMT
Sender: [e-mail]
Reply-To: [e-mail] (Tom Horsley)
Lines: 157
Approved: [e-mail] (Brandon S. Allbery - comp.sources.misc)

Posting-number: Volume 6, Issue 13
Submitted-by: [e-mail] (Tom Horsley)
Archive-name: u16.patch1

[This just about clears out the comp.sources.misc queue -- there are 4 more
submissions to go, but they require further thought.  ++bsa]

This is patch number 1 (and hopefully patch number last) to the u16
16 bit uncompress for the PC.
----------------------------cut here-------------------------------
*** u16.c.orig    Tue Jan  3 15:34:51 1989
--- u16.c    Tue Jan  3 15:34:28 1989
***************
*** 6,11 ****
--- 6,23 ----
   * Parts written (other parts plagarized) by Tom Horsley
   *   ([e-mail])
   *   Dec 1988.
+  *
+  * Version 1.1
+  *
+  * Bug1 - fixed Dec 22, 1988
+  *   The initial check for end of file that attempted to just use
+  *   the current byte pointer was not good enough. Because the byte
+  *   pointer might look within bounds, but a large code might eat
+  *   some garbage bits beyond the end as it is assembled.
+  *
+  *   The fix was to add a new variable to keep track of the count
+  *   of bits in the buffer and always check to see that there are
+  *   enough bits for at least one new code to be extracted.
   */
  #include <stdio.h>
  #include <fcntl.h>
***************
*** 82,88 ****
   * message you get with the -H option.
   */
  unsigned char buf[MAXBUF] = "\
! u16 - 16 bit LZW uncompress for the IBM PC\n\
  u16 [-H] [files...]\n\
  \n\
  -H\tPrint this message and exit.\n\
--- 94,100 ----
   * message you get with the -H option.
   */
  unsigned char buf[MAXBUF] = "\
! u16 - 16 bit LZW uncompress for the IBM PC, version 1.1\n\
  u16 [-H] [files...]\n\
  \n\
  -H\tPrint this message and exit.\n\
***************
*** 97,102 ****
--- 109,119 ----
  on the ends of file names.\n"
  ;

+ /* Number of bits in the buffer.
+  */
+ long          bitsinbuf = 0;
+ long          n_bits;
+
  /* Number of bytes of file data resident in buf.
   */
  int          bufsize = 0;
***************
*** 167,172 ****
--- 184,190 ----
       eofmark = &buf[bufsize];
        } else {
       bufsize += cursize;
+      bitsinbuf += (((long)cursize) << 3);
        }
     }
     if (eofmark == NULL) {
***************
*** 203,210 ****
         * have recieved a clear code then flush the current size code
         * and advance to next size.
         */
!       while (cb.codep != vartab[curvartab].origp) xcode(&cb);
!       if (cb.bufp >= endbuf) return(-1L);
        if (clear_flg > 0) {
       curvartab = 0;
       clear_flg = 0;
--- 221,230 ----
         * have recieved a clear code then flush the current size code
         * and advance to next size.
         */
!       while (cb.codep != vartab[curvartab].origp) {
!      xcode(&cb);
!      bitsinbuf -= n_bits;
!       }
        if (clear_flg > 0) {
       curvartab = 0;
       clear_flg = 0;
***************
*** 220,232 ****
        (*vartab[curvartab].initp)(&cb);
        vartab[curvartab].origp = cb.codep;
        maxcode = vartab[curvartab].maxcode;
  #ifdef DEBUG
        fprintf(stderr,
       "switching to %d bit codes, bytes_out = %ld, free_ent = %ld\n",
!      vartab[curvartab].n_bits,bytes_out, free_ent);
  #endif
     }
!    return (long)(xcode(&cb));
  }

  /* Decompress stdin to stdout.    This routine adapts to the codes in
--- 240,258 ----
        (*vartab[curvartab].initp)(&cb);
        vartab[curvartab].origp = cb.codep;
        maxcode = vartab[curvartab].maxcode;
+       n_bits = (long)vartab[curvartab].n_bits;
  #ifdef DEBUG
        fprintf(stderr,
       "switching to %d bit codes, bytes_out = %ld, free_ent = %ld\n",
!      n_bits,bytes_out, free_ent);
  #endif
     }
!    bitsinbuf -= n_bits;
!    if (bitsinbuf < 0) {
!       return -1L;
!    } else {
!       return (long) xcode(&cb);
!    }
  }

  /* Decompress stdin to stdout.    This routine adapts to the codes in
***************
*** 270,276 ****
--- 296,304 ----
     /* Read the iniital buffer worth of data and check magic numbers
      * and flags.
      */
+    bitsinbuf = 0;
     ReadBuf();
+    bitsinbuf -= 3 * 8;
     if (bufsize < 3) {
        fputs("u16: Missing file header.\n",stderr);
        return 1;
***************
*** 298,303 ****
--- 326,332 ----
     /*
      * initialize the first 256 entries in the table.
      */
+    n_bits = vartab[0].n_bits;
     maxcode = vartab[0].maxcode;
     for ( code = 255; code >= 0; code-- ) {
        tab_prefixof(code) = 0;
--------------------------- cut here --------------------------------
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