Update to gobook@27cf7b490526a0a0751ff12ed7cc4acd8afa2edc

ch13/bzip/bzip2.c

@@ -2,6 +2,14 @@
 // License: https://creativecommons.org/licenses/by-nc-sa/4.0/
 
 // See page 362.
+//
+// The version of this program that appeared in the first and second
+// printings did not comply with the proposed rules for passing
+// pointers between Go and C, described here:
+// https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md
+//
+// The version below, which appears in the third printing,
+// has been corrected.  See bzip2.go for explanation.
 
 //!+
 /* This file is gopl.io/ch13/bzip/bzip2.c,         */
@@ -17,12 +25,7 @@ int bz2compress(bz_stream *s, int action,
   int r = BZ2_bzCompress(s, action);
   *inlen -= s->avail_in;
   *outlen -= s->avail_out;
-
-  /* "C code may store a Go pointer in C memory subject to rule 2: 
-   * it must stop storing the pointer before it returns to Go." */
-  s->next_in = NULL;
-  s->next_out = NULL;
-  
+  s->next_in = s->next_out = NULL;
   return r;
 }