Add a "G" option to force gzip, disabling transparency in gzread().

If the input is not a gzip stream with this option, an error will
be returned.

1 files changed, 17 insertions, 3 deletions

diff --git a/gzlib.c b/gzlib.c
index 2f9cdd4e..4c1aa83a 100644
--- a/gzlib.c
+++ b/gzlib.c
@@ -153,6 +153,9 @@ local gzFile gz_open(const void *path, int fd, const char *mode) {
             case 'F':
                 state->strategy = Z_FIXED;
                 break;
+            case 'G':
+                state->direct = -1;
+                break;
             case 'T':
                 state->direct = 1;
                 break;
@@ -168,14 +171,25 @@ local gzFile gz_open(const void *path, int fd, const char *mode) {
         return NULL;
     }
 
-    /* can't force transparent read */
+    /* direct is 0, 1 if "T", or -1 if "G" (last "G" or "T" wins) */
     if (state->mode == GZ_READ) {
-        if (state->direct) {
+        if (state->direct == 1) {
+            /* can't force a transparent read */
             free(state);
             return NULL;
         }
-        state->direct = 1;      /* for empty file */
+        if (state->direct == 0)
+            /* default when reading is auto-detect of gzip vs. transparent --
+               start with a transparent assumption in case of an empty file */
+            state->direct = 1;
+    }
+    else if (state->direct == -1) {
+        /* "G" has no meaning when writing -- disallow it */
+        free(state);
+        return NULL;
     }
+    /* if reading, direct == 1 for auto-detect, -1 for gzip only; if writing or
+       appending, direct == 0 for gzip, 1 for transparent (copy in to out) */
 
     /* save the path name for error messages */
 #ifdef WIDECHAR