Sign-extend negative long longs correctly.

1 files changed, 23 insertions, 17 deletions

diff --git a/bits.c b/bits.c
index 8530c8c6..20a0dfea 100644
--- a/bits.c
+++ b/bits.c
@@ -8,6 +8,8 @@
  */
 #include <assert.h>
 
+#define DEBUG
+
 #include "bits.h"
 #ifdef DEBUG
 #include <stdio.h>
@@ -15,12 +17,10 @@
 #include "gpsd.h"
 #endif /* DEBUG */
 
-
-
 #define BITS_PER_BYTE	8
 
 unsigned long long ubits(char buf[], unsigned int start, unsigned int width)
-/* extract a bitfield from the buffer as an unsigned big-endian long */
+/* extract a bitfield from the buffer as an unsigned big-endian long long */
 {
     unsigned long long fld = 0;
     unsigned int i;
@@ -31,25 +31,25 @@ unsigned long long ubits(char buf[], unsigned int start, unsigned int width)
 	fld <<= BITS_PER_BYTE;
 	fld |= (unsigned char)buf[i];
     }
-#ifdef DEBUG
+#ifdef UDEBUG
     printf("Extracting %d:%d from %s: segment 0x%llx = %lld\n", start, width,
 	   gpsd_hexdump(buf, 12), fld, fld);
-#endif /* DEBUG */
+#endif /* UDEBUG */
 
     end = (start + width) % BITS_PER_BYTE;
     if (end != 0) {
 	fld >>= (BITS_PER_BYTE - end);
-#ifdef DEBUG
+#ifdef UDEBUG
 	printf("After downshifting by %d bits: 0x%llx = %lld\n", 
 	       BITS_PER_BYTE - end, fld, fld);
-#endif /* DEBUG */
+#endif /* UDEBUG */
     }
 
     fld &= ~(0xffffffff << width);
-#ifdef DEBUG
+#ifdef UDEBUG
     printf("After selecting out the bottom %u bits: 0x%llx = %lld\n", 
 	   width, fld, fld);
-#endif /* DEBUG */
+#endif /* UDEBUG */
 
     return fld;
 }
@@ -57,16 +57,22 @@ unsigned long long ubits(char buf[], unsigned int start, unsigned int width)
 signed long long sbits(char buf[], unsigned int start, unsigned int width)
 /* extract a bitfield from the buffer as a signed big-endian long */
 {
-    unsigned long long un = ubits(buf, start, width);
-    signed long long fld;
+    unsigned long long fld = ubits(buf, start, width);
 
+#ifdef DEBUG
+    (void)fprintf(stderr, "sbits(%d, %d) extracts %llx\n", start, width, fld);
+#endif /* DEBUG */
     /*@ +relaxtypes */
-    if (un & (1 << width))
-	fld = -(un & ~(1 << width));
-    else
-	fld = (signed long long)un;
-    
-    return fld;
+    if (fld & (1 << (width-1))) {
+#ifdef DEBUG
+	(void)fprintf(stderr, "%llx is signed\n", fld);
+#endif /* DEBUG */
+	fld |= (-1LL << (width-1));
+    }
+#ifdef DEBUG
+    (void)fprintf(stderr, "sbits(%d, %d) returns %lld\n", start, width, (signed long long)fld);
+#endif /* DEBUG */
+    return (signed long long)fld;
     /*@ -relaxtypes */
 }