Casting a negative value to unsigned makes it positive; I'm not sure

that will do the right thing here.  Instead, cast its negative (which is
positive) to unsigned, use that value as the adjustment, and flip the
signs of the subsequent adjustment operations.

svn path=/trunk/; revision=43105

1 files changed, 16 insertions, 3 deletions

diff --git a/wiretap/file_wrappers.c b/wiretap/file_wrappers.c
index 300f7348b6..210acc581b 100644
--- a/wiretap/file_wrappers.c
+++ b/wiretap/file_wrappers.c
@@ -928,9 +928,22 @@ file_seek(FILE_T file, gint64 offset, int whence, int *err)
 		 */
 		unsigned had = (unsigned)(file->next - file->out);
 		if (-offset <= had) {
-			file->have -= (unsigned)offset;
-			file->next += offset;
-			file->pos += offset;
+			/*
+			 * Offset is negative, so -offset is
+			 * non-negative, and -offset is
+			 * <= an unsigned and thus fits in an
+			 * unsigned.  Get that value and
+			 * adjust appropriately.
+			 *
+			 * (Casting offset to unsigned makes
+			 * it positive, which is not what we
+			 * would want, so we cast -offset
+			 * instead.)
+			 */
+			unsigned adjustment = (unsigned)(-offset);
+			file->have += adjustment;
+			file->next -= adjustment;
+			file->pos -= adjustment;
 			return file->pos;
 		}
 	}