Index: tclUnixFCmd.c
===================================================================
RCS file: /cvsroot/tcl/tcl/unix/tclUnixFCmd.c,v
retrieving revision 1.58
diff -u -r1.58 tclUnixFCmd.c
--- tclUnixFCmd.c	2 Nov 2006 15:58:09 -0000	1.58
+++ tclUnixFCmd.c	3 Nov 2006 09:18:22 -0000
@@ -540,7 +540,9 @@
     blockSize = 4096;
 #endif
 #endif
-
+    if (blockSize <= 0) {
+        blockSize = 4096;
+    }
     buffer = ckalloc(blockSize);
     while (1) {
 	nread = (size_t) read(srcFd, buffer, blockSize);
