Ensure and document that SEEK works on unbuffered ports only.

RELEASE

@@ -191,6 +191,8 @@ We manage the project using SourceForge:
    This release adds support for SRFI 42.
 
 ** Bug fixes
+   - SEEK currently works on unbuffered ports only. Check this in the
+     implementation and oopsify it in the manual.
    - Adjust documentation of some low-level regexp procedures
    - Removed message argument form errno-error
    - After fork/pipe, make the ports returned by the pipe the

doc/scsh-manual/syscalls.tex

@@ -572,6 +572,7 @@ this is dependent on the OS implementation.
 The return value is the resulting position of the I/O cursor in the I/O stream.
 \oops{The current implementation doesn't handle \var{offset} arguments
         that are not immediate integers (\ie, representable in 30 bits).}
+\oops{The current implementation doesn't handle buffered ports.}
 \end{desc}
 
 

scsh/syscalls.scm

@@ -375,6 +375,12 @@
 (define seek/end 2)
 
 (define (seek fd/port offset . maybe-whence)
+  (if (and (open-input-port? fd/port)
+           (> (byte-vector-length (port-buffer fd/port)) 1))
+      (error "Seek does currently not work on buffered ports" fd/port))
+  (if (and (open-output-port? fd/port)
+           (> (byte-vector-length (port-buffer fd/port)) 0))
+      (error "Seek does currently not work on buffered ports" fd/port))
   (let ((whence (:optional maybe-whence seek/set))
 	(fd (if (integer? fd/port) fd/port (port->fdes fd/port))))
     (%fd-seek fd offset whence)))