Stream-seeking functionality.

Bugfix in stdio_flush: check FILE * handle for null.

Minor cleanups.

* eval.c (eval_init): Register seek_stream as intrinsic.

* stream.c (from_start_k, from_current_k, from_end_k): New symbol
variables.
(strm_whence): New enum.
(strm_ops): New member, seek.
(stdio_maybe_write_error): Renamed to stdio_maybe_error; takes
new string argument to describe action.
(stdio_put_string, stdio_put_char, stdio_put_byte): Updated call to
stdio_maybe_error.
(stdio_flush): Updated call to stdio_maybe_error. Check
handle for null.
(stdio_seek): New static function.
(stdio_ops): Added stdio_seek.
(pipe_ops, string_in_ops, byte_in_ops, string_out_ops,
strlist_out_ops, dir_ops): Added explicit zero entries and comments for
unimplemented functions.
(seek_stream): New function.
(stream_init): New keyword symbols interned.

* stream.h (from_start_k, from_current_k, from_end_k): New
variables declared.
(dev_k, ino_k, mode_k, nlink_k, uid_k, gid_k, rdev_k, size_k,
blksize_k, blocks_k, atime_k, mtime_k, ctime_k, s_ifmt, s_iflnk,
s_ifreg, s_ifblk, s_ifdir, s_ifchr, s_ififo, s_isuid, s_isgid, s_isvtx,
s_irwxu, s_irusr, s_iwusr, s_ixusr, s_irwxg, s_irgrp,
s_iwgrp): Existing extern variables declared.
(seek_stream): New function declared.

* txr.1: Documented seek-stream.

1 files changed, 34 insertions, 0 deletions

diff --git a/txr.1 b/txr.1
index 571e2271..6d98aab7 100644
--- a/txr.1
+++ b/txr.1
@@ -10241,6 +10241,40 @@ Calling this function causes all accumulated data inside <stream> to be passed
 to the operating system. If called on streams for which this function is not
 meaningful, it does nothing.
 
+.SS Function seek-stream
+
+.TP
+Syntax:
+
+  (seek-stream <stream> <offset> <whence>)
+
+.TP
+Description:
+
+The seek-stream function is meaningful for file streams. It changes the
+current read/write position within <stream>.  It can also be used
+to determine the current position: see the notes about the
+return value below. 
+
+The <offset> argument is a positive or negative integer which gives a
+displacement that is measured from the point identified by the <whence>
+argument.
+
+Note that for text files, there isn't necessarily a 1:1 correspondence between
+characters and positions due to line-ending conversions and conversions
+to and from UTF-8.
+
+The <whence> argument is one of three keywords: :from-start, :from-current
+and :from-end. These denote the start of the file, the current position
+and the end of the file.
+
+If <offset> is zero, and <whence> is :from-current, then
+seek-stream returns the current absolute position within the
+stream, if it can successfully obtain it. Otherwise, it
+returns t if it is successful.
+
+On failure, it throws an exception of type stream-error.
+
 .SH FILESYSTEM ACCESS
 
 .SS Function stat