Fix a large number of bugs.

[mLib] / man / lbuf.3

.\" -*-nroff-*-
.TH lbuf 3 "6 July 1999" "Straylight/Edgeware" "mLib utilities library"
.SH "NAME"
lbuf \- split lines out of asynchronously received blocks
.\" @lbuf_flush
.\" @lbuf_close
.\" @lbuf_free
.\" @lbuf_snarf
.\" @lbuf_setsize
.\" @lbuf_init
.\" @lbuf_destroy
.SH "SYNOPSIS"
.nf
.B "#include <mLib/lbuf.h>"

.BI "void lbuf_flush(lbuf *" b ", char *" p ", size_t " len );
.BI "void lbuf_close(lbuf *" b );
.BI "size_t lbuf_free(lbuf *" b ", char **" p );
.BI "void lbuf_snarf(lbuf *" b ", const void *" p ", size_t " sz );
.BI "void lbuf_setsize(lbuf *" b ", size_t " sz );
.BI "void lbuf_init(lbuf *" b ,
.BI "               void (*" func ")(char *" s ", void *" p ),
.BI "               void *" p );
.BI "void lbuf_destroy(lbuf *" b );
.fi
.SH "DESCRIPTION"
The declarations in
.B <mLib/lbuf.h>
implement a handy object called a
.IR "line buffer" .
Given unpredictably-sized chunks of data, the line buffer extracts
completed lines of text and passes them to a caller-supplied function.
This is useful in nonblocking network servers, for example: the server
can feed input from a client into a line buffer as it arrives and deal
with completed text lines as they appear without having to wait for
newline characters.
.PP
The state of a line buffer is stored in an object of type
.BR lbuf .
This is a structure which must be allocated by the caller.  The
structure should normally be considered opaque (see the section on
.B Disablement
for an exception to this).
.SS "Initialization and finalization"
The function
.B lbuf_init
initializes a line buffer ready for use.  It is given three arguments:
.TP
.BI "lbuf *" b
A pointer to the block of memory to use for the line buffer.  The line
buffer will allocate memory to store incoming data automatically: this
structure just contains bookkeeping information.
.TP
.BI "void (*" func ")(char *" s ", void *" p )
The
.I line-handler
function to which the line buffer should pass completed lines of text.
.TP
.BI "void *" p
A pointer argument to be passed to the function when a completed line of
text arrives.
.PP
The amount of memory set aside for reading lines is configurable.  It
may be set by calling
.B lbuf_setsize
at any time when the buffer is empty.  The default limit is 256 bytes.
Lines longer than the limit are truncated.  By default, the buffer is
allocated from the current arena,
.BR arena_global (3);
this may be changed by altering the buffer's
.B a
member to refer to a different arena at any time when the buffer is
unallocated.
.PP
A line buffer must be destroyed after use by calling
.BR lbuf_destroy ,
passing it the address of the buffer block.
.SS "Inserting data into the buffer"
There are two interfaces for inserting data into the buffer.  One's much
simpler than the other, although it's less expressive.
.PP
The simple interface is
.BR lbuf_snarf .
This function is given three arguments: a pointer
.I b
to a line buffer structure; a pointer
.I p
to a chunk of data to read; and the size
.I sz
of the chunk of data.  The data is pushed through the line buffer and
any complete lines are passed on to the line handler.
.PP
The complex interface is the pair of functions
.B lbuf_free
and
.BR lbuf_flush .
.PP
The 
.B lbuf_free
function returns the address and size of a free portion of the line
buffer's memory into which data may be written.  The function is passed
the address 
.I b
of the line buffer.  Its result is the size of the free area, and it
writes the base address of this free space to the location pointed to by
the argument
.IR p .
The caller's data must be written to ascending memory locations starting
at
.BI * p
and no data may be written beyond the end of the free space.  However,
it isn't necessary to completely fill the buffer.
.PP
Once the free area has had some data written to it,
.B lbuf_flush
is called to examine the new data and break it into text lines.  This is
given three arguments:
.TP
.BI "lbuf *" b
The address of the line buffer.
.TP
.BI "char *" p
The address at which the new data has been written.  This must be the
base address returned from
.BR lbuf_free .
.TP
.BI "size_t " len
The number of bytes which have been written to the buffer.
.PP
The
.B lbuf_flush
function breaks the new data into lines as described below, and passes
each one in turn to the line-handler function.
.PP
The
.B lbuf_snarf
function is trivially implemented in terms of the more complex
.BR lbuf_free / lbuf_flush
interface.
.SS "Line breaking"
The line buffer considers a line to end with either a simple linefeed
character (the normal Unix convention) or a carriage-return/linefeed
pair (the Internet convention).
.PP
The line buffer has a fixed amount of memory available to it.  This is
deliberate, to prevent a trivial attack whereby a remote user sends a
stream of data containing no newline markers, wasting the server's
memory.  Instead, the buffer will truncate overly long lines (silently)
and return only the initial portion.  It will ignore the rest of the
line completely.
.SS "Line-handler functions"
Completed lines, as already said, are passed to the caller's
line-handler function.  The function is given two arguments:
the address
.I s
of the line which has just been read, and the pointer
.I p
which was set up in the call to
.BR lbuf_init .
The line passed is null-terminated, and has had its trailing newline
stripped.  The area of memory in which the string is located may be
overwritten by the line-handler function, although writing beyond the
terminating zero byte is not permitted.
.PP
The line pointer argument
.I s
may be null to signify end-of-file.  See the next section.
.SS "Flushing the remaining data"
When the client program knows that there's no more data arriving (for
example, an end-of-file condition exists on its data source) it should
call the function
.BR lbuf_close
to flush out the remaining data in the buffer as one last (improperly
terminated) line.  This will pass the remaining text to the line
handler, if there is any, and then call the handler one final time with
a null pointer rather than the address of a text line to inform it of
the end-of-file.
.SS "Disablement"
The line buffer is intended to be used in higher-level program objects,
such as the buffer selector described in
.BR selbuf (3).
Unfortunately, a concept from this high level needs to exist at the line
buffer level, which complicates the description somewhat.  The idea is
that, when a line-handler attached to some higher-level object decides
that it's read enough, it can
.I disable
the object so that it doesn't see any more data.
.PP
Clearly, since an
.B lbuf_flush
call can emit more than one line, so it must be aware that the line
handler isn't interested in any more lines.  However, this fact must
also be signalled to the higher-level object so that it can detach
itself from its data source.
.PP
Rather than invent some complex interface for this, the line buffer
exports one of its structure members,
.BR flags .
A higher-level object wishing to disable the line buffer simply clears
the bit
.B LBUF_ENABLE
in the flags word.
.PP
Disabling a buffer causes an immediate return from
.BR lbuf_flush .
However, it is not permitted for the functions
.B lbuf_flush
or
.B lbuf_close
to be called on a disabled buffer.  (This condition isn't checked for;
it'll just do the wrong thing.)  Furthermore, the
.B lbuf_snarf
function does not handle disablement at all, because it would complicate
the interface so much that it wouldn't have any advantage over the more
general
.BR lbuf_free / lbuf_flush .
.SH "SEE ALSO"
.BR selbuf (3),
.BR mLib (3).
.SH "AUTHOR"
Mark Wooding, <mdw@nsict.org>