2 .TH lbuf 3 "6 July 1999" "Straylight/Edgeware" "mLib utilities library"
4 lbuf \- split lines out of asynchronously received blocks
14 .B "#include <mLib/lbuf.h>"
18 .B "\h'4n'LBUF_STRICTCRLF,"
21 .B "#define LBUF_ENABLE ..."
24 .B "\h'4n'unsigned f;"
28 .B "typedef void lbuf_func(char *" s ", size_t " len ", void *" p );
30 .BI "void lbuf_flush(lbuf *" b ", char *" p ", size_t " len );
31 .BI "void lbuf_close(lbuf *" b );
32 .BI "size_t lbuf_free(lbuf *" b ", char **" p );
33 .BI "void lbuf_snarf(lbuf *" b ", const void *" p ", size_t " sz );
34 .BI "void lbuf_setsize(lbuf *" b ", size_t " sz );
35 .BI "void lbuf_init(lbuf *" b ", lbuf_func *" func ", void *" p );
36 .BI "void lbuf_destroy(lbuf *" b );
41 implement a handy object called a
43 Given unpredictably-sized chunks of data, the line buffer extracts
44 completed lines of text and passes them to a caller-supplied function.
45 This is useful in nonblocking network servers, for example: the server
46 can feed input from a client into a line buffer as it arrives and deal
47 with completed text lines as they appear without having to wait for
50 The state of a line buffer is stored in an object of type
52 This is a structure which must be allocated by the caller. The
53 structure should normally be considered opaque (see the section on
55 for an exception to this).
56 .SS "Initialization and finalization"
59 initializes a line buffer ready for use. It is given three arguments:
62 A pointer to the block of memory to use for the line buffer. The line
63 buffer will allocate memory to store incoming data automatically: this
64 structure just contains bookkeeping information.
66 .BI "lbuf_func *" func
69 function to which the line buffer should pass completed lines of text.
71 .B "Line-handler functions"
72 below for a description of this function.
75 A pointer argument to be passed to the function when a completed line of
78 The amount of memory set aside for reading lines is configurable. It
81 at any time when the buffer is empty. The default limit is 256 bytes.
82 Lines longer than the limit are truncated. By default, the buffer is
83 allocated from the current arena,
85 this may be changed by altering the buffer's
87 member to refer to a different arena at any time when the buffer is
90 A line buffer must be destroyed after use by calling
92 passing it the address of the buffer block.
93 .SS "Inserting data into the buffer"
94 There are two interfaces for inserting data into the buffer. One's much
95 simpler than the other, although it's less expressive.
97 The simple interface is
99 This function is given three arguments: a pointer
101 to a line buffer structure; a pointer
103 to a chunk of data to read; and the size
105 of the chunk of data. The data is pushed through the line buffer and
106 any complete lines are passed on to the line handler.
108 The complex interface is the pair of functions
115 function returns the address and size of a free portion of the line
116 buffer's memory into which data may be written. The function is passed
119 of the line buffer. Its result is the size of the free area, and it
120 writes the base address of this free space to the location pointed to by
123 The caller's data must be written to ascending memory locations starting
126 and no data may be written beyond the end of the free space. However,
127 it isn't necessary to completely fill the buffer.
129 Once the free area has had some data written to it,
131 is called to examine the new data and break it into text lines. This is
132 given three arguments:
135 The address of the line buffer.
138 The address at which the new data has been written. This must be the
139 base address returned from
143 The number of bytes which have been written to the buffer.
147 function breaks the new data into lines as described below, and passes
148 each one in turn to the line-handler function.
152 function is trivially implemented in terms of the more complex
153 .BR lbuf_free / lbuf_flush
156 By default, the line buffer considers a line to end with either a simple
157 linefeed character (the normal Unix convention) or a
158 carriage-return/linefeed pair (the Internet convention). This can be
159 changed by modifying the
163 structure: the default value is
166 .BR LBUF_STRICTCRLF ,
167 only a carriage-return/linefeed pair will terminate a line. Any other
168 value is a single character which is considered to be the line terminator.
170 The line buffer has a fixed amount of memory available to it. This is
171 deliberate, to prevent a trivial attack whereby a remote user sends a
172 stream of data containing no newline markers, wasting the server's
173 memory. Instead, the buffer will truncate overly long lines (silently)
174 and return only the initial portion. It will ignore the rest of the
176 .SS "Line-handler functions"
177 Completed lines, as already said, are passed to the caller's
178 line-handler function. It is given three arguments: the address
180 of the line which has just been read; the length
182 of the line (not including the null terminator), and the pointer
184 which was set up in the call to
186 The line passed is null-terminated, and has had its trailing newline
187 stripped. The area of memory in which the string is located may be
188 overwritten by the line-handler function, although writing beyond the
189 terminating zero byte is not permitted.
191 The line pointer argument
193 may be null to signify end-of-file; in this case, the length
195 will be zero. See the next section.
196 .SS "Flushing the remaining data"
197 When the client program knows that there's no more data arriving (for
198 example, an end-of-file condition exists on its data source) it should
201 to flush out the remaining data in the buffer as one last (improperly
202 terminated) line. This will pass the remaining text to the line
203 handler, if there is any, and then call the handler one final time with
204 a null pointer rather than the address of a text line to inform it of
207 The line buffer is intended to be used in higher-level program objects,
208 such as the buffer selector described in
210 Unfortunately, a concept from this high level needs to exist at the line
211 buffer level, which complicates the description somewhat. The idea is
212 that, when a line-handler attached to some higher-level object decides
213 that it's read enough, it can
215 the object so that it doesn't see any more data.
219 call can emit more than one line, it must be aware that the line handler
220 isn't interested in any more lines. However, this fact must also be
221 signalled to the higher-level object so that it can detach itself from
224 Rather than invent some complex interface for this, the line buffer
225 exports one of its structure members, a flags word called
227 A higher-level object wishing to disable the line buffer simply clears
232 Disabling a buffer causes an immediate return from
234 However, it is not permitted for the functions
238 to be called on a disabled buffer. (This condition isn't checked for;
239 it'll just do the wrong thing.) Furthermore, the
241 function does not handle disablement at all, because it would complicate
242 the interface so much that it wouldn't have any advantage over the more
244 .BR lbuf_free / lbuf_flush .
249 Mark Wooding, <mdw@distorted.org.uk>
~mdw / mLib / blob