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05fbeb03 | 1 | .\" -*-nroff-*- |
fbf20b5b | 2 | .TH lbuf 3 "6 July 1999" "Straylight/Edgeware" "mLib utilities library" |
05fbeb03 | 3 | .SH "NAME" |
4 | lbuf \- split lines out of asynchronously received blocks | |
5 | .\" @lbuf_flush | |
6 | .\" @lbuf_close | |
7 | .\" @lbuf_free | |
8 | .\" @lbuf_snarf | |
31e83d07 | 9 | .\" @lbuf_setsize |
05fbeb03 | 10 | .\" @lbuf_init |
31e83d07 | 11 | .\" @lbuf_destroy |
05fbeb03 | 12 | .SH "SYNOPSIS" |
13 | .nf | |
14 | .B "#include <mLib/lbuf.h>" | |
15 | ||
4729aa69 MW |
16 | .B "enum {" |
17 | .B "\h'4n'LBUF_CRLF," | |
18 | .B "\h'4n'LBUF_STRICTCRLF," | |
19 | .B "\h'4n'..." | |
20 | .B "};" | |
21 | .B "#define LBUF_ENABLE ..." | |
22 | ||
23 | .B "typedef struct {" | |
24 | .B "\h'4n'unsigned f;" | |
25 | .B "\h'4n'..." | |
26 | .B "} lbuf;" | |
27 | ||
28 | .B "typedef void lbuf_func(char *" s ", size_t " len ", void *" p ); | |
29 | ||
05fbeb03 | 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 ); | |
cededfbe | 34 | .BI "void lbuf_setsize(lbuf *" b ", size_t " sz ); |
83c63e03 | 35 | .BI "void lbuf_init(lbuf *" b ", lbuf_func *" func ", void *" p ); |
cededfbe | 36 | .BI "void lbuf_destroy(lbuf *" b ); |
05fbeb03 | 37 | .fi |
38 | .SH "DESCRIPTION" | |
39 | The declarations in | |
40 | .B <mLib/lbuf.h> | |
41 | implement a handy object called a | |
42 | .IR "line buffer" . | |
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 | |
48 | newline characters. | |
49 | .PP | |
50 | The state of a line buffer is stored in an object of type | |
51 | .BR lbuf . | |
52 | This is a structure which must be allocated by the caller. The | |
53 | structure should normally be considered opaque (see the section on | |
54 | .B Disablement | |
55 | for an exception to this). | |
56 | .SS "Initialization and finalization" | |
57 | The function | |
58 | .B lbuf_init | |
59 | initializes a line buffer ready for use. It is given three arguments: | |
60 | .TP | |
ff76c38f | 61 | .BI "lbuf *" b |
31e83d07 | 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. | |
05fbeb03 | 65 | .TP |
83c63e03 | 66 | .BI "lbuf_func *" func |
05fbeb03 | 67 | The |
68 | .I line-handler | |
69 | function to which the line buffer should pass completed lines of text. | |
83c63e03 | 70 | See |
71 | .B "Line-handler functions" | |
72 | below for a description of this function. | |
05fbeb03 | 73 | .TP |
ff76c38f | 74 | .BI "void *" p |
05fbeb03 | 75 | A pointer argument to be passed to the function when a completed line of |
76 | text arrives. | |
77 | .PP | |
cededfbe | 78 | The amount of memory set aside for reading lines is configurable. It |
79 | may be set by calling | |
80 | .B lbuf_setsize | |
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, | |
84 | .BR arena_global (3); | |
85 | this may be changed by altering the buffer's | |
86 | .B a | |
87 | member to refer to a different arena at any time when the buffer is | |
88 | unallocated. | |
89 | .PP | |
90 | A line buffer must be destroyed after use by calling | |
91 | .BR lbuf_destroy , | |
92 | passing it the address of the buffer block. | |
05fbeb03 | 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. | |
96 | .PP | |
97 | The simple interface is | |
98 | .BR lbuf_snarf . | |
99 | This function is given three arguments: a pointer | |
100 | .I b | |
101 | to a line buffer structure; a pointer | |
102 | .I p | |
d0053e2e | 103 | to a chunk of data to read; and the size |
05fbeb03 | 104 | .I sz |
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. | |
107 | .PP | |
108 | The complex interface is the pair of functions | |
d0053e2e | 109 | .B lbuf_free |
05fbeb03 | 110 | and |
d0053e2e | 111 | .BR lbuf_flush . |
05fbeb03 | 112 | .PP |
d4efbcd9 | 113 | The |
05fbeb03 | 114 | .B lbuf_free |
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 | |
d4efbcd9 | 117 | the address |
31e83d07 | 118 | .I b |
05fbeb03 | 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 | |
121 | the argument | |
122 | .IR p . | |
123 | The caller's data must be written to ascending memory locations starting | |
124 | at | |
125 | .BI * p | |
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. | |
128 | .PP | |
129 | Once the free area has had some data written to it, | |
130 | .B lbuf_flush | |
131 | is called to examine the new data and break it into text lines. This is | |
132 | given three arguments: | |
133 | .TP | |
ff76c38f | 134 | .BI "lbuf *" b |
05fbeb03 | 135 | The address of the line buffer. |
136 | .TP | |
ff76c38f | 137 | .BI "char *" p |
05fbeb03 | 138 | The address at which the new data has been written. This must be the |
139 | base address returned from | |
140 | .BR lbuf_free . | |
141 | .TP | |
ff76c38f | 142 | .BI "size_t " len |
05fbeb03 | 143 | The number of bytes which have been written to the buffer. |
144 | .PP | |
145 | The | |
146 | .B lbuf_flush | |
147 | function breaks the new data into lines as described below, and passes | |
148 | each one in turn to the line-handler function. | |
149 | .PP | |
150 | The | |
151 | .B lbuf_snarf | |
152 | function is trivially implemented in terms of the more complex | |
d0053e2e | 153 | .BR lbuf_free / lbuf_flush |
05fbeb03 | 154 | interface. |
155 | .SS "Line breaking" | |
83c63e03 | 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 | |
160 | .B delim | |
161 | member of the | |
162 | .B lbuf | |
163 | structure: the default value is | |
164 | .BR LBUF_CRLF . | |
165 | If set to | |
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. | |
05fbeb03 | 169 | .PP |
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 | |
175 | line completely. | |
176 | .SS "Line-handler functions" | |
177 | Completed lines, as already said, are passed to the caller's | |
4729aa69 | 178 | line-handler function. It is given three arguments: the address |
05fbeb03 | 179 | .I s |
83c63e03 | 180 | of the line which has just been read; the length |
181 | .I len | |
182 | of the line (not including the null terminator), and the pointer | |
05fbeb03 | 183 | .I p |
184 | which was set up in the call to | |
d0053e2e | 185 | .BR lbuf_init . |
05fbeb03 | 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. | |
190 | .PP | |
191 | The line pointer argument | |
192 | .I s | |
83c63e03 | 193 | may be null to signify end-of-file; in this case, the length |
194 | .I len | |
195 | will be zero. See the next section. | |
05fbeb03 | 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 | |
199 | call the function | |
200 | .BR lbuf_close | |
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 | |
205 | the end-of-file. | |
206 | .SS "Disablement" | |
207 | The line buffer is intended to be used in higher-level program objects, | |
208 | such as the buffer selector described in | |
209 | .BR selbuf (3). | |
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 | |
214 | .I disable | |
215 | the object so that it doesn't see any more data. | |
216 | .PP | |
217 | Clearly, since an | |
218 | .B lbuf_flush | |
528c8b4d | 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 | |
222 | its data source. | |
05fbeb03 | 223 | .PP |
224 | Rather than invent some complex interface for this, the line buffer | |
3bc42912 | 225 | exports one of its structure members, a flags word called |
226 | .BR f . | |
05fbeb03 | 227 | A higher-level object wishing to disable the line buffer simply clears |
228 | the bit | |
229 | .B LBUF_ENABLE | |
3bc42912 | 230 | in this flags word. |
05fbeb03 | 231 | .PP |
232 | Disabling a buffer causes an immediate return from | |
233 | .BR lbuf_flush . | |
234 | However, it is not permitted for the functions | |
235 | .B lbuf_flush | |
236 | or | |
237 | .B lbuf_close | |
238 | to be called on a disabled buffer. (This condition isn't checked for; | |
239 | it'll just do the wrong thing.) Furthermore, the | |
240 | .B lbuf_snarf | |
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 | |
243 | general | |
244 | .BR lbuf_free / lbuf_flush . | |
245 | .SH "SEE ALSO" | |
246 | .BR selbuf (3), | |
247 | .BR mLib (3). | |
248 | .SH "AUTHOR" | |
9b5ac6ff | 249 | Mark Wooding, <mdw@distorted.org.uk> |