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1 | .\" -*-nroff-*- |
2 | .TH lbuf 3 "6 July 1999" mLib |
3 | .SH "NAME" |
4 | lbuf \- split lines out of asynchronously received blocks |
5 | .\" @lbuf_flush |
6 | .\" @lbuf_close |
7 | .\" @lbuf_free |
8 | .\" @lbuf_snarf |
9 | .\" @lbuf_init |
10 | .SH "SYNOPSIS" |
11 | .nf |
12 | .B "#include <mLib/lbuf.h>" |
13 | |
14 | .BI "void lbuf_flush(lbuf *" b ", char *" p ", size_t " len ); |
15 | .BI "void lbuf_close(lbuf *" b ); |
16 | .BI "size_t lbuf_free(lbuf *" b ", char **" p ); |
17 | .BI "void lbuf_snarf(lbuf *" b ", const void *" p ", size_t " sz ); |
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18 | .BI "void lbuf_setsize(lbuf *" b ", size_t " sz ); |
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19 | .BI "void lbuf_init(lbuf *" b , |
20 | .BI " void (*" func ")(char *" s ", void *" p ), |
21 | .BI " void *" p ); |
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22 | .BI "void lbuf_destroy(lbuf *" b ); |
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23 | .fi |
24 | .SH "DESCRIPTION" |
25 | The declarations in |
26 | .B <mLib/lbuf.h> |
27 | implement a handy object called a |
28 | .IR "line buffer" . |
29 | Given unpredictably-sized chunks of data, the line buffer extracts |
30 | completed lines of text and passes them to a caller-supplied function. |
31 | This is useful in nonblocking network servers, for example: the server |
32 | can feed input from a client into a line buffer as it arrives and deal |
33 | with completed text lines as they appear without having to wait for |
34 | newline characters. |
35 | .PP |
36 | The state of a line buffer is stored in an object of type |
37 | .BR lbuf . |
38 | This is a structure which must be allocated by the caller. The |
39 | structure should normally be considered opaque (see the section on |
40 | .B Disablement |
41 | for an exception to this). |
42 | .SS "Initialization and finalization" |
43 | The function |
44 | .B lbuf_init |
45 | initializes a line buffer ready for use. It is given three arguments: |
46 | .TP |
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47 | .BI "lbuf *" b |
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48 | A pointer to the block of memory to use for the line buffer. This is |
49 | all the memory the line buffer requires. |
50 | .TP |
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51 | .BI "void (*" func ")(char *" s ", void *" p ) |
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52 | The |
53 | .I line-handler |
54 | function to which the line buffer should pass completed lines of text. |
55 | .TP |
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56 | .BI "void *" p |
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57 | A pointer argument to be passed to the function when a completed line of |
58 | text arrives. |
59 | .PP |
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60 | The amount of memory set aside for reading lines is configurable. It |
61 | may be set by calling |
62 | .B lbuf_setsize |
63 | at any time when the buffer is empty. The default limit is 256 bytes. |
64 | Lines longer than the limit are truncated. By default, the buffer is |
65 | allocated from the current arena, |
66 | .BR arena_global (3); |
67 | this may be changed by altering the buffer's |
68 | .B a |
69 | member to refer to a different arena at any time when the buffer is |
70 | unallocated. |
71 | .PP |
72 | A line buffer must be destroyed after use by calling |
73 | .BR lbuf_destroy , |
74 | passing it the address of the buffer block. |
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75 | .SS "Inserting data into the buffer" |
76 | There are two interfaces for inserting data into the buffer. One's much |
77 | simpler than the other, although it's less expressive. |
78 | .PP |
79 | The simple interface is |
80 | .BR lbuf_snarf . |
81 | This function is given three arguments: a pointer |
82 | .I b |
83 | to a line buffer structure; a pointer |
84 | .I p |
85 | to a chunk of data to read; and the size |
86 | .I sz |
87 | of the chunk of data. The data is pushed through the line buffer and |
88 | any complete lines are passed on to the line handler. |
89 | .PP |
90 | The complex interface is the pair of functions |
91 | .I lbuf_free |
92 | and |
93 | .IR lbuf_flush . |
94 | .PP |
95 | The |
96 | .B lbuf_free |
97 | function returns the address and size of a free portion of the line |
98 | buffer's memory into which data may be written. The function is passed |
99 | the address |
100 | .I l |
101 | of the line buffer. Its result is the size of the free area, and it |
102 | writes the base address of this free space to the location pointed to by |
103 | the argument |
104 | .IR p . |
105 | The caller's data must be written to ascending memory locations starting |
106 | at |
107 | .BI * p |
108 | and no data may be written beyond the end of the free space. However, |
109 | it isn't necessary to completely fill the buffer. |
110 | .PP |
111 | Once the free area has had some data written to it, |
112 | .B lbuf_flush |
113 | is called to examine the new data and break it into text lines. This is |
114 | given three arguments: |
115 | .TP |
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116 | .BI "lbuf *" b |
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117 | The address of the line buffer. |
118 | .TP |
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119 | .BI "char *" p |
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120 | The address at which the new data has been written. This must be the |
121 | base address returned from |
122 | .BR lbuf_free . |
123 | .TP |
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124 | .BI "size_t " len |
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125 | The number of bytes which have been written to the buffer. |
126 | .PP |
127 | The |
128 | .B lbuf_flush |
129 | function breaks the new data into lines as described below, and passes |
130 | each one in turn to the line-handler function. |
131 | .PP |
132 | The |
133 | .B lbuf_snarf |
134 | function is trivially implemented in terms of the more complex |
135 | .B lbuf_free / lbuf_flush |
136 | interface. |
137 | .SS "Line breaking" |
138 | The line buffer considers a line to end with either a simple linefeed |
139 | character (the normal Unix convention) or a carriage-return/linefeed |
140 | pair (the Internet convention). |
141 | .PP |
142 | The line buffer has a fixed amount of memory available to it. This is |
143 | deliberate, to prevent a trivial attack whereby a remote user sends a |
144 | stream of data containing no newline markers, wasting the server's |
145 | memory. Instead, the buffer will truncate overly long lines (silently) |
146 | and return only the initial portion. It will ignore the rest of the |
147 | line completely. |
148 | .SS "Line-handler functions" |
149 | Completed lines, as already said, are passed to the caller's |
150 | line-handler function. The function is given two arguments: |
151 | the address |
152 | .I s |
153 | of the line which has just been read, and the pointer |
154 | .I p |
155 | which was set up in the call to |
156 | .B lbuf_init . |
157 | The line passed is null-terminated, and has had its trailing newline |
158 | stripped. The area of memory in which the string is located may be |
159 | overwritten by the line-handler function, although writing beyond the |
160 | terminating zero byte is not permitted. |
161 | .PP |
162 | The line pointer argument |
163 | .I s |
164 | may be null to signify end-of-file. See the next section. |
165 | .SS "Flushing the remaining data" |
166 | When the client program knows that there's no more data arriving (for |
167 | example, an end-of-file condition exists on its data source) it should |
168 | call the function |
169 | .BR lbuf_close |
170 | to flush out the remaining data in the buffer as one last (improperly |
171 | terminated) line. This will pass the remaining text to the line |
172 | handler, if there is any, and then call the handler one final time with |
173 | a null pointer rather than the address of a text line to inform it of |
174 | the end-of-file. |
175 | .SS "Disablement" |
176 | The line buffer is intended to be used in higher-level program objects, |
177 | such as the buffer selector described in |
178 | .BR selbuf (3). |
179 | Unfortunately, a concept from this high level needs to exist at the line |
180 | buffer level, which complicates the description somewhat. The idea is |
181 | that, when a line-handler attached to some higher-level object decides |
182 | that it's read enough, it can |
183 | .I disable |
184 | the object so that it doesn't see any more data. |
185 | .PP |
186 | Clearly, since an |
187 | .B lbuf_flush |
188 | call can emit more than one line, so it must be aware that the line |
189 | handler isn't interested in any more lines. However, this fact must |
190 | also be signalled to the higher-level object so that it can detach |
191 | itself from its data source. |
192 | .PP |
193 | Rather than invent some complex interface for this, the line buffer |
194 | exports one of its structure members, |
195 | .BR flags . |
196 | A higher-level object wishing to disable the line buffer simply clears |
197 | the bit |
198 | .B LBUF_ENABLE |
199 | in the flags word. |
200 | .PP |
201 | Disabling a buffer causes an immediate return from |
202 | .BR lbuf_flush . |
203 | However, it is not permitted for the functions |
204 | .B lbuf_flush |
205 | or |
206 | .B lbuf_close |
207 | to be called on a disabled buffer. (This condition isn't checked for; |
208 | it'll just do the wrong thing.) Furthermore, the |
209 | .B lbuf_snarf |
210 | function does not handle disablement at all, because it would complicate |
211 | the interface so much that it wouldn't have any advantage over the more |
212 | general |
213 | .BR lbuf_free / lbuf_flush . |
214 | .SH "SEE ALSO" |
215 | .BR selbuf (3), |
216 | .BR mLib (3). |
217 | .SH "AUTHOR" |
218 | Mark Wooding, <mdw@nsict.org> |