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