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[paraslash.git] / buffer_tree.h
1 /**
2 * \file buffer_tree.h Buffer tree management.
3 *
4 * \par Buffer trees and buffer tree nodes.
5 * The buffer tree API offers a more powerful method than standard unix pipes
6 * for managing the data flow from the producer of the data (e.g. the network
7 * receiver) to its consumer(s) (e.g. a sound card).
8 *
9 * A buffer tree consists of buffer tree nodes linked via certain parent/child
10 * relationships.
11 *
12 * Each data buffer starts its way from the root of the buffer tree. At each
13 * node the data is investigated and possibly changed. New data is then fed to
14 * each child. Everything happens within one single-treaded process. There are
15 * no file descriptors and no calls to read() or write().
16 *
17 * Whenever a node in the buffer tree creates output, either by creating a new
18 * buffer or by pushing down buffers received from its parent, references to
19 * that buffer are created for all children of the node. The buffer tree code
20 * tries hard to avoid to copy buffer contents, but is forced to do so in case
21 * there are alignment constraints.
22 *
23 * Communication between nodes is possible via the btr_exec_up() function.
24 * For example, in para_audiod the alsa writer asks all parent nodes
25 * for for the number of channels and the sample rate of the current
26 * audio file.
27 *
28 * Buffer pools - An alternative to malloc/free buffer management.
29 *
30 * Non-leaf nodes usually create output to be processed by their children. The
31 * data must be fed through the output channel(s) of the node in order to make
32 * that data available to each child.
33 *
34 * The easiest way to do so is to malloc() a buffer, fill it, and then call
35 * btr_add_output(). This adds references to that buffer to all children. The
36 * buffer is automatically freed if no buffer tree node is using it any more.
37 *
38 * This approach, while being simple, has some drawbacks, especially affecting
39 * the root nodes of the buffer tree. Often the data source which is
40 * represented by a root node does not know in advance how much data will be
41 * available. Therefore the allocated buffer is either larger than what can
42 * currently be read, or is too small so that multiple buffers have to be used.
43 *
44 * While this could be worked around by using a large buffer and calling
45 * realloc() afterwards to shrink the buffer according to how much has been
46 * read, there is a second problem which comes from the alignment constraints
47 * of some filters, mainly the decoders like mp3dec. These need a minimal
48 * amount of data to proceed, and most of them even need this amount as one
49 * contiguous buffer, i.e. not spread out over two or more buffers.
50 *
51 * Although the buffer tree code handles this case just fine, it can be
52 * expensive because two or more buffers must be merged by copying buffer
53 * contents around in order to satisfy the constraint.
54 *
55 * This is where buffer pools come into play. Buffer pools try to satisfy
56 * alignment constraints without copying buffer content whenever possible. To
57 * avoid spreading out the input data over the address space like in the
58 * malloc/free approach, a fixed large contiguous buffer (the area) is used
59 * instead. A buffer pool consists basically of an area and two pointers, the
60 * read head and the write head.
61 *
62 * Once a buffer pool has been created, its node, e.g. a receiver, obtains the
63 * current value of the write head and writes new data to this location. Then
64 * it calls btr_add_output_pool() to tell much data it has written. This
65 * advances the write head accordingly, and it also creates references to the
66 * newly written part of the area for the children of the node to consume.
67 *
68 * Child nodes consume data by working through their input queue, which is a
69 * list of buffer references. Once the content of a buffer is no longer needed
70 * by a child node, the child calls btr_consume() to indicate the amount of
71 * data which can be dropped from the child's point of view. If no reference
72 * to some region of the buffer pool area remains, the read head of the buffer
73 * pool advances, making space available for the receiver node to fill.
74 *
75 * No matter if malloc() or a buffer pool is used, the buffer tree code takes
76 * care of alignment constraints imposed by the consumers. In the buffer pool
77 * case, automatic merging of references to contiguous buffers is performed.
78 * memcpy is only used if a constraint can not be satisfied by using the
79 * remaining part of the area only. This only happens when the end of the area
80 * is reached.
81 */
82
83 struct btr_node;
84 struct btr_pool;
85
86 /**
87 * The three different types of buffer tree nodes.
88 *
89 * Usually, there is exactly one node in the buffer tree, the root node, which
90 * has no parent. Every node different from the root node has exactly one
91 * parent. The root node represents a data source. Root nodes are thus used by
92 * the receivers of paraslash. Also, reading from stdin is realized as the root
93 * node of a buffer tree.
94 *
95 * Each node may have arbitrary many children, including none. Nodes with no
96 * children are called leaf nodes. They represent a data sink, like the alsa or
97 * the file writer.
98 *
99 * Hence there are three different types of buffer tree nodes: The root node
100 * and the leaf nodes and nodes which have both a parent and at least one
101 * child. Such a node is called an internal node.
102 *
103 * Internal nodes represent filters through which data buffers flow, possibly
104 * while being altered on their way to the children of the node. Examples of
105 * internal nodes are audio file decoders (mp3dec, oggdec, ...), but also the
106 * check for a wav header is implemented as an internal buffer tree node.
107 */
108 enum btr_node_type {
109 /* This node has no parent. */
110 BTR_NT_ROOT,
111 /* Node has parent and at least one child. */
112 BTR_NT_INTERNAL,
113 /* Node has no children. */
114 BTR_NT_LEAF,
115 };
116
117 /**
118 * Per node handler used for inter node communication.
119 *
120 * Each node in the buffer tree may optionally provide a command handler for
121 * execution of commands by other nodes of the tree.
122 *
123 * It is dependent on the node in question which commands are supported and how
124 * they work. In any case, the input for the command handler is some string and
125 * its output is also a string which is returned via the \a result pointer of
126 * the handler.
127 *
128 * This mechanism is used in para_audiod e.g. by the alsa writer which needs to
129 * know the sample rate of its input known to e.g. the mp3dec node further up
130 * in the buffer tree.
131 */
132 typedef int (*btr_command_handler)(struct btr_node *btrn,
133 const char *command, char **result);
134
135 /**
136 * Structure for creating new buffer tree nodes.
137 *
138 * btr_new_node() takes a pointer to such a structure.
139 *
140 * There are four different combinations of \a parent and child:
141 *
142 * 1. both \p NULL. This creates a new buffer tree with a single isolated node.
143 *
144 * 2. \a parent != \p NULL, \a child == NULL. This creates a new leaf node by
145 * adding the new node to the list of children of the given parent node.
146 *
147 * 3. \a parent == NULL, \a child != NULL. The new node becomes the new root of
148 * the buffer tree. The child must be old root.
149 *
150 * 4. both != NULL. This creates a new internal node. \a child must be child of
151 * p. This mode of operation is currently not needed and is thus not yet
152 * implemented.
153 */
154 struct btr_node_description {
155 /** Name of the new node. */
156 const char *name;
157 /** Parent of the new node. */
158 struct btr_node *parent;
159 /** Child of the new node. */
160 struct btr_node *child;
161 /** Used for inter node communication. Optional. */
162 btr_command_handler handler;
163 /** Points usually to the struct that contains the node pointer. */
164 void *context;
165 };
166
167 size_t btr_pool_size(struct btr_pool *btrp);
168 struct btr_pool *btr_pool_new(const char *name, size_t area_size);
169 void btr_pool_free(struct btr_pool *btrp);
170 size_t btr_pool_get_buffer(struct btr_pool *btrp, char **result);
171 int btr_pool_get_buffers(struct btr_pool *btrp, struct iovec iov[2]);
172 void btr_add_output_pool(struct btr_pool *btrp, size_t size,
173 struct btr_node *btrn);
174 size_t btr_pool_unused(struct btr_pool *btrp);
175 void btr_copy(const void *src, size_t n, struct btr_pool *btrp,
176 struct btr_node *btrn);
177
178 struct btr_node *btr_new_node(struct btr_node_description *bnd);
179 void btr_remove_node(struct btr_node *btrn);
180 void btr_free_node(struct btr_node *btrn);
181 void btr_add_output(char *buf, size_t size, struct btr_node *btrn);
182 size_t btr_get_input_queue_size(struct btr_node *btrn);
183 size_t btr_get_output_queue_size(struct btr_node *btrn);
184 bool btr_no_parent(struct btr_node *btrn);
185 size_t btr_next_buffer(struct btr_node *btrn, char **bufp);
186 void btr_consume(struct btr_node *btrn, size_t numbytes);
187 int btr_exec_up(struct btr_node *btrn, const char *command, char **value_result);
188 void btr_splice_out_node(struct btr_node *btrn);
189 void btr_pushdown(struct btr_node *btrn);
190 void *btr_context(struct btr_node *btrn);
191 void btr_merge(struct btr_node *btrn, size_t dest_size);
192 void btr_log_tree(struct btr_node *btrn, int loglevel);
193 void btr_pushdown_one(struct btr_node *btrn);
194 bool btr_inplace_ok(struct btr_node *btrn);
195 int btr_node_status(struct btr_node *btrn, size_t min_iqs,
196 enum btr_node_type type);
197 void btr_get_node_start(struct btr_node *btrn, struct timeval *tv);
198 struct btr_node *btr_search_node(const char *name, struct btr_node *root);