buffer_tree.h

   1 /**
   2  * Buffer trees and buffer tree nodes.
   3  *
   4  * The buffer tree API offers a more powerful method than standard unix pipes
   5  * for managing the data flow from the producer of the data (e.g. the network
   6  * receiver) to its consumer(s) (e.g. a sound card).
   7  *
   8  * Each data buffer starts its way from the root of the buffer tree. At each
   9  * child node the data is investigated and new data is fed to each child.
  10  * Everything happens within one single-treaded process. There are no file
  11  * descriptors, no calls to read() or write().
  12  *
  13  * A buffer tree consists of buffer tree nodes linked via certain parent/child
  14  * relationships.
  15  *
  16  * Whenever a node in the buffer tree creates output, either by creating a new
  17  * buffer or by pushing down buffers received from its parent, references to
  18  * that buffer are created for all children of the node. The buffer tree code
  19  * tries hard to avoid to copy buffer contents, but is forced to do so in case
  20  * there are alignment constraints.
  21  *
  22  * Communication between nodes is possible via the btr_exec_up() function.
  23  * For example, in para_audiod the alsa writer asks all parent nodes
  24  * for for the number of channels and the sample rate of the current
  25  * audio file.
  26  */
  27 struct btr_node;
  28
  29 /**
  30  * Buffer pools - An alternative to malloc/free buffer management.
  31  *
  32  * Non-leaf nodes usually create output to be processed by their children.  The
  33  * data must be fed through the output channel(s) of the node in order to make
  34  * that data available to each child.
  35  *
  36  * The easiest way to do so is to malloc() a buffer, fill it, and then call
  37  * btr_add_output(). This adds references to that buffer to all children. The
  38  * buffer is automatically freed if no buffer tree node is using it any more.
  39  *
  40  * This approach, while simple, has some drawbacks, especially affecting the
  41  * root nodes of the buffer tree. Often the data source which is represented by
  42  * a root node does not know in advance how much data will be available.
  43  * Therefore the allocated buffer is either larger than what can currently be
  44  * read, or is too small so that multiple buffers have to be used.
  45  *
  46  * While this could be worked around by using a large buffer and calling
  47  * realloc() afterwards to shrink the buffer according to how much has been
  48  * read, there is a second problem which comes from the alignment constraints
  49  * of some filters, mainly the decoders. These need a minimal amount of data to
  50  * proceed, and most of them even need this amount as one contiguous buffer,
  51  * i.e.  not spread out over two or more buffers.
  52  *
  53  * Although the buffer tree code handles this case just fine, it can be
  54  * expensive because two or more buffers must be combined by copying buffer
  55  * contents around in order to satisfy the constraint.
  56  *
  57  * This is where buffer pools come into play. Buffer pools try to satisfy
  58  * alignment constraints without copying buffer content whenever possible. To
  59  * avoid spreading out the input data over the address space like in the
  60  * malloc/free approach, a fixed large contiguous buffer (the area) is used
  61  * instead. A buffer pool consists basically of an area and two pointers, the
  62  * read head and the write head.
  63  *
  64  * Once a buffer pool has been created, its node, e.g. a receiver, obtains the
  65  * current value of the write head and writes new data to this location. Then
  66  * it calls btr_add_output_pool() to tell much data it has written. This
  67  * advances the write head accordingly, and it also creates references to the
  68  * newly written part of the area for the children of the node to consume.
  69  *
  70  * Child nodes consume data by working through their input queue, which is a
  71  * list of buffer references. Once the content of a buffer is no longer needed
  72  * by a child node, the child calls btr_consume() to indicate the amount of
  73  * data which can be dropped from the child's point of view.  If no reference
  74  * to some region of the buffer pool area remains, the read head of the buffer
  75  * pool advances, making space available for the receiver node to fill.
  76  *
  77  * No matter if malloc() or a buffer pool is used, the buffer tree code takes
  78  * care of alignment constraints imposed by the consumers. In the buffer pool
  79  * case, automatic merging of references to contiguous buffers is performed.
  80  * memcpy is only used if a constraint can not be satisfied by using the
  81  * remaining part of the area only. This only happens when the end of the area
  82  * is reached.
  83  */
  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 void btr_pool_allocate(struct btr_pool *btrp, size_t size);
 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 bool btr_no_children(struct btr_node *btrn);
 183 size_t btr_bytes_pending(struct btr_node *btrn);
 184 size_t btr_get_input_queue_size(struct btr_node *btrn);
 185 bool btr_no_parent(struct btr_node *btrn);
 186 size_t btr_next_buffer(struct btr_node *btrn, char **bufp);
 187 void btr_consume(struct btr_node *btrn, size_t numbytes);
 188 int btr_exec(struct btr_node *btrn, const char *command, char **value_result);
 189 int btr_exec_up(struct btr_node *btrn, const char *command, char **value_result);
 190 void btr_splice_out_node(struct btr_node *btrn);
 191 void btr_pushdown(struct btr_node *btrn);
 192 void *btr_context(struct btr_node *btrn);
 193 void btr_merge(struct btr_node *btrn, size_t dest_size);
 194 bool btr_eof(struct btr_node *btrn);
 195 void btr_log_tree(struct btr_node *btrn, int loglevel);
 196 int btr_pushdown_one(struct btr_node *btrn);
 197 bool btr_inplace_ok(struct btr_node *btrn);
 198 int btr_node_status(struct btr_node *btrn, size_t min_iqs,
 199                 enum btr_node_type type);
 200 void btr_get_node_start(struct btr_node *btrn, struct timeval *tv);
 201 struct btr_node *btr_search_node(const char *name, struct btr_node *root);