doc/API.txt

   1 Babeltrace API documentation
   2
   3 Babeltrace provides trace read and write libraries, as well as a trace
   4 converter. A plugin can be created for any trace format to allow its
   5 conversion to/from another trace format.
   6
   7 The main format expected to be converted to/from is the Common Trace
   8 Format (CTF). The latest version of the CTF specification can be found at:
   9         git tree:   git://git.efficios.com/ctf.git
  10         gitweb:     http://git.efficios.com/?p=ctf.git
  11
  12 This document describes the main concepts to use the libbabeltrace,
  13 which exposes the Babeltrace trace reading capability.
  14
  15
  16 TERMINOLOGY
  17 ¯¯¯¯¯¯¯¯¯¯¯
  18
  19 * A "callback" is a reference to a piece of executable code (such as a
  20   function) that is passed as an argument to another piece of code
  21   (like another function).
  22
  23 * A "context" is a structure that represents an object in which a trace
  24   collection is opened.
  25
  26 * An "iterator" is a structure that enables the user to traverse a trace.
  27
  28 * A "trace handle" is a unique identifier representing a trace file.
  29   It allows the user to manipulate a trace directly.
  30
  31
  32
  33 USAGE
  34 ¯¯¯¯¯¯
  35
  36 Context:
  37
  38 In order to use libbabeltrace to read a trace, the first step is to create a
  39 context structure and to add a trace to it. This is done using the
  40 bt_context_create() and bt_context_add_trace() functions. As long as this
  41 context structure is allocated and the trace is valid, the trace can be
  42 manipulated by the library.
  43
  44 The context can be destroyed by calling one more bt_context_put() than
  45 bt_context_get(), functions which respectively decrement and increment the
  46 refcount of the context. These functions ensures that the context won't be
  47 destroyed when it is in use.
  48
  49 Once a trace is added to the context, it can be read and seeked using iterators
  50 and callbacks.
  51
  52
  53 Iterator:
  54
  55 An iterator can be created using the bt_iter_create() function. As of now, only
  56 ctf iterator are supported. These are used to traverse a ctf-formatted trace.
  57 Such iterators can be created with bt_ctf_iter_create().
  58
  59 While creating an iterator, a begin and an end position may be specified. To do
  60 so, one or two struct bt_iter_pos must be passed. Such struct have two
  61 attributes: type and u. "type" is the seek type, can be either:
  62         BT_SEEK_TIME
  63         BT_SEEK_RESTORE
  64         BT_SEEK_CUR
  65         BT_SEEK_BEGIN
  66         BT_SEEK_END
  67 and "u" is a union of the seek time (if using BT_SEEK_TIME) and the restore
  68 position (if using BT_SEEK_RESTORE).
  69
  70 Once the iterator is created, various functions become available. We have
  71 bt_ctf_iter_read_event() which returns the ctf event of the trace where the
  72 iterator is set. There is also bt_ctf_iter_destroy() which frees the iterator.
  73 Note that only one iterator can be created in a context at the same time. If
  74 more than one iterator is being created for the same context, the second
  75 creation will return NULL. The previous iterator must be destroyed before
  76 creation of the new iterator. In the future, creation of multiples iterators
  77 will be allowed.
  78
  79 Finally, we have the bt_ctf_get_iter() function which returns a struct bt_iter
  80 with which the iterator can be moved using one of these functions:
  81         bt_iter_next(),         moves the iterator to the next event
  82         bt_iter_set_pos(),      moves the iterator to the specified position
  83
  84 To get the current position (struct bt_iter_pos) of the iterator, the function
  85 bt_iter_get_pos() must be used. To create an arbitrary position based on a
  86 specific time, bt_iter_create_time_pos() is the function to use. The
  87 bt_iter_pos structure returned by these two functions must be freed with
  88 bt_iter_free_pos() after use.
  89
  90
  91 CTF Event:
  92
  93 A CTF event is obtained from an iterator via the bt_ctf_iter_read_event()
  94 function or via the call_data parameter of a callback. To read the data of a
  95 CTF event :
  96         * bt_ctf_event_name()           returns the name of the event;
  97         * bt_ctf_get_timestamp()        returns the timestamp of the event
  98                                         offsetted with the system clock
  99                                         source (in ns);
 100         * bt_ctf_get_cycles()           returns the timestamp of the event as
 101                                         written in the packet (in cycles).
 102
 103 The payload of an event is divided in various scopes depending on the type of
 104 information. There are six top-level scopes (defined in the bt_ctf_scope enum)
 105 which can be accessed by the bt_ctf_get_top_level_scope() function :
 106         BT_TRACE_PACKET_HEADER          = 0,
 107         BT_STREAM_PACKET_CONTEXT        = 1,
 108         BT_STREAM_EVENT_HEADER          = 2,
 109         BT_STREAM_EVENT_CONTEXT         = 3,
 110         BT_EVENT_CONTEXT                = 4,
 111         BT_EVENT_FIELDS                 = 5.
 112
 113 In order to access a field or a field list, the user needs to pass a scope as
 114 argument, this scope can be a top-level scope or a scope relative to an
 115 arbitrary field in the case of compound types (array, sequence, structure or
 116 variant)
 117
 118 For more information on each scope, see the CTF specifications.
 119
 120 The function to get a field list is the bt_ctf_get_field_list(). The function
 121 to get the definition of a specific field is bt_ctf_get_field().
 122
 123 Once the field is obtained, we can obtain its name and type using the
 124 bt_ctf_field_name() and bt_ctf_field_type() functions respectively. The
 125 possible types are defined in the ctf_type_id enum:
 126         CTF_TYPE_UNKNOWN = 0,
 127         CTF_TYPE_INTEGER,
 128         CTF_TYPE_FLOAT,
 129         CTF_TYPE_ENUM,
 130         CTF_TYPE_STRING,
 131         CTF_TYPE_STRUCT,
 132         CTF_TYPE_UNTAGGED_VARIANT,
 133         CTF_TYPE_VARIANT,
 134         CTF_TYPE_ARRAY,
 135         CTF_TYPE_SEQUENCE,
 136         NR_CTF_TYPES.
 137
 138 Depending on the field type, we can get informations about the field with these
 139 functions:
 140         * bt_ctf_get_index()            return the element at the index
 141                                         position of an array of a sequence;
 142
 143         * bt_ctf_get_array_len()        return the length of an array;
 144
 145         * bt_ctf_get_int_signedness()   return the signedness of an integer;
 146
 147         * bt_ctf_get_int_base()         return the base of an integer;
 148
 149         * bt_ctf_get_int_byte_order()   return the byte order of an integer;
 150
 151         * bt_ctf_get_int_len()          return the size in bits of an integer;
 152
 153         * bt_ctf_get_encoding()         return the encoding of an int or a
 154                                         string defined in the
 155                                         ctf_string_encoding enum:
 156                                                 CTF_STRING_NONE = 0,
 157                                                 CTF_STRING_UTF8,
 158                                                 CTF_STRING_ASCII,
 159                                                 CTF_STRING_UNKNOWN.
 160
 161 These functions give access to the value associated with a field :
 162         * bt_ctf_get_uint64();
 163         * bt_ctf_get_int64();
 164         * bt_ctf_get_char_array();
 165         * bt_ctf_get_string().
 166
 167 If the field does not exist or is not of the type requested, the value returned
 168 with these four functions is undefined. To check if an error occured, use the
 169 bt_ctf_field_get_error() function after accessing a field. If no error
 170 occured, the function will return 0.
 171
 172 It is also possible to access the declaration fields, the same way as the
 173 definition ones. bt_ctf_get_event_decl_list() sets a list to an array of
 174 bt_ctf_event_decl pointers and bt_ctf_get_event_decl_fields() sets a list to an
 175 array of bt_ctf_field_decl pointers.  From the first type, the name of the
 176 event can be obtained with bt_ctf_get_decl_event_name().  For the second type,
 177 the field decl name is obtained with bt_ctf_get_decl_field_name().
 178
 179 The declaration functions allow the user to list the events, fields and
 180 contexts fields enabled in the trace once it is opened, whereas the definition
 181 functions apply on the current event being read.
 182
 183
 184 Callback:
 185
 186 The iterator allow the user to read the trace, in order to access the events
 187 and fields, the user can either call the functions listed previously on each
 188 event, or register callbacks functions that are called when specific (or all)
 189 events are read.
 190
 191 This is done with the bt_ctf_iter_add_callback() function. It requires a valid
 192 ctf iterator as the first argument. Here are all arguments:
 193         iter: trace collection iterator (input)
 194         event: event to target. 0 for all events.
 195         private_data: private data pointer to pass to the callback
 196         flags: specific flags controlling the behavior of this callback
 197                 (or'd).
 198         callback: function pointer to call
 199         depends: struct bt_dependency detailing the required computation
 200                 results.  Ends with 0.
 201         weak_depends: struct bt_dependency detailing the optional computation
 202                 results that can be optionally consumed by this
 203                 callback.
 204         provides: struct bt_dependency detailing the computation results
 205                 provided by this callback.
 206                 Ends with 0.
 207
 208 "depends", "weak_depends" and "provides" memory is handled by the babeltrace
 209 library after this call succeeds or fails. These objects can still be used by
 210 the caller until the babeltrace iterator is destroyed, but they belong to the
 211 babeltrace library.
 212
 213 As of now the flags and dependencies are not used, the callbacks are
 214 processed in FIFO order.
 215
 216 Note: once implemented, the dependency graph will be calculated when
 217 bt_ctf_iter_read_event() is executed after a bt_ctf_iter_add_callback(). It is
 218 valid to create/add callbacks/read/add more callbacks/read some more.
 219
 220 The callback function passed to bt_ctf_iter_add_callback() must return a
 221 bt_cb_ret value:
 222         BT_CB_OK                = 0,
 223         BT_CB_OK_STOP           = 1,
 224         BT_CB_ERROR_STOP        = 2,
 225         BT_CB_ERROR_CONTINUE    = 3.
 226
 227
 228 Trace handle:
 229
 230 When a trace is added to a context, bt_context_add_trace() returns a trace
 231 handle id.  This id is associated with its corresponding trace handle.  With
 232 that id, it is possible to manipulate directly the trace.
 233
 234         * bt_trace_handle_get_path()
 235                 -> returns the path of the trace handle (path to the trace).
 236
 237         * bt_trace_handle_get_timestamp_begin()
 238         * bt_trace_handle_get_timestamp_end()
 239                 -> return the creation/destruction timestamps (in ns or cycles
 240                         depending on the type specified) of the buffers of a
 241                         trace.
 242
 243         * bt_ctf_event_get_handle_id()
 244                 -> returns the handle id associated with an event.
 245
 246
 247 For more information on CTF, see the CTF documentation.