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1 | #ifndef BABELTRACE_REF_H |
2 | #define BABELTRACE_REF_H | |
de3dd40e PP |
3 | |
4 | /* | |
83509119 | 5 | * BabelTrace: common reference counting |
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6 | * |
7 | * Copyright (c) 2015 EfficiOS Inc. and Linux Foundation | |
8 | * Copyright (c) 2015 Philippe Proulx <pproulx@efficios.com> | |
83509119 | 9 | * Copyright (c) 2015 Jérémie Galarneau <jeremie.galarneau@efficios.com> |
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10 | * |
11 | * Permission is hereby granted, free of charge, to any person obtaining a copy | |
12 | * of this software and associated documentation files (the "Software"), to deal | |
13 | * in the Software without restriction, including without limitation the rights | |
14 | * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell | |
15 | * copies of the Software, and to permit persons to whom the Software is | |
16 | * furnished to do so, subject to the following conditions: | |
17 | * | |
18 | * The above copyright notice and this permission notice shall be included in | |
19 | * all copies or substantial portions of the Software. | |
20 | * | |
21 | * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR | |
22 | * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, | |
23 | * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE | |
24 | * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER | |
25 | * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, | |
26 | * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE | |
27 | * SOFTWARE. | |
28 | */ | |
29 | ||
26162993 PP |
30 | #ifdef __cplusplus |
31 | extern "C" { | |
32 | #endif | |
33 | ||
b62d3f21 PP |
34 | /** |
35 | @defgroup refs Reference counting management | |
36 | @ingroup apiref | |
37 | @brief Common reference counting management for all Babeltrace objects. | |
38 | ||
9197569d PP |
39 | @code |
40 | #include <babeltrace/ref.h> | |
41 | @endcode | |
42 | ||
b62d3f21 PP |
43 | The macros and functions of this module are everything that is needed |
44 | to handle the <strong><em>reference counting</em></strong> of | |
45 | Babeltrace objects. | |
46 | ||
223f6c6a | 47 | Any Babeltrace object can be shared by multiple owners thanks to |
b62d3f21 | 48 | <a href="https://en.wikipedia.org/wiki/Reference_counting">reference |
223f6c6a | 49 | counting</a>. |
b62d3f21 | 50 | |
223f6c6a PP |
51 | The Babeltrace C API complies with the following key principles: |
52 | ||
53 | 1. When you call an API function which accepts a Babeltrace object | |
54 | pointer as a parameter, the API function <strong>borrows the | |
55 | reference</strong> for the <strong>duration of the function</strong>. | |
56 | ||
57 | @image html ref-count-user-calls.png | |
58 | ||
59 | The API function can also get a new reference if the system needs a | |
60 | more persistent reference, but the ownership is <strong>never | |
61 | transferred</strong> from the caller to the API function. | |
62 | ||
63 | In other words, the caller still owns the object after calling any | |
64 | API function: no function "steals" the user's reference (except | |
65 | bt_put()). | |
66 | ||
67 | 2. An API function which \em returns a Babeltrace object pointer to the | |
68 | user returns a <strong>new reference</strong>. The caller becomes an | |
69 | owner of the object. | |
70 | ||
71 | @image html ref-count-api-returns.png | |
72 | ||
73 | It is your responsibility to discard the object when you don't | |
74 | need it anymore with bt_put(). | |
75 | ||
76 | For example, see bt_value_array_get(). | |
77 | ||
78 | 3. A Babeltrace object pointer received as a parameter in a user | |
79 | function called back from an API function is a | |
80 | <strong>borrowed</strong>, or <strong>weak reference</strong>: if you | |
09ac5ce7 | 81 | need a reference which is more persistent than the duration of the |
223f6c6a PP |
82 | user function, call bt_get() on the pointer. |
83 | ||
84 | @image html ref-count-callback.png | |
85 | ||
86 | For example, see bt_value_map_foreach(). | |
b62d3f21 PP |
87 | |
88 | The two macros BT_PUT() and BT_MOVE() operate on \em variables rather | |
89 | than pointer values. You should use BT_PUT() instead of bt_put() when | |
90 | possible to avoid "double puts". For the same reason, you should use use | |
91 | BT_MOVE() instead of performing manual reference moves between | |
92 | variables. | |
93 | ||
94 | @file | |
95 | @brief Reference counting management macros and functions. | |
96 | @sa refs | |
97 | ||
98 | @addtogroup refs | |
99 | @{ | |
100 | */ | |
101 | ||
102 | /** | |
34d0da31 PP |
103 | @brief Calls bt_put() on a variable named \p _var, then |
104 | sets \p _var to \c NULL. | |
b62d3f21 PP |
105 | |
106 | Using this macro is considered safer than calling bt_put() because it | |
107 | makes sure that the variable which used to contain a reference to a | |
108 | Babeltrace object is set to \c NULL so that a future BT_PUT() or | |
109 | bt_put() call will not cause another, unwanted reference decrementation. | |
110 | ||
34d0da31 PP |
111 | @param[in,out] _var Name of a variable containing a |
112 | Babeltrace object's address (this address | |
113 | can be \c NULL). | |
b62d3f21 | 114 | |
34d0da31 PP |
115 | @post <strong>If \p _var does not contain \p NULL</strong>, |
116 | its reference count is decremented. | |
117 | @post \p _var contains \c NULL. | |
b62d3f21 PP |
118 | |
119 | @sa BT_MOVE(): Transfers the ownership of a Babeltrace object from a | |
120 | variable to another. | |
121 | */ | |
122 | #define BT_PUT(_var) \ | |
83509119 | 123 | do { \ |
b62d3f21 PP |
124 | bt_put(_var); \ |
125 | (_var) = NULL; \ | |
de3dd40e PP |
126 | } while (0) |
127 | ||
b62d3f21 | 128 | /** |
34d0da31 PP |
129 | @brief Transfers the ownership of a Babeltrace object from a variable |
130 | named \p _var_src to a variable named \p _var_dst. | |
b62d3f21 PP |
131 | |
132 | This macro implements the following common pattern: | |
133 | ||
134 | 1. Call bt_put() on \p _var_dst to make sure the previous reference | |
135 | held by \p _var_dst is discarded. | |
136 | 2. Assign \p _var_src to \p _var_dst. | |
137 | 3. Set \p _var_src to \c NULL to avoid future, unwanted reference | |
138 | decrementation of \p _var_src. | |
139 | ||
140 | @warning | |
141 | You must \em not use this macro when both \p _var_dst and | |
142 | \p _var_src contain the same Babeltrace object address and the reference | |
143 | count of this object is 1. The initial call to bt_put() on \p _var_dst | |
144 | would destroy the object and leave a dangling pointer in \p _var_dst. | |
145 | ||
34d0da31 PP |
146 | @param[in,out] _var_dst Name of the destination variable, containing |
147 | either the address of a Babeltrace object to | |
148 | put first, or \c NULL. | |
149 | @param[in,out] _var_src Name of the source variable, containing | |
150 | either the address of a Babeltrace object to | |
151 | move, or \c NULL. | |
b62d3f21 PP |
152 | |
153 | @pre <strong>If \p _var_dst and \p _var_src contain the same | |
34d0da31 PP |
154 | value which is not \c NULL</strong>, this object's reference |
155 | count is greater than 1. | |
b62d3f21 PP |
156 | @post <strong>If \c _var_dst is not \c NULL</strong>, its reference |
157 | count is decremented. | |
34d0da31 PP |
158 | @post \p _var_dst is equal to the value of \p _var_src \em before |
159 | you called this macro. | |
b62d3f21 PP |
160 | @post \p _var_src is \c NULL. |
161 | ||
162 | @sa BT_PUT(): Calls bt_put() on a variable, then sets it to \c NULL. | |
163 | */ | |
164 | #define BT_MOVE(_var_dst, _var_src) \ | |
165 | do { \ | |
166 | bt_put(_var_dst); \ | |
167 | (_var_dst) = (_var_src); \ | |
168 | (_var_src) = NULL; \ | |
e693822d PP |
169 | } while (0) |
170 | ||
b62d3f21 PP |
171 | /** |
172 | @brief Increments the reference count of the Babeltrace object \p obj. | |
173 | ||
174 | @param[in] obj Babeltrace object of which to get a new reference | |
175 | (can be \c NULL). | |
176 | @returns \p obj | |
177 | ||
178 | @post <strong>If \c obj is not \c NULL</strong>, its reference | |
179 | count is incremented. | |
180 | ||
181 | @sa bt_put(): Decrements the reference count of a Babeltrace object. | |
182 | */ | |
b82cd9f0 | 183 | void *bt_get(void *obj); |
de3dd40e | 184 | |
b62d3f21 PP |
185 | /** |
186 | @brief Decrements the reference count of the Babeltrace object | |
187 | \p obj. | |
188 | ||
189 | When the object's reference count reaches 0, the object can no longer | |
190 | be accessed and is considered \em destroyed. | |
191 | ||
192 | @remarks | |
193 | You should use the BT_PUT() macro instead of calling bt_put() since the | |
194 | former is generally safer. | |
195 | ||
196 | @param[in] obj Babeltrace object of which to drop a reference | |
197 | (can be \c NULL). | |
198 | ||
199 | @post <strong>If \c obj is not \c NULL</strong>, its reference | |
200 | count is decremented. | |
201 | ||
202 | @sa BT_PUT(): Calls bt_put() on a variable, then sets it to \c NULL. | |
203 | @sa BT_MOVE(): Transfers the ownership of a Babeltrace object from a | |
204 | variable to another. | |
205 | @sa bt_get(): Increments the reference count of a Babeltrace object. | |
206 | */ | |
83509119 | 207 | void bt_put(void *obj); |
de3dd40e | 208 | |
b62d3f21 PP |
209 | /** |
210 | @} | |
211 | */ | |
212 | ||
26162993 PP |
213 | #ifdef __cplusplus |
214 | } | |
215 | #endif | |
216 | ||
83509119 | 217 | #endif /* BABELTRACE_REF_H */ |