src/common/optional.h

   1 /*
   2  * Copyright (C) 2019 - Jérémie Galarneau <jeremie.galarneau@efficios.com>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License, version 2 only,
   6  * as published by the Free Software Foundation.
   7  *
   8  * This program is distributed in the hope that it will be useful, but WITHOUT
   9  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  10  * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
  11  * more details.
  12  *
  13  * You should have received a copy of the GNU General Public License along
  14  * with this program; if not, write to the Free Software Foundation, Inc.,
  15  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  16  */
  17
  18 #ifndef LTTNG_OPTIONAL_H
  19 #define LTTNG_OPTIONAL_H
  20
  21 #include <stdint.h>
  22 #include <assert.h>
  23
  24 /*
  25  * Define wrapper structure representing an optional value.
  26  *
  27  * This macro defines an "is_set" boolean field that must be checked
  28  * when accessing the optional field. This "is_set" field provides
  29  * the semantics that would be expected of a typical "raw pointer" field
  30  * which would be checked for NULL.
  31  *
  32  * Prefer using this macro where "special" values would be used, e.g.
  33  * -1ULL for uint64_t types.
  34  *
  35  * Declaration example:
  36  * struct my_struct {
  37  *      int a;
  38  *      LTTNG_OPTIONAL(int, b);
  39  * };
  40  *
  41  * Usage example:
  42  * struct my_struct foo = LTTNG_OPTIONAL_INIT;
  43  *
  44  * LTTNG_OPTIONAL_SET(&foo.b, 42);
  45  * if (foo.b.is_set) {
  46  *      printf("%d", foo.b.value);
  47  * }
  48  *
  49  * LTTNG_OPTIONAL_UNSET(&foo.b);
  50  */
  51 #define LTTNG_OPTIONAL(type) \
  52         struct {             \
  53                 uint8_t is_set; \
  54                 type value;  \
  55         }
  56
  57 /*
  58  * Alias used for communication structures. If the layout of an LTTNG_OPTIONAL
  59  * is changed, the original layout should still be used for communication
  60  * purposes.
  61  *
  62  * LTTNG_OPTIONAL_COMM should be combined with the LTTNG_PACKED macro when
  63  * used for IPC / network communication.
  64  */
  65 #define LTTNG_OPTIONAL_COMM LTTNG_OPTIONAL
  66
  67 /*
  68  * This macro is available as a 'convenience' to allow sites that assume
  69  * an optional value is set to assert() that it is set when accessing it.
  70  *
  71  * Since this returns the 'optional' by value, it is not suitable for all
  72  * wrapped optional types. It is meant to be used with PODs.
  73  */
  74 #define LTTNG_OPTIONAL_GET(optional)                    \
  75         ({                                              \
  76                 assert(optional.is_set);                \
  77                 optional.value;                         \
  78         })
  79
  80 /*
  81  * Initialize an optional field.
  82  *
  83  * The wrapped field is set to the value it would gave if it had static storage
  84  * duration.
  85  */
  86 #define LTTNG_OPTIONAL_INIT { .is_set = 0 }
  87
  88 /* Set the value of an optional field. */
  89 #define LTTNG_OPTIONAL_SET(field_ptr, val) \
  90         (field_ptr)->value = val;          \
  91         (field_ptr)->is_set = 1;
  92
  93 /* Put an optional field in the "unset" (NULL-ed) state. */
  94 #define LTTNG_OPTIONAL_UNSET(field_ptr)    \
  95         (field_ptr)->is_set = 0;
  96
  97 #endif /* LTTNG_OPTIONAL_H */