src/common/optional.h

   1 /*
   2  * Copyright (C) 2019 Jérémie Galarneau <jeremie.galarneau@efficios.com>
   3  *
   4  * SPDX-License-Identifier: GPL-2.0-only
   5  *
   6  */
   7
   8 #ifndef LTTNG_OPTIONAL_H
   9 #define LTTNG_OPTIONAL_H
  10
  11 #include <stdint.h>
  12 #include <assert.h>
  13
  14 /*
  15  * Define wrapper structure representing an optional value.
  16  *
  17  * This macro defines an "is_set" boolean field that must be checked
  18  * when accessing the optional field. This "is_set" field provides
  19  * the semantics that would be expected of a typical "raw pointer" field
  20  * which would be checked for NULL.
  21  *
  22  * Prefer using this macro where "special" values would be used, e.g.
  23  * -1ULL for uint64_t types.
  24  *
  25  * Declaration example:
  26  * struct my_struct {
  27  *      int a;
  28  *      LTTNG_OPTIONAL(int, b);
  29  * };
  30  *
  31  * Usage example:
  32  * struct my_struct foo = LTTNG_OPTIONAL_INIT;
  33  *
  34  * LTTNG_OPTIONAL_SET(&foo.b, 42);
  35  * if (foo.b.is_set) {
  36  *      printf("%d", foo.b.value);
  37  * }
  38  *
  39  * LTTNG_OPTIONAL_UNSET(&foo.b);
  40  */
  41 #define LTTNG_OPTIONAL(type) \
  42         struct {             \
  43                 uint8_t is_set; \
  44                 type value;  \
  45         }
  46
  47 /*
  48  * Alias used for communication structures. If the layout of an LTTNG_OPTIONAL
  49  * is changed, the original layout should still be used for communication
  50  * purposes.
  51  *
  52  * LTTNG_OPTIONAL_COMM should be combined with the LTTNG_PACKED macro when
  53  * used for IPC / network communication.
  54  */
  55 #define LTTNG_OPTIONAL_COMM LTTNG_OPTIONAL
  56
  57 /*
  58  * This macro is available as a 'convenience' to allow sites that assume
  59  * an optional value is set to assert() that it is set when accessing it.
  60  *
  61  * Since this returns the 'optional' by value, it is not suitable for all
  62  * wrapped optional types. It is meant to be used with PODs.
  63  */
  64 #define LTTNG_OPTIONAL_GET(optional)                    \
  65         ({                                              \
  66                 assert((optional).is_set);              \
  67                 (optional).value;                       \
  68         })
  69
  70 /*
  71  * Initialize an optional field.
  72  *
  73  * The wrapped field is set to the value it would gave if it had static storage
  74  * duration.
  75  */
  76 #define LTTNG_OPTIONAL_INIT { .is_set = 0 }
  77
  78 /* Set the value of an optional field. */
  79 #define LTTNG_OPTIONAL_SET(field_ptr, val)      \
  80         do {                                    \
  81                 (field_ptr)->value = (val);     \
  82                 (field_ptr)->is_set = 1;        \
  83         } while (0)
  84
  85 /* Put an optional field in the "unset" (NULL-ed) state. */
  86 #define LTTNG_OPTIONAL_UNSET(field_ptr)         \
  87         do {                                    \
  88                 (field_ptr)->is_set = 0;        \
  89         } while (0)
  90
  91 #endif /* LTTNG_OPTIONAL_H */