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
  13 /*
  14  * Define wrapper structure representing an optional value.
  15  *
  16  * This macro defines an "is_set" boolean field that must be checked
  17  * when accessing the optional field. This "is_set" field provides
  18  * the semantics that would be expected of a typical "raw pointer" field
  19  * which would be checked for NULL.
  20  *
  21  * Prefer using this macro where "special" values would be used, e.g.
  22  * -1ULL for uint64_t types.
  23  *
  24  * Declaration example:
  25  * struct my_struct {
  26  *      int a;
  27  *      LTTNG_OPTIONAL(int) b;
  28  * };
  29  *
  30  * Usage example:
  31  * struct my_struct foo = LTTNG_OPTIONAL_INIT;
  32  *
  33  * LTTNG_OPTIONAL_SET(&foo.b, 42);
  34  * if (foo.b.is_set) {
  35  *      printf("%d", foo.b.value);
  36  * }
  37  *
  38  * LTTNG_OPTIONAL_UNSET(&foo.b);
  39  */
  40 #define LTTNG_OPTIONAL(type) \
  41         struct {             \
  42                 uint8_t is_set; \
  43                 type value;  \
  44         }
  45
  46 /*
  47  * Alias used for communication structures. If the layout of an LTTNG_OPTIONAL
  48  * is changed, the original layout should still be used for communication
  49  * purposes.
  50  *
  51  * LTTNG_OPTIONAL_COMM should be combined with the LTTNG_PACKED macro when
  52  * used for IPC / network communication.
  53  */
  54 #define LTTNG_OPTIONAL_COMM LTTNG_OPTIONAL
  55
  56 /*
  57  * This macro is available as a 'convenience' to allow sites that assume
  58  * an optional value is set to LTTNG_ASSERT() that it is set when accessing it.
  59  *
  60  * Since this returns the 'optional' by value, it is not suitable for all
  61  * wrapped optional types. It is meant to be used with PODs.
  62  */
  63 #define LTTNG_OPTIONAL_GET(optional)                    \
  64         ({                                              \
  65                 LTTNG_ASSERT((optional).is_set);                \
  66                 (optional).value;                       \
  67         })
  68
  69 /*
  70  * This macro is available as a 'convenience' to allow sites that assume
  71  * an optional value is set to LTTNG_ASSERT() that it is set when fecthing the
  72  * underlying value's address.
  73  */
  74 #define LTTNG_OPTIONAL_GET_PTR(optional)                        \
  75         ({                                              \
  76                 LTTNG_ASSERT((optional).is_set);                \
  77                 &(optional).value;                      \
  78         })
  79
  80 /*
  81  * Initialize an optional field as unset.
  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_UNSET { .is_set = 0 }
  87
  88 /*
  89  * Initialize an optional field as 'set' with a given value.
  90  */
  91 #define LTTNG_OPTIONAL_INIT_VALUE(val) { .is_set = 1, .value = val }
  92
  93 /* Set the value of an optional field. */
  94 #define LTTNG_OPTIONAL_SET(field_ptr, val)      \
  95         do {                                    \
  96                 (field_ptr)->is_set = 1;        \
  97                 (field_ptr)->value = (val);     \
  98         } while (0)
  99
 100 /* Put an optional field in the "unset" (NULL-ed) state. */
 101 #define LTTNG_OPTIONAL_UNSET(field_ptr)         \
 102         do {                                    \
 103                 (field_ptr)->is_set = 0;        \
 104         } while (0)
 105
 106 #endif /* LTTNG_OPTIONAL_H */