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Merge branch 'doc/add_UUID_description_in_gattc_demo_backport_v4.2' into 'release/v4.2'

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docs:add 128-bit UUID description backport v4.2

See merge request espressif/esp-idf!9224
This commit is contained in:
Jiang Jiang Jian 2020-07-08 16:14:45 +08:00
commit ccc4f1337b
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examples/bluetooth/bluedroid/ble/gatt_client/tutorial/Gatt_Client_Example_Walkthrough.md
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@ -484,6 +484,25 @@ Where,
```c
#define REMOTE_SERVICE_UUID 0x00FF
```
If UUID of the service application the user is interested in is 128-bit, then there is one note below for the user which is relevant with the little-endian storage mode of the processor architecture.
The struct of UUID is defined as:
```c
typedef struct {
#define ESP_UUID_LEN_16 2
#define ESP_UUID_LEN_32 4
#define ESP_UUID_LEN_128 16
uint16_t len; /*!< UUID length, 16bit, 32bit or 128bit */
union {
uint16_t uuid16; /*!< 16bit UUID */
uint32_t uuid32; /*!< 32bit UUID */
uint8_t uuid128[ESP_UUID_LEN_128]; /*!< 128bit UUID */
} uuid; /*!< UUID */
} __attribute__((packed)) esp_bt_uuid_t;
```
Note: In little-endian storage mode, you can define service UUID directly in the normal order if it's a 16-bit or a 32-bit UUID, but if service UUID is 128-bit, there is minor difference. For example, if the UUID of the service application that the user is interested in is 12345678-a1b2-c3d4-e5f6-9fafd205e457, `REMOTE_SERVICE_UUID` should be defined as {0x57,0xE4,0x05,0xD2,0xAF,0x9F,0xF6,0xE5,0xD4,0xC3,0xB2,0xA1,0x78,0x56,0x34,0x12}.
The services are then discovered as follows:
```c