BLE gatt_client ( do while(0) )

filipESP
Posts: 59
Joined: Tue Nov 06, 2018 12:42 pm

BLE gatt_client ( do while(0) )

Postby filipESP » Fri Feb 15, 2019 3:19 pm

Hi.
I wonder about the do{}while(0) loop but I don't know what is the adventage of using this code in the esp_gattc_cb() function.
Could anybody explain it?

ESP_Sprite
Posts: 4106
Joined: Thu Nov 26, 2015 4:08 am

Re: BLE gatt_client ( do while(0) )

Postby ESP_Sprite » Sat Feb 16, 2019 4:50 am

Is this part of a macro (#define bla)? If so, this is probably what's happening: https://gcc.gnu.org/onlinedocs/cpp/Swal ... colon.html

filipESP
Posts: 59
Joined: Tue Nov 06, 2018 12:42 pm

Re: BLE gatt_client ( do while(0) )

Postby filipESP » Mon Feb 25, 2019 4:53 pm

I mean about this code:

Code: Select all

    do {
        int idx;
        for (idx = 0; idx < PROFILE_NUM; idx++) {
            if (gattc_if == ESP_GATT_IF_NONE || /* ESP_GATT_IF_NONE, not specify a certain gatt_if, need to call every profile cb function */
                    gattc_if == gl_profile_tab[idx].gattc_if) {
                if (gl_profile_tab[idx].gattc_cb) {
                    gl_profile_tab[idx].gattc_cb(event, gattc_if, param);
                }
            }
        }
    } while (0);

phatpaul
Posts: 67
Joined: Fri Aug 24, 2018 1:14 pm

Re: BLE gatt_client ( do while(0) )

Postby phatpaul » Tue Feb 26, 2019 3:46 pm

Maybe so that they could declare a new variable int idx; vs. declaring it at the top of the function.

That would have also been possible without the do{}while(0).
You could have just started a new section with a curly bracket i.e.

Code: Select all

void function() 
{
  blah;
  blah;

  {
    int idx;
    for (idx = 0; idx < PROFILE_NUM; idx++) {
      blah;
    }
  }
}

filipESP
Posts: 59
Joined: Tue Nov 06, 2018 12:42 pm

Re: BLE gatt_client ( do while(0) )

Postby filipESP » Wed Feb 27, 2019 10:02 am

OK, and what is the difference between use:

Code: Select all

void function() { blah; blah; { int idx; for (idx = 0; idx < PROFILE_NUM; idx++) { blah; } } }
and:

Code: Select all

void function() { int idx; blah; blah; { for (idx = 0; idx < PROFILE_NUM; idx++) { blah; } } }
?

phatpaul
Posts: 67
Joined: Fri Aug 24, 2018 1:14 pm

Re: BLE gatt_client ( do while(0) )

Postby phatpaul » Wed Feb 27, 2019 2:11 pm

I think this discussion is getting off-topic from the forum ESP32 IDF, but it's interesting...

In the trivial case that you provided, I don't think there is much if any difference in function, performance, or memory usage.

But in some cases you may want to wait to put variables on the stack until they are needed. This is good practice to keep the stack small.

Code: Select all

void function() 
{
  blah;
  blah;

  {
    char big_buffer1[255] = {0};
    int idx;
    for (idx = 0; idx < PROFILE_NUM; idx++) {
      blah;  // i.e. do something with big_buffer1
    }
  } // big_buffer1 is released from the stack automatically when execution exits this section
  
  {
    char big_buffer2[255] = {0};
    int idx;
    for (idx = 0; idx < PROFILE_NUM; idx++) {
      blah;  // i.e. do something with big_buffer2
    }
  } // big_buffer2 is released from the stack automatically when execution exits this section
}
Notice that there is never a time that both big_buffer1 and big_buffer2 are both on the stack.

Note you could have achieved the same by making each section into a function, but for style/readability this explicit inline may be preferred.

ESP_Tianhao
Posts: 28
Joined: Thu Jan 05, 2017 10:46 am

Re: BLE gatt_client ( do while(0) )

Postby ESP_Tianhao » Tue Mar 05, 2019 4:16 am

Agree with phatpaul.

I tend to keep the variables just in the section which use it. Not only related to stack use, but also make code looks more logical.

But there's another discussion about `{}` and `do {} while {0}`, which is better? :roll:

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