Generated prototype incorrectly prefixed with `extern "C"` when comment contains `//`

[Rob58329]

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Describe the problem

A stray curly-brace in // comments or #if (false) sections causes the function prototypes generated during the "sketch pre-processing" stage of the compilation operation to be incorrectly prefixed with extern "C"

For example:

extern "C" void setup();

To reproduce

Compile the following sketch:

#if (false) // The below lines should not be enabled, and so should not have any effect 
#error "Should not get to here"
// {               // either delete this line,
// extern "C" {}   // or delete this line, and the below code is compiled fine!
#endif

void setup() {
  Serial.begin(74880);
}

void loop() {
  delay(5000);
  Serial.println("Hello");
}

Examine the .cpp file generated by the compilation.

🐛 Generated function prototypes are prefixed with extern "C"

---

Compile the following sketch:

#if (false) // The below lines should not be enabled, and so should not have any effect 
#error "Should not get to here"
// {               // either delete this line,
// extern "C" {}   // or delete this line, and the below code works fine!
#endif

ADC_MODE(ADC_VCC);

void setup() {
  Serial.begin(74880);
}

void loop() {
  delay(5000);
  Serial.print("raw_battery (should be between 3000 and 3300)="); Serial.println(ESP.getVcc());
  // unless you DELETE one of the above two COMMENT lines, you will get "=65535" and not "=3017"
}

Examine the .cpp file generated by the compilation.

🐛 Generated function prototypes are prefixed with extern "C"

Expected behavior

Generated function prototypes should be correct regardless of the contents of comments:

void setup();

Arduino CLI version

Original report

Arduino IDE v1.8.16

Last verified with

d563755e

Operating system

Windows

Operating system version

11

Additional context

I note that there are several discussions regarding curly-braces for Arduino IDE but none of these seem to cover this specific issue:

• https://forum.arduino.cc/t/ide-bug-with-braces-that-arent-part-of-the-code/206063
• https://forum.arduino.cc/t/curly-braces-in-strings-or-quotes/212575

Additional reports

• https://forum.arduino.cc/t/arduino-ide-causes-a-compilation-error-even-if-extern-c-is-inside-a-comment/1126819

Related

• https://github.com/arduino/arduino-cli/issues/1618

Issue checklist

• [X] I searched for previous reports in the issue tracker
• [X] I verified the problem still occurs when using the nightly build
• [X] My report contains all necessary details

[per1234]

Thanks @Rob58329!

In case it will be useful to the developers, I'll provide a minimal demo of the bug:

$ arduino-cli version
arduino-cli.exe  Version: nightly-20211216 Commit: d458040 Date: 2021-12-16T01:27:31Z

$ SKETCH_PATH="/tmp/ExternCBug"
$ mkdir "$SKETCH_PATH"
$ printf "// {\n// extern \"C\" {}\nvoid setup() {}\nvoid loop() {}\n" > "$SKETCH_PATH"/ExternCBug.ino
$ arduino-cli compile --fqbn arduino:avr:uno --preprocess "$SKETCH_PATH"
#include <Arduino.h>
#line 1 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
// {
// extern "C" {}
#line 3 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
 extern "C" void setup();
#line 4 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
 extern "C" void loop();
#line 3 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
void setup() {}
void loop() {}

Note that the generated setup and loop function prototypes unexpectedly are prefixed by extern "C"

Preprocessed sketch is as expected after removing the first comment from the sketch:

$ printf "// extern \"C\" {}\nvoid setup() {}\nvoid loop() {}\n" > "$SKETCH_PATH"/ExternCBug.ino
$ arduino-cli compile --fqbn arduino:avr:uno --preprocess "$SKETCH_PATH"
#include <Arduino.h>
#line 1 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
// extern "C" {}
#line 2 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
void setup();
#line 3 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
void loop();
#line 2 "C:\\Users\\per\\AppData\\Local\\Temp\\ExternCBug\\ExternCBug.ino"
void setup() {}
void loop() {}