How to specify global build defines and options

To create globally usable macro definitions for a Sketch, create a file with a name based on your Sketch’s file name followed by .globals.h in the Sketch folder. For example, if the main Sketch file is named LowWatermark.ino, its global .h file would be LowWatermark.ino.globals.h. This file will be implicitly included with every module built for your Sketch. Do not directly include it in any of your sketch files or in any other source files. There is no need to create empty/dummy files, when not used.

This global .h also supports embedding compiler command-line options in a unique “C” block comment. Compiler options are placed in a “C” block comment starting with /*@create-file:build.opt@. This signature line must be alone on a single line. The block comment ending */ should also be alone on a single line. In between, place your compiler command-line options just as you would have for the GCC @file command option.

Actions taken in processing comment block to create build.opt

  • for each line, white space is trimmed

  • blank lines are skipped

  • lines starting with *, //, or # are skipped

  • the remaining results are written to build tree/core/build.opt

  • multiple /*@create-file:build.opt@ */ comment blocks are not allowed

  • build.opt is finished with a -include ... command, which references the global .h its contents were extracted from.

Example Sketch: LowWatermark.ino

#include <umm_malloc/umm_malloc.h>  // has prototype for umm_free_heap_size_min()

void setup() {
#ifdef MYTITLE1
  Serial.printf("\r\n" MYTITLE1 MYTITLE2 "\r\n");
  Serial.println("ERROR:  MYTITLE1 not present");
  Serial.printf("Heap Low Watermark %u\r\n", umm_free_heap_size_min());

void loop() {}

Global .h file: LowWatermark.ino.globals.h

// An embedded build.opt file using a "C" block comment. The starting signature
// must be on a line by itself. The closing block comment pattern should be on a
// line by itself. Each line within the block comment will be space trimmed and
// written to build.opt, skipping blank lines and lines starting with '//', '*'
// or '#'.

 * this line is ignored
 # this line is ignored
-DMYTITLE1="\"Running on \""


#if !defined(__ASSEMBLER__)
// Defines kept away from assembler modules
// i.e. Defines for .cpp, .ino, .c ... modules

#if defined(__cplusplus)
// Defines kept private to .cpp and .ino modules
//#pragma message("__cplusplus has been seen")
#define MYTITLE2 "Empty"

#if !defined(__cplusplus) && !defined(__ASSEMBLER__)
// Defines kept private to .c modules
#define MYTITLE2 "Full"

#if defined(__ASSEMBLER__)
// Defines kept private to assembler modules


Separate production and debug build options

If your production and debug build option requirements are different, adding mkbuildoptglobals.extra_flags={build.debug_port} to platform.local.txt will create separate build option groups for debugging and production. For the production build option group, the “C” block comment starts with /*@create-file:build.opt@, as previously defined. For the debugging group, the new “C” block comment starts with /*@create-file:build.opt:debug@. You make your group selection through “Arduino->Tools->Debug port” by selecting or disabling the “Debug port.”

Options common to both debug and production builds must be included in both groups. Neither of the groups is required. You may also omit either or both.

Reminder with this change, any old “sketch” with only a “C” block comment starting with /*@create-file:build.opt@ would not use a build.opt file for the debug case. Update old sketches as needed.

Updated Global .h file: LowWatermark.ino.globals.h

// Debug build options
-DMYTITLE1="\"Running on \""


// Removing the optimization for "sibling and tail recursive calls" may fill
// in some gaps in the stack decoder report. Preserves the stack frames
// created at each level as you call down to the next.

// Production build options
-DMYTITLE1="\"Running on \""


#if defined(__cplusplus)
#define MYTITLE2 "Empty"

#if !defined(__cplusplus) && !defined(__ASSEMBLER__)
#define MYTITLE2 "Full"

// Global Debug defines
// ...
// Global Production defines
// ...


Aggressively cache compiled core

This feature appeared with the release of Arduino IDE 1.8.2. The feature “Aggressively Cache Compiled core” refers to sharing a single copy of core.a across all Arduino IDE Sketch windows. This feature is on by default. core.a is an archive file containing the compiled objects of ./core/esp8266/*. Created after your 1ST successful compilation. All other open sketch builds use this shared file. When you close all Arduino IDE windows, the core archive file is deleted.

This feature is not compatible with using global defines or compiler command-line options. Without mediation, bad builds could result, when left enabled. When #define changes require rebuilding core.a and multiple Sketches are open, they can no longer reliably share one cached core.a. In a simple case: The 1st Sketch to be built has its version of core.a cached. Other sketches will use this cached version for their builds.

There are two solutions to this issue:

  1. Do nothing, and rely on aggressive cache workaround built into the script.

  2. Turn off the “Aggressively Cache Compiled core” feature, by setting compiler.cache_core=false.

Using “compiler.cache_core=false”

There are two ways to turn off the “Aggressively Cache Compiled core” feature: This can be done with the Arduino IDE command-line or a text editor.

Using the Arduino IDE command-line from a system command line, enter the following:

arduino --pref compiler.cache_core=false --save-prefs

For the text editor, you need to find the location of preferences.txt. From the Arduino IDE, go to File->Preferences. Make note of the path to prefereces.txt. You cannot edit the file while the Arduino IDE is running. Close all Arduino IDE windows and edit the file preferences.txt. Change compiler.cache_core=true to compiler.cache_core=false and save. Then each sketch will maintain its own copy of core.a built with the customization expressed by their respective build.opt file.

The “workaround”

When the “Aggressively Cache Compiled core” feature is enabled and the global define file is detected, a workaround will turn on and stay on. When you switch between Sketch windows, core will be recompiled and the cache updated. The workaround logic is reset when Arduino IDE is completely shutdown and restarted.

The workaround is not perfect. These issues may be of concern:

  1. Dirty temp space. Arduino build cache files left over from a previous run or boot.

  2. Arduino command-line options:

    • override default preferences.txt file.

    • override a preference, specifically compiler.cache_core.

  3. Multiple versions of the Arduino IDE running

Dirty temp space

A minor concern, the workaround is always on. Not an issue for build accuracy, but core.a maybe rebuild more often than necessary.

Some operating systems are better at cleaning up their temp space than others at reboot after a crash. At least for Windows®, you may need to manually delete the Arduino temp files and directories after a crash. Otherwise, the workaround logic may be left on. There is no harm in the workaround being stuck on, the build will be correct; however, the core files will occasionally be recompiled when not needed.

For some Windows® systems the temp directory can be found near C:\Users\<user id>\AppData\Local\Temp\arduino*. Note AppData is a hidden directory. For help with this do an Internet search on windows disk cleanup. Or, type disk cleanup in the Windows® taskbar search box.

With Linux, this problem could occur after an Arduino IDE crash. The problem would be cleared after a reboot. Or you can manually cleanup the /tmp/ directory before restarting the Arduino IDE.

Arduino command-line option overrides

If you are building with compiler.cache_core=true no action is needed. If false the script would benefit by knowing that.

When using either of these two command-line options:

./arduino --preferences-file other-preferences.txt
./arduino --pref compiler.cache_core=false

Hints for discovering the value of compiler.cache_core, can be provided by specifying mkbuildoptglobals.extra_flags=... in platform.local.txt.

Examples of hints:

mkbuildoptglobals.extra_flags=--preferences_sketch            # assume file preferences.txt in the sketch folder
mkbuildoptglobals.extra_flags=--preferences_sketch "pref.txt" # is relative to the sketch folder
mkbuildoptglobals.extra_flags=--preferences_file "other-preferences.txt" # relative to IDE or full path

If required, remember to quote file or file paths.

Multiple versions of the Arduino IDE running

You can run multiple Arduino IDE windows as long as you run one version of the Arduino IDE at a time. When testing different versions, completely exit one before starting the next version. For example, Arduino IDE 1.8.19 and Arduino IDE 2.0 work with different temp and build paths. With this combination, the workaround logic sometimes fails to enable.

At the time of this writing, when Arduino IDE 2.0 rc5 exits, it leaves the temp space dirty. This keeps the workaround active the next time the IDE is started. If this is an issue, manually delete the temp files.

Custom build environments

Some custom build environments may have already addressed this issue by other means. If you have a custom build environment that does not require this feature and would like to turn it off, you can add the following lines to the platform.local.txt used in your build environment:


Other build confusion

  1. Renaming a file does not change the last modified timestamp, possibly causing issues when adding a file by renaming and rebuilding. A good example of this problem would be to have then fixed a typo in file name LowWatermark.ino.globals.h. You need to touch (update timestamp) the file so a “rebuild all” is performed.

  2. When a .h file is renamed in the sketch folder, a copy of the old file remains in the build sketch folder. This can create confusion if you missed an edit in updating an #include in one or more of your modules. That module will continue to use the stale version of the .h until you restart the IDE or other major changes that would cause the IDE to delete and recopy the contents from the source Sketch directory. Changes on the IDE Tools board settings may cause a complete rebuild, clearing the problem. This may be the culprit for “What! It built fine last night!”