`build_flags`

Type: String | Multiple: Yes

These flags/options affect the preprocessing, compilation, assembly, and linking processes for C and C++ code. All compiler and linker flags can be used. Despite the name, CPPDEFINES (C PreProcesor) rows also apply to the C compiler.

For more detailed information about available compiler flags/options, please visit GCC Command Options official documentation.

This option can also be set by the global environment variable PLATFORMIO_BUILD_FLAGS.

Scopes (SCons Variables)

Format	Affects build variable	Description
`-D name`	CPPDEFINES	Predefine name as a macro, with definition 1.
`-D name=definition`	CPPDEFINES	The contents of definition are tokenized and processed as if they appeared during translation phase three in a `#define` directive.
`-U name`	CPPDEFINES	Cancel any previous definition of name, either built in or provided with a `-D` option.
`-Wp,option`	CPPFLAGS	Bypass the compiler driver and pass option directly through to the preprocessor
`-Wall`	CCFLAGS	Turn on all optional warnings which are desirable for normal code.
`-Werror`	CCFLAGS	Make all warnings into hard errors. With this option, if any source code triggers warnings, the compilation will be aborted.
`-w`	CCFLAGS	Suppress all warnings, including those which GNU CPP issues by default.
`-include file`	CCFLAGS	Process file as if `#include "file"` appeared as the first line of the primary source file.
`-Idir`	CPPPATH	Add the directory dir to the list of directories to be searched for header files.
`-Wa,option`	ASFLAGS, CCFLAGS	Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.
`-Wl,option`	LINKFLAGS	Pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas.
`-llibrary`	LIBS	Search the library named library when linking
`-Ldir`	LIBPATH	Add directory dir to the list of directories to be searched for `-l`.

Examples 

[env:specific_defines]
build_flags =
  -DFOO -DBAR=1
  -D BUILD_ENV_NAME=$PIOENV
  -D CURRENT_TIME=$UNIX_TIME
  -DFLOAT_VALUE=1.23457e+07

[env:specific_inclibs]
build_flags =
  -I/opt/include
  -I"relative/path with space"
  -I"C:\windows\dir"
  -I"${platformio.packages_dir}/framework-foo/include"
  -L/opt/lib
  -lfoo

[env:ignore_incremental_builds]
; We dynamically change the value of "LAST_BUILD_TIME" macro,
; PlatformIO will not cache objects
build_flags = -DLAST_BUILD_TIME=$UNIX_TIME

Note

If you need to control build flags that are specific for debug configuration please refer to debug_build_flags.

Built-in Variables 

You can inject the built-in variables into your build flags, such as:

$PYTHONEXE, full path to current Python interpreter
$UNIX_TIME, current time in Unix format
$PIOENV, name of build environment from “platformio.ini” (Project Configuration File)
$PIOPLATFORM, name of development platform
$PIOFRAMEWORK, a list of frameworks
$PROJECT_DIR, project directory
$PROJECT_CORE_DIR, PlatformIO Core directory, see core_dir
$PROJECT_BUILD_DIR, project build directory per all environments
$BUILD_DIR, build directory per current environment

See the full list of PlatformIO variables.

Please use target envdump for the pio run --target command to see ALL variable values for a build environment.

Stringification 

Sometimes you may want to convert a macro argument into a valid C string constant. In this case, you need to wrap a value with double quotes (") and escape double quotes (" -> \\") in the constant value.

Here is an example of a macro definition that uses stringification and build_flags. Please note that we enclosed the flag in the single quotes to prevent the shell evaluation:

platformio.ini

[env:myenv]
build_flags =
    '-D MYSTRING="Text is \\"Quoted\\""'
    '-D MYFONT=&roboto14'

src/main.cpp

#include <stdio.h>

#define CONFIG_LV_FONT_DEFAULT MYFONT

int main(void) {
    printf("MYSTRING=<%s>\n", MYSTRING);
    return(0);
}

If you use Advanced Scripting, we recommend benefiting from the env.StringifyMacro(value) helper function. In this case, you don’t need to apply any escaping, PlatformIO will do this for you:

platformio.ini

[env:myenv]
extra_scripts = myscript.py

myscript.py

Import("env")

env.Append(CPPDEFINES=[
    ("MYSTRING", env.StringifyMacro('Text is "Quoted"')),
])

Dynamic build flags 

PlatformIO allows users to run an external command/script which outputs build flags into STDOUT by prepending the shell command with a ! character. PlatformIO will automatically replace commands with their output when appending flags to build environments.

You can use any shell or programming language.

This external command will be called on each pio run command before building/uploading process.

Use cases:

Macro with the latest VCS revision/tag “on-the-fly”

Generate dynamic headers (*.h)

Process media content before generating SPIFFS image

Make some changes to source code or related libraries

Note

If you need more advanced control and would like to apply changes to a PlatformIO Build System environment, please refer to Advanced Scripting.

Example:

[env:generate_flags_with_external_command]
build_flags = !cmd_or_path_to_script

; Unix only, get output from internal command
build_flags = !echo '-D COMMIT_HASH=\\"'$(git log -1 --format=%%h)'\\"'

Use Case: Create a “PIO_SRC_REV” macro with the latest Git revision

This example includes a separate file named git_rev_macro.py, to be placed in the same directory as platformio.ini.

platformio.ini:

[env:git_revision_macro]
build_flags = !python git_rev_macro.py

git_rev_macro.py:

import subprocess

revision = (
    subprocess.check_output(["git", "rev-parse", "HEAD"])
    .strip()
    .decode("utf-8")
)
print("'-DGIT_REV=\"%s\"'" % revision)

build_flags

Scopes (SCons Variables)

Examples

Built-in Variables

Stringification

Dynamic build flags