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Duktape config header, duk_config.h

Purpose of duk_config.h

duk_config.h is an external configuration header ("config header") which provides all platform, compiler, and architecture specific features so that the main Duktape source code can compile without relying on platform specific headers or functionality. The header also provides active Duktape config options (DUK_USE_xxx) for enabling/disabling various optional Duktape features.

The external duk_config.h config header replaces the built-in platform and feature detection used by Duktape 1.2 and prior. Moving the built-in platform and configuration logic into an external header avoids the need to use compiler command line defines (-DDUK_OPT_xxx) which were needed both when compiling Duktape and the application. An external config header is also easier to manually adapt to exotic environments without the need to change Duktape internals such as platform detection. Finally, a config header is a much better match to Unix distributions than relying on compiler command line defines.

While the external config header provides much more flexibility it also needs a bit more thought especially when adapting Duktape to an exotic environment.

This document describes various approaches on creating a config header and updating it when a new Duktape release is taken into use.

Coming up with a duk_config.h

duk_config.h is always external to Duktape main source code so that it's always possible, if necessary as a last resort, to manually edit the configuration file or even create one from scratch.

As such there are multiple ways to come up with a config header; for common platforms you don't usually need to do much while for more exotic platforms more manual work may be needed. There's no "right way", but the more manual modifications are made, the more effort is needed to deal with Duktape updates.

The basic options are:

  • Use default duk_config.h in distribution: Duktape distributable includes a default duk_config.h which is compatible with Duktape 1.2: it autodetects the platform, compiler, and architecture, and resolves active config options through feature options (DUK_OPT_xxx). This header should work "out of the box" for Linux, OS X, Windows, and also for several more exotic platforms, and is a drop-in replacement for Duktape 1.2 behavior. If you're using one of the supported platforms, this should be your default choice.
  • Use default duk_config.h with manual modifications: You can modify the default duk_config.h directly if only a small change is needed. Such changes can be manual, or scripted using e.g. sed. Using scripting is less error prone when Duktape is upgraded and the source duk_config.h changes (which is usual for new versions). See separate section below on how to tweak a header using a script.
  • Use genconfig.py to create an autodetect duk_config.h: You can use genconfig.py to create a custom autodetecting duk_config.h and specify config option overrides on genconfig command line. See separate section below on how to use genconfig.
  • Use genconfig.py to create a barebones duk_config.h: While the autodetect duk_config.h is convenient, it won't work on exotic platforms. To support exotic platforms, genconfig.py can generate a template duk_config.h for a specified platform, compiler, and architecture combination (each can be either specified or left as "autodetect") which should match your target as closely as possible. You can then modify the header manually or through scripting.
  • Edit the genconfig metadata and regenerate duk_config.h: You can also add support for your custom platform directly into the genconfig metadata. For example, to support a custom compiler, you'll need to add a compiler-specific C header snippets to detect the compiler and to override default macros which are inappropriate for that compiler. The duk_config.h can then be regenerated using updated metadata.
  • Write a duk_config.h from scratch: You could also write a duk_config.h from scratch, but because there are quite many typedefs, macros, and config options, it's probably easiest to modify the default or genconfig-generated duk_config.h.

NOTE: In future Duktape versions the DUK_OPT_xxx feature options will be removed altogether so that config options are only controlled through DUK_USE_xxx options, avoiding the two-level structure which is no longer well justified if DUK_USE_xxx options are configured directly in the config header.

Using genconfig

Overview of genconfig

Genconfig (config/genconfig.py) is a helper script which provides several commands related to config handling:

  • Generate a duk_config.h for a specified platform, compiler, and architecture. Each can be specified explicitly (e.g. use "gcc" for the compiler) or be left up to automatic compile-time detection. The default duk_config.h is generated with everything left up to automatic detection. A barebones, target specific header can be generated by defining platform, compiler, and architecture explicitly.
  • Generate documentation for feature and config options.

Config headers are generated based on config option and target metadata files, and manually edited header snippets which are combined to create a final header. Documentation is generated based on config option metadata. Metadata is expressed as YAML files for easy editing and good diff/merge behavior.

Generating an autodetect duk_config.h

To generate an autodetect header suitable for directly supported platforms (matches Duktape 1.2 platform support):

# The --metadata option can point to a metadata directory or a tar.gz
# file with packed metadata (included in end user distributable).

$ cd duktape-1.4.0
$ python config/genconfig.py \
    --metadata config/genconfig_metadata.tar.gz \
    --output /tmp/duk_config.h \
    duk-config-header

# The same command using unpacked metadata present in Duktape source repo.

$ cd duktape
$ python config/genconfig.py \
    --metadata config/ \
    --output /tmp/duk_config.h \
    duk-config-header

The resulting header in /tmp/duk_config.h can then either be used as is or edited manually or through scripting.

You can override individual defines using in several ways (see "Option overrides" section below for more details): C compiler format (-D and -U options), YAML config through a file or inline, or verbatim fixup header through a file or inline.

If you're building Duktape as a DLL, you should use the --dll option:

$ python config/genconfig.py \
    --metadata config/ \
    --dll \
    --output /tmp/duk_config.h \
    duk-config-header

DLL builds cannot be detected automatically and they affect symbol visibility attributes on Windows. The -dll option creates a header which assumes that a DLL will be built. If legacy feature option support is enabled, you can still use DUK_OPT_DLL_BUILD and DUK_OPT_NO_DLL_BUILD during compilation to override this default.

Some changes such as reworking #include statements cannot be represented as override files; you'll need to edit the resulting config header manually or using some scripting approach.

Generating a barebones duk_config.h

To generate a barebones header you need to specify a platform, compiler, and architecture for genconfig:

$ python config/genconfig.py \
    --metadata config/ \
    --platform linux \
    --compiler gcc \
    --architecture x64 \
    --output /tmp/duk_config.h \
    duk-config-header

The barebones header in /tmp/duk_config.h can then either be used as is or edited manually or through scripting.

The platform, compiler, and architecture names map to genconfig header snippet files. Duktape config options will be assigned their default values specified in config option metadata files in config/config-options/.

You can override individual defines using in several ways (see "Option overrides" section below for more details): C compiler format (-D and -U options), YAML config through a file or inline, or verbatim fixup header through a file or inline.

Some changes such as reworking #include statements cannot be represented as override files; you'll need to edit the resulting config header manually or using some scripting approach.

Genconfig option overrides

Genconfig provides multiple ways of overriding config options when generating an autodetect or barebones duk_config.h header:

  • C compiler format:

    -DDUK_USE_TRACEBACK_DEPTH=100
-DDUK_USE_JX
-UDUK_USE_JC

  • YAML config read from a file or given inline on the command line:

    --option-file my_config.yaml
--option-yaml 'DUK_USE_FASTINT: true'

  • Verbatim fixup header lines read from a file or given inline on the command line:

    --fixup-file my_custom.h
--fixup-line '#undef DUK_USE_JX'

These option formats can be mixed which allows you to specify an option baseline (say --option-file low_memory.yaml) and then apply further overrides in various ways. All forced options in C compiler format and YAML format are processed first, with the last override winning. Fixup headers are then emitted in order.

C compiler format

The usual C compiler like format is supported because it's quite familiar. In this example a low memory base configuration is read from a YAML config file, and a few options are then tweaked using the C compiler format. An autodetect header is then generated:

$ cd duktape
$ python config/genconfig.py \
    --metadata config/ \
    --option-file low_memory.yaml \
    -DDUK_USE_TRACEBACK_DEPTH=100 \
    -UDUK_USE_JX -UDUK_USE_JC \
    --output /tmp/duk_config.h \
    duk-config-header

YAML config

A YAML config file allows options to be specified in a structured, programmatic manner. An example YAML config file, my_config.yaml could contain:

DUK_USE_OS_STRING: "\"hack-os\""  # force os name for Duktape.env
DUK_USE_ALIGN_BY: 8  # force align-by-8
DUK_USE_FASTINT: true
DUK_UNREF:
  verbatim: "#define DUK_UNREF(x) do { (void) (x); } while (0)"

This file, another override file, and a few inline YAML forced options could be used as follows to generate a barebones header:

$ cd duktape
$ python config/genconfig.py \
    --metadata config/ \
    --platform linux \
    --compiler gcc \
    --architecture x64 \
    --option-file my_config.yaml \
    --option-file more_overrides.yaml \
    --option-yaml 'DUK_USE_JX: false' \
    --option-yaml 'DUK_USE_JC: false' \
    --output /tmp/duk_config.h \
    barebones-header

For inline YAML, multiple forced options can be given either by using a YAML value with multiple keys, or by using multiple options:

# Multiple values for one option
--option-yaml '{ DUK_USE_JX: false, DUK_USE_DEBUG: true }'

# Multiple options
--option-yaml 'DUK_USE_JX: false' \
--option-yaml 'DUK_USE_DEBUG: true'

The YAML format for specifying options is simple: the top level value must be an object whose keys are define names to override. Values are as follows:

  • false: #undef option:

    # Produces: #undef DUK_USE_DEBUG
DUK_USE_DEBUG: false

  • true: #define option:

    # Produces: #define DUK_USE_DEBUG
DUK_USE_DEBUG: true

  • number: decimal value for define:

    # Produces: #define DUK_USE_TRACEBACK_DEPTH 10
DUK_USE_TRACEBACK_DEPTH: 10

# Produces: #define DUK_USE_TRACEBACK_DEPTH 100000L
# (a long constant is used automatically if necessary)
DUK_USE_TRACEBACK_DEPTH: 100000

  • string: verbatim string used as the define value:

    # Produces: #define DUK_USE_TRACEBACK_DEPTH (10 + 7)
DUK_USE_TRACEBACK_DEPTH: "(10 + 7)"

# Produces: #define DUK_USE_OS_STRING "linux"
DUK_USE_OS_STRING: "\"linux\""

  • C string for value:

    # Produces: #define DUK_USE_OS_STRING "linux"
DUK_USE_OS_STRING:
  string: "linux"

  • verbatim text for entire define:

    # Produces: #define DUK_UNREF(x) do {} while (0)
DUK_UNREF:
  verbatim: "#define DUK_UNREF(x) do {} while (0)"

Fixup header

In addition to YAML-based option overrides, genconfig has an option for appending direct "fixup headers" to deal with situations which cannot be handled with individual option overrides. For example, you may want to inject specific environment sanity checks, or set config option values based on environment #ifdefs. This mechanism is similar to Duktape 1.2.x duk_custom.h header, and you can in fact use duk_custom.h headers directly as inputs.

Fixup headers are emitted after all individual option overrides (in either C compiler or YAML format) have been resolved, but before emitting option sanity checks (if enabled).

For example, to generate a barebones header with two fixup headers:

$ python config/genconfig.py \
    --metadata config/ \
    --platform linux \
    --compiler gcc \
    --architecture x64 \
    --fixup-file my_env_strings.h \
    --fixup-file my_no_json_fastpath.h \
    --output /tmp/duk_config.h \
    barebones-header

The my_env_strings.h fixup header could be:

/* Force OS string. */
#undef DUK_USE_OS_STRING
#if !defined(__WIN32__)
#error this header is Windows only
#endif
#define DUK_USE_OS_STRING "windows"

/* Force arch string. */
#undef DUK_USE_ARCH_STRING
#if !defined(__amd64__)
#error this header is x64 only
#endif
#define DUK_USE_ARCH_STRING "x64"

/* Force compiler string. */
#undef DUK_USE_COMPILER_STRING
#if !defined(__GNUC__)
#error this header is gcc only
#endif
#if defined(__cplusplus__)
#define DUK_USE_COMPILER_STRING "g++"
#else
#define DUK_USE_COMPILER_STRING "gcc"
#endif

The example fixup header uses dynamic detection and other environment checks which cannot be easily expressed using individual option overrides.

The my_no_json_fastpath.h fixup header could be:

/* Disable JSON fastpath for reduced footprint. */
#undef DUK_USE_JSON_STRINGIFY_FASTPATH

This could have also been expressed using a simple override, e.g. as -UDUK_USE_JSON_STRINGIFY_FASTPATH.

Fixup headers are appended verbatim so they must be valid C header files, contain appropriate newlines, and must #undef any defines before redefining them if necessary. Fixup headers can only be used to tweak C preprocessor defines, they naturally cannot un-include headers or un-typedef types.

There's also a command line option to append a single fixup line for convenience:

# Append two lines to forcibly enable fastints
--fixup-line '#undef DUK_USE_FASTINT' \
--fixup-line '#define DUK_USE_FASTINT'

These can be mixed with --fixup-file options and are processed in sequence.

Modifying a duk_config.h manually or using scripting

The basic approach when using scripted modifications is to take a base header (either an autodetect or barebones header) and then make specific changes using a script. The advantage of doing so is that if the base header is updated, the script may often still be valid without any manual changes.

Scripting provides much more flexibility than tweaking individual options in genconfig, but the cost is more complicated maintenance over time.

Using diff/patch

  • Make the necessary changes to the base header manually.

  • Use diff to store the changes:

    $ diff -u duk_config.h.base duk_config.h.edited > edits.diff

  • In your build script:

    $ cp duk_config.h.base duk_config.h
$ patch duk_config.h edits.diff

  • If the patch fails (e.g. there is too much offset), you need to rebuild the diff file manually.

Using sed (or awk, etc) to modify an option in-place

If an option is defined on a single line in the base header (this is true for Duktape config options in the genconfig "barebones" header when feature option support is disabled, for example), e.g. either as:

#define DUK_USE_FOO

or as:

#undef DUK_USE_FOO

you can use sed to easily flip such an option:

# enable shuffle torture
cat duk_config.h.base | \
    sed -r -e 's/^#\w+\s+DUK_USE_SHUFFLE_TORTURE.*$/#define DUK_USE_SHUFFLE_TORTURE  \/*forced*\//' \
    > duk_config.h

The above example would flip DUK_USE_SHUFFLE_TORTURE on, regardless of its previous setting. You can also use a more verbose sed format which is easier to read especially if there are multiple changes:

cat duk_config.h.base | sed -r -e '
s/^#\w+\s+DUK_USE_SHUFFLE_TORTURE.*$/#define DUK_USE_SHUFFLE_TORTURE  \/*forced*\//
s/^#\w+\s+DUK_USE_OS_STRING.*$/#define DUK_USE_OS_STRING "my-custom-os"  \/*forced*\//
' > duk_config.h

This approach won't work if the defined option is defined/undefined multiple times or if the define has a multiline value.

For more stateful changes you can use awk or other scripting languages (Python, Perl, etc).

Modifying defines at __OVERRIDE_DEFINES__

Instead of modifying options in-place as in the sed example above, you can simply append additional preprocessor directives to undefine/redefine options as necessary. This is much easier to maintain in version updates than when modifications are made in-place.

Genconfig has a direct option to append "fixups" after the main generated header:

# my_custom.h is applied after generated header; functionally similar
# to Duktape 1.2.x duk_custom.h

$ python config/ genconfig.py [...] --fixup-file my_custom.h [...]

A genconfig-generated barebones header also has the following line near the end for detecting where to add override defines; this is easy to detect reliably:

/* __OVERRIDE_DEFINES__ */

The __OVERRIDE_DEFINES__ line is near the end of the file, before any automatically generated option sanity checks (which are optional) so that the sanity checks will be applied after your tweaks have been done:

#!/bin/bash

CONFIG_IN=duk_config.h.base
CONFIG_OUT=duk_config.h.new

cat $CONFIG_IN | sed -e '
/^\/\* __OVERRIDE_DEFINES__ \*\/$/ {
    r my_overrides.h
    d
}' > $CONFIG_OUT

Modifying defines near the end of the file is relatively easy but has a few limitations:

  • You can't change typedefs this way because there's no way to un-typedef.
  • You can't undo any #include directives executed.

Modifying defines at the end of the file

Another simple approach is to simply assume that an #endif line (include guard) is the last line in the file, i.e. there are no trailing empty lines. Changes will then be applied after option sanity checks which is not ideal:

#!/bin/bash

CONFIG_IN=duk_config.h.base
CONFIG_OUT=duk_config.h.new

if tail -1 $CONFIG_IN | grep endif ; then
    echo "Final line of $CONFIG_IN is an #endif as expected, modifying config"
else
    echo "Final line of $CONFIG_IN is not an #endif!"
    exit 1
fi

head -n -1 $CONFIG_IN > $CONFIG_OUT
cat >> $CONFIG_OUT <<EOF
/*
 *  Config hacks for platform XYZ.
 */

#undef DUK_USE_FASTINT  /* undef first to avoid redefine */
#define DUK_USE_FASTINT

/* compiler on XYZ has a custom "unreferenced" syntax */
#undef DUK_UNREF
#define DUK_UNREF(x) do { __foo_compiler_unreferenced((x)); } while (0)

#endif  /* DUK_CONFIG_H_INCLUDED */
EOF

echo "Wrote new config to $CONFIG_OUT, diff -u:"
diff -u $CONFIG_IN $CONFIG_OUT

Dealing with #include files

Include files are often a portability problem on exotic targets:

  • System headers may be missing. You may need to provide replacement functions for even very basic features like string formatting functions.
  • System headers may be present but in non-standard include paths. Duktape can't easily autodetect such paths because there's no "#include if available" directive: an #include either succeeds or causes compilation to fail.
  • System headers may be present but broken in some fashion so you want to avoid them entirely.
  • Sometimes custom programming environments have "SDK headers" that conflict with standard headers so that you can't include them both at the same time. It may be necessary to include the SDK headers but provide manual declarations for the system functions needed.

In such cases you may need to replace all the #include statements of a base header file and provide alternate include files or manual declarations.

Keeping a manually created duk_config.h up-to-date

When new Duktape versions are released, the set of config options and other macros required of the duk_config.h config header may change. This is the case for even minor version updates, though incompatible changes are of course avoided when possible.

Nevertheless, when a new version is taken into use, you may need to update your config header to match. How to do that depends on how you created the config header:

  • If you're using the default header, no changes should be necessary. You should check out new DUK_OPT_xxx feature options and decide if you want to use any of them.
  • If you're using a script to modify the default or genconfig-generated header, you should ensure your script works when the source header is updated to the new Duktape release.
  • If you're editing a config header manually, you should look at the diff between the previous and new default config header to see what defines have changed, and then implement matching changes in your updated header.

Adding a new compiler, platform, or architecture

Adding a new platform "Acme OS"

  • Add a new detection snippet config/helper-snippets/DUK_F_ACMEOS.h.in.
  • Create a new config/platforms/platform_acmeos.h.in. Platform files should have the necessary #include statements, select the Date provider, and can override various broken platform calls. For example, if realloc() doesn't handle NULL and/or zero size correctly, you can override that. Compare to existing platform files for reference.
  • Add the platform to config/platforms.yaml, reference DUK_F_ACMEOS for detection.

That should be enough for an autogenerated duk_config.h to support Acme OS detection.

Adding a compiler or an architecture

The process is similar for compilers and architectures; see existing files for reference.

Notes

Byte order

Byte order is a awkward to detect automatically:

  • Sometimes byte order is best determined based on architecture, especially for architectures with a fixed byte order. Some architectures can support multiple endianness modes, however, and it depends on the platform which one is used.
  • Sometimes byte order is best determined from compiler defines; for example GCC and Clang provide built-in defines which mostly provide the necessary endianness information without the need to use system headers.
  • Sometimes byte order is best determined from platform #include headers. There's a lot of variability in what defines are available, and where the related headers are located.

To allow endianness to be determined in each phase, platform, architecture, and compiler files should only define endianness when not already defined:

#if !defined(DUK_USE_BYTE_ORDER)
#define DUK_USE_BYTE_ORDER 1
#endif

Alignment

Alignment is similar to byte order for detection: it can be sometimes detected from architecture, sometimes from platform, etc. There are architectures where alignment requirements are configurable, e.g. on X86 it's up to the operating system to decide if AC (Alignment Check) is enabled for application code.

As a result, platform, architecture, and compiler files should avoid redefinition:

#if !defined(DUK_USE_ALIGN_BY)
#define DUK_USE_ALIGN_BY 4
#endif

Defines provided by duk_config.h

The role of duk_config.h is to provide all typedefs, macros, structures, system headers, etc, which are platform dependent. Duktape internals can then just assume these are in place and will remain clean of any detection.

These typedefs, macros, etc, include:

  • Including platform specific headers (#include <...>) needed by any of the config header macros, including:
    • Standard library functions like sprintf() and memset()
    • Math functions like acos()
    • Any other functions called by macros defined in duk_config.h, e.g. the functions needed by a custom Date provider
  • Typedefs for integer and floating point types (duk_uint8_t, etc), and their limit defines.
  • Some IEEE double constants, including NaN and Infinity, because some constants cannot be reliably expressed as constants in all compilers.
  • Wrapper macros for platform functions, covering string operations, file I/O, math, etc. For example: DUK_FOPEN(), DUK_SPRINTF(), DUK_ACOS()), etc. Typically these are just mapped 1:1 to platform functions, but sometimes tweaks are needed.
  • Various compiler specific macros: unreachable code, unreferenced variable, symbol visibility attributes, inlining control, etc.
  • Duktape config options, DUK_USE_xxx, including a possible custom Date provider.

The required defines and typedefs are also available in a machine parseable metadata form:

  • config/other-defines/c_types.yaml: required integer and other types and their limits.
  • config/other-defines/platform_functions.yaml: required platform function wrappers.
  • config/other-defines/other_defines.yaml: compiler specific macros and other misc defines.
  • config/config-options/DUK_USE_*.yaml: Duktape config options.

Motivation for duk_config.h

Duktape 1.2 feature option benefits

  • Works out of the box for many targets
    • With default options and a supported platform just compile and run
    • Preprocessor-based detection works well with cross compilation compared to e.g. autoconf or similar
  • Feature options only needed to deviate from defaults
    • No need to read through all feature options to start using
    • Learn relevant feature options when they become relevant

Duktape 1.2 feature option problems

  • Monolithic detection
    • One large file which becomes more and more difficult to maintain
    • Doesn't serve mainline platforms well: clutter from exotic platforms
    • Doesn't server exotic platforms well: doesn't support nearly all exotic platforms, and difficult to maintain a patched version
  • Difficult to support proprietary or broken platforms
    • Cannot easily merge support to mainline
    • Even if could, some hacks needed by broken platforms may be extreme
  • Preprocessor detection is not always possible
    • Some platform features may not be detectable through preprocessor defines
    • Some detection code may go wrong because a platform provides incorrect defines
  • Awkward requirement to provide same feature options (DUK_OPT_xxx) for both Duktape and application compilation
    • Easy to forget when compiling an application
    • Error prone to maintain option lists for Duktape and application when they're compiled separately
    • Difficult to install as a system library unless using default feature options: how would custom options be passed to applications?

Nice-to-have features

Should use a configuration header (duk_config.h):

  • Common model for many libraries, works well with distributions

Should provide a default configuration header which works out of the box:

  • Similar to Duktape 1.2: automatic detection of at least mainline platforms
  • Platform support for automatic detection can be narrowed from Duktape 1.2

Default configuration header should be 1.2 backwards compatible (initially):

  • In other words, current DUK_OPT_xxx feature options should be supported
  • Allows easier transition and avoids the need to bump the major version

Should document DUK_USE_xxx options and other defines expected from duk_config.h so that a configuration header can be built manually:

  • Human readable documentation and/or programmatic metadata
  • If programmatic metadata, automatic generation of option documentation

Should provide a configuration utility for generating template headers:

  • Such a template can then more easily be edited manually
  • Allow forced deviations from defaults directly in the utility so that a generated header is already customized