Provided by: dpkg-dev_1.19.0.5ubuntu2.4_all 
NAME
dpkg-buildflags - returns build flags to use during package build
SYNOPSIS
dpkg-buildflags [option...] [command]
DESCRIPTION
dpkg-buildflags is a tool to retrieve compilation flags to use during build of Debian
packages. The default flags are defined by the vendor but they can be extended/overridden
in several ways:
1. system-wide with /etc/dpkg/buildflags.conf;
2. for the current user with $XDG_CONFIG_HOME/dpkg/buildflags.conf where
$XDG_CONFIG_HOME defaults to $HOME/.config;
3. temporarily by the user with environment variables (see section ENVIRONMENT);
4. dynamically by the package maintainer with environment variables set via
debian/rules (see section ENVIRONMENT).
The configuration files can contain four types of directives:
SET flag value
Override the flag named flag to have the value value.
STRIP flag value
Strip from the flag named flag all the build flags listed in value.
APPEND flag value
Extend the flag named flag by appending the options given in value. A space is
prepended to the appended value if the flag's current value is non-empty.
PREPEND flag value
Extend the flag named flag by prepending the options given in value. A space is
appended to the prepended value if the flag's current value is non-empty.
The configuration files can contain comments on lines starting with a hash (#). Empty
lines are also ignored.
COMMANDS
--dump Print to standard output all compilation flags and their values. It prints one flag
per line separated from its value by an equal sign (“flag=value”). This is the
default action.
--list Print the list of flags supported by the current vendor (one per line). See the
SUPPORTED FLAGS section for more information about them.
--status
Display any information that can be useful to explain the behaviour of
dpkg-buildflags (since dpkg 1.16.5): relevant environment variables, current
vendor, state of all feature flags. Also print the resulting compiler flags with
their origin.
This is intended to be run from debian/rules, so that the build log keeps a clear
trace of the build flags used. This can be useful to diagnose problems related to
them.
--export=format
Print to standard output commands that can be used to export all the compilation
flags for some particular tool. If the format value is not given, sh is assumed.
Only compilation flags starting with an upper case character are included, others
are assumed to not be suitable for the environment. Supported formats:
sh Shell commands to set and export all the compilation flags in the
environment. The flag values are quoted so the output is ready for
evaluation by a shell.
cmdline
Arguments to pass to a build program's command line to use all the
compilation flags (since dpkg 1.17.0). The flag values are quoted in shell
syntax.
configure
This is a legacy alias for cmdline.
make Make directives to set and export all the compilation flags in the
environment. Output can be written to a makefile fragment and evaluated
using an include directive.
--get flag
Print the value of the flag on standard output. Exits with 0 if the flag is known
otherwise exits with 1.
--origin flag
Print the origin of the value that is returned by --get. Exits with 0 if the flag
is known otherwise exits with 1. The origin can be one of the following values:
vendor the original flag set by the vendor is returned;
system the flag is set/modified by a system-wide configuration;
user the flag is set/modified by a user-specific configuration;
env the flag is set/modified by an environment-specific configuration.
--query
Print any information that can be useful to explain the behaviour of the program:
current vendor, relevant environment variables, feature areas, state of all feature
flags, and the compiler flags with their origin (since dpkg 1.19.0).
For example:
Vendor: Debian
Environment:
DEB_CFLAGS_SET=-O0 -Wall
Area: qa
Features:
bug=no
canary=no
Area: reproducible
Features:
timeless=no
Flag: CFLAGS
Value: -O0 -Wall
Origin: env
Flag: CPPFLAGS
Value: -D_FORTIFY_SOURCE=2
Origin: vendor
--query-features area
Print the features enabled for a given area (since dpkg 1.16.2). The only
currently recognized areas on Debian and derivatives are future, qa, reproducible,
sanitize and hardening, see the FEATURE AREAS section for more details. Exits with
0 if the area is known otherwise exits with 1.
The output is in RFC822 format, with one section per feature. For example:
Feature: pie
Enabled: yes
Feature: stackprotector
Enabled: yes
--help Show the usage message and exit.
--version
Show the version and exit.
SUPPORTED FLAGS
CFLAGS Options for the C compiler. The default value set by the vendor includes -g and the
default optimization level (-O2 usually, or -O0 if the DEB_BUILD_OPTIONS
environment variable defines noopt).
CPPFLAGS
Options for the C preprocessor. Default value: empty.
CXXFLAGS
Options for the C++ compiler. Same as CFLAGS.
OBJCFLAGS
Options for the Objective C compiler. Same as CFLAGS.
OBJCXXFLAGS
Options for the Objective C++ compiler. Same as CXXFLAGS.
GCJFLAGS
Options for the GNU Java compiler (gcj). A subset of CFLAGS.
FFLAGS Options for the Fortran 77 compiler. A subset of CFLAGS.
FCFLAGS
Options for the Fortran 9x compiler. Same as FFLAGS.
LDFLAGS
Options passed to the compiler when linking executables or shared objects (if the
linker is called directly, then -Wl and , have to be stripped from these options).
Default value: empty.
New flags might be added in the future if the need arises (for example to support other
languages).
FEATURE AREAS
Each area feature can be enabled and disabled in the DEB_BUILD_OPTIONS and
DEB_BUILD_MAINT_OPTIONS environment variable's area value with the ‘+’ and ‘-’ modifier.
For example, to enable the hardening “pie” feature and disable the “fortify” feature you
can do this in debian/rules:
export DEB_BUILD_MAINT_OPTIONS=hardening=+pie,-fortify
The special feature all (valid in any area) can be used to enable or disable all area
features at the same time. Thus disabling everything in the hardening area and enabling
only “format” and “fortify” can be achieved with:
export DEB_BUILD_MAINT_OPTIONS=hardening=-all,+format,+fortify
future
Several compile-time options (detailed below) can be used to enable features that should
be enabled by default, but cannot due to backwards compatibility reasons.
lfs This setting (disabled by default) enables Large File Support on 32-bit
architectures where their ABI does not include LFS by default, by adding
-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 to CPPFLAGS.
qa
Several compile-time options (detailed below) can be used to help detect problems in the
source code or build system.
bug This setting (disabled by default) adds any warning option that reliably detects
problematic source code. The warnings are fatal. The only currently supported
flags are CFLAGS and CXXFLAGS with flags set to -Werror=array-bounds,
-Werror=clobbered, -Werror=implicit-function-declaration and
-Werror=volatile-register-var.
canary This setting (disabled by default) adds dummy canary options to the build flags, so
that the build logs can be checked for how the build flags propagate and to allow
finding any omission of normal build flag settings. The only currently supported
flags are CPPFLAGS, CFLAGS, OBJCFLAGS, CXXFLAGS and OBJCXXFLAGS with flags set to
-D__DEB_CANARY_flag_random-id__, and LDFLAGS set to -Wl,-z,deb-canary-random-id.
sanitize
Several compile-time options (detailed below) can be used to help sanitize a resulting
binary against memory corruptions, memory leaks, use after free, threading data races and
undefined behavior bugs. Note: these options should not be used for production builds as
they can reduce reliability for conformant code, reduce security or even functionality.
address
This setting (disabled by default) adds -fsanitize=address to LDFLAGS and
-fsanitize=address -fno-omit-frame-pointer to CFLAGS and CXXFLAGS.
thread This setting (disabled by default) adds -fsanitize=thread to CFLAGS, CXXFLAGS and
LDFLAGS.
leak This setting (disabled by default) adds -fsanitize=leak to LDFLAGS. It gets
automatically disabled if either the address or the thread features are enabled, as
they imply it.
undefined
This setting (disabled by default) adds -fsanitize=undefined to CFLAGS, CXXFLAGS
and LDFLAGS.
hardening
Several compile-time options (detailed below) can be used to help harden a resulting
binary against memory corruption attacks, or provide additional warning messages during
compilation. Except as noted below, these are enabled by default for architectures that
support them.
format This setting (enabled by default) adds -Wformat -Werror=format-security to CFLAGS,
CXXFLAGS, OBJCFLAGS and OBJCXXFLAGS. This will warn about improper format string
uses, and will fail when format functions are used in a way that represent possible
security problems. At present, this warns about calls to printf and scanf functions
where the format string is not a string literal and there are no format arguments,
as in printf(foo); instead of printf("%s", foo); This may be a security hole if the
format string came from untrusted input and contains ‘%n’.
fortify
This setting (enabled by default) adds -D_FORTIFY_SOURCE=2 to CPPFLAGS. During code
generation the compiler knows a great deal of information about buffer sizes (where
possible), and attempts to replace insecure unlimited length buffer function calls
with length-limited ones. This is especially useful for old, crufty code.
Additionally, format strings in writable memory that contain ‘%n’ are blocked. If
an application depends on such a format string, it will need to be worked around.
Note that for this option to have any effect, the source must also be compiled with
-O1 or higher. If the environment variable DEB_BUILD_OPTIONS contains noopt, then
fortify support will be disabled, due to new warnings being issued by glibc 2.16
and later.
stackprotector
This setting (enabled by default if stackprotectorstrong is not in use) adds
-fstack-protector --param=ssp-buffer-size=4 to CFLAGS, CXXFLAGS, OBJCFLAGS,
OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS. This adds safety checks against stack
overwrites. This renders many potential code injection attacks into aborting
situations. In the best case this turns code injection vulnerabilities into denial
of service or into non-issues (depending on the application).
This feature requires linking against glibc (or another provider of
__stack_chk_fail), so needs to be disabled when building with -nostdlib or
-ffreestanding or similar.
stackprotectorstrong
This setting (enabled by default) adds -fstack-protector-strong to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS. This is a stronger
variant of stackprotector, but without significant performance penalties.
Disabling stackprotector will also disable this setting.
This feature has the same requirements as stackprotector, and in addition also
requires gcc 4.9 and later.
relro This setting (enabled by default) adds -Wl,-z,relro to LDFLAGS. During program
load, several ELF memory sections need to be written to by the linker. This flags
the loader to turn these sections read-only before turning over control to the
program. Most notably this prevents GOT overwrite attacks. If this option is
disabled, bindnow will become disabled as well.
bindnow
This setting (disabled by default) adds -Wl,-z,now to LDFLAGS. During program load,
all dynamic symbols are resolved, allowing for the entire PLT to be marked read-
only (due to relro above). The option cannot become enabled if relro is not
enabled.
pie This setting (with no default since dpkg 1.18.23, and injected by default by gcc on
the amd64, arm64, armel, armhf, i386, kfreebsd-amd64, kfreebsd-i386, mips, mipsel,
mips64el, ppc64el, s390x, sparc and sparc64 Debian architectures) adds the required
options via gcc specs files if needed to enable or disable PIE. When enabled and
injected by gcc, adds nothing. When enabled and not injected by gcc, adds -fPIE to
CFLAGS, CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS, and -fPIE
-pie to LDFLAGS. When disabled and injected by gcc, adds -fno-PIE to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS, and -fno-PIE
-no-pie to LDFLAGS.
Position Independent Executable are needed to take advantage of Address Space
Layout Randomization, supported by some kernel versions. While ASLR can already be
enforced for data areas in the stack and heap (brk and mmap), the code areas must
be compiled as position-independent. Shared libraries already do this (-fPIC), so
they gain ASLR automatically, but binary .text regions need to be build PIE to gain
ASLR. When this happens, ROP (Return Oriented Programming) attacks are much harder
since there are no static locations to bounce off of during a memory corruption
attack.
PIE is not compatible with -fPIC, so in general care must be taken when building
shared objects. But because the PIE flags emitted get injected via gcc specs files,
it should always be safe to unconditionally set them regardless of the object type
being compiled or linked.
Static libraries can be used by programs or other shared libraries. Depending on
the flags used to compile all the objects within a static library, these libraries
will be usable by different sets of objects:
none Cannot be linked into a PIE program, nor a shared library.
-fPIE Can be linked into any program, but not a shared library (recommended).
-fPIC Can be linked into any program and shared library.
If there is a need to set these flags manually, bypassing the gcc specs injection,
there are several things to take into account. Unconditionally and explicitly
passing -fPIE, -fpie or -pie to a build-system using libtool is safe as these flags
will get stripped when building shared libraries. Otherwise on projects that build
both programs and shared libraries you might need to make sure that when building
the shared libraries -fPIC is always passed last (so that it overrides any previous
-PIE) to compilation flags such as CFLAGS, and -shared is passed last (so that it
overrides any previous -pie) to linking flags such as LDFLAGS. Note: This should
not be needed with the default gcc specs machinery.
Additionally, since PIE is implemented via a general register, some register
starved architectures (but not including i386 anymore since optimizations
implemented in gcc >= 5) can see performance losses of up to 15% in very text-
segment-heavy application workloads; most workloads see less than 1%. Architectures
with more general registers (e.g. amd64) do not see as high a worst-case penalty.
reproducible
The compile-time options detailed below can be used to help improve build reproducibility
or provide additional warning messages during compilation. Except as noted below, these
are enabled by default for architectures that support them.
timeless
This setting (enabled by default) adds -Wdate-time to CPPFLAGS. This will cause
warnings when the __TIME__, __DATE__ and __TIMESTAMP__ macros are used.
fixdebugpath
This setting (enabled by default) adds -fdebug-prefix-map=BUILDPATH=. to CFLAGS,
CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS and FCFLAGS where BUILDPATH is
set to the top-level directory of the package being built. This has the effect of
removing the build path from any generated debug symbols.
ENVIRONMENT
There are 2 sets of environment variables doing the same operations, the first one
(DEB_flag_op) should never be used within debian/rules. It's meant for any user that wants
to rebuild the source package with different build flags. The second set
(DEB_flag_MAINT_op) should only be used in debian/rules by package maintainers to change
the resulting build flags.
DEB_flag_SET
DEB_flag_MAINT_SET
This variable can be used to force the value returned for the given flag.
DEB_flag_STRIP
DEB_flag_MAINT_STRIP
This variable can be used to provide a space separated list of options that will be
stripped from the set of flags returned for the given flag.
DEB_flag_APPEND
DEB_flag_MAINT_APPEND
This variable can be used to append supplementary options to the value returned for
the given flag.
DEB_flag_PREPEND
DEB_flag_MAINT_PREPEND
This variable can be used to prepend supplementary options to the value returned
for the given flag.
DEB_BUILD_OPTIONS
DEB_BUILD_MAINT_OPTIONS
These variables can be used by a user or maintainer to disable/enable various area
features that affect build flags. The DEB_BUILD_MAINT_OPTIONS variable overrides
any setting in the DEB_BUILD_OPTIONS feature areas. See the FEATURE AREAS section
for details.
DEB_VENDOR
This setting defines the current vendor. If not set, it will discover the current
vendor by reading /etc/dpkg/origins/default.
DEB_BUILD_PATH
This variable sets the build path (since dpkg 1.18.8) to use in features such as
fixdebugpath so that they can be controlled by the caller. This variable is
currently Debian and derivatives-specific.
FILES
Configuration files
/etc/dpkg/buildflags.conf
System wide configuration file.
$XDG_CONFIG_HOME/dpkg/buildflags.conf or
$HOME/.config/dpkg/buildflags.conf
User configuration file.
Packaging support
/usr/share/dpkg/buildflags.mk
Makefile snippet that will load (and optionally export) all flags supported by
dpkg-buildflags into variables (since dpkg 1.16.1).
EXAMPLES
To pass build flags to a build command in a makefile:
$(MAKE) $(shell dpkg-buildflags --export=cmdline)
./configure $(shell dpkg-buildflags --export=cmdline)
To set build flags in a shell script or shell fragment, eval can be used to interpret the
output and to export the flags in the environment:
eval "$(dpkg-buildflags --export=sh)" && make
or to set the positional parameters to pass to a command:
eval "set -- $(dpkg-buildflags --export=cmdline)"
for dir in a b c; do (cd $dir && ./configure "$@" && make); done
Usage in debian/rules
You should call dpkg-buildflags or include buildflags.mk from the debian/rules file to
obtain the needed build flags to pass to the build system. Note that older versions of
dpkg-buildpackage (before dpkg 1.16.1) exported these flags automatically. However, you
should not rely on this, since this breaks manual invocation of debian/rules.
For packages with autoconf-like build systems, you can pass the relevant options to
configure or make(1) directly, as shown above.
For other build systems, or when you need more fine-grained control about which flags are
passed where, you can use --get. Or you can include buildflags.mk instead, which takes
care of calling dpkg-buildflags and storing the build flags in make variables.
If you want to export all buildflags into the environment (where they can be picked up by
your build system):
DPKG_EXPORT_BUILDFLAGS = 1
include /usr/share/dpkg/buildflags.mk
For some extra control over what is exported, you can manually export the variables (as
none are exported by default):
include /usr/share/dpkg/buildflags.mk
export CPPFLAGS CFLAGS LDFLAGS
And you can of course pass the flags to commands manually:
include /usr/share/dpkg/buildflags.mk
build-arch:
$(CC) -o hello hello.c $(CPPFLAGS) $(CFLAGS) $(LDFLAGS)