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NAME | SYNOPSIS | DESCRIPTION | COMMANDS | SUPPORTED FLAGS | FEATURE AREAS | ENVIRONMENT | FILES | EXAMPLES | COLOPHON |
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dpkg-buildflags(1) dpkg suite dpkg-buildflags(1)
dpkg-buildflags - returns build flags to use during package build
dpkg-buildflags [option...] [command]
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 /usr/local/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.
--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.
ASFLAGS
Options for the assembler. Default value: empty. Since dpkg
1.21.0.
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.
DFLAGS
Options for the D compiler (ldc or gdc). Since dpkg 1.20.6.
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).
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.
optimize
Several compile-time options (detailed below) can be used to help
optimize a resulting binary (since dpkg 1.21.0). Note: enabling
all these options can result in unreproducible binary artifacts.
lto This setting (since dpkg 1.21.0; disabled by default) enables
Link Time Optimization by adding -flto=auto -ffat-lto-objects
to CFLAGS, CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS,
FFLAGS, FCFLAGS and LDFLAGS.
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 global default since dpkg 1.18.23, as
it is enabled by default now by gcc on the amd64, arm64,
armel, armhf, hurd-i386, i386, kfreebsd-amd64, kfreebsd-i386,
mips, mipsel, mips64el, powerpc, ppc64, ppc64el, riscv64,
s390x, sparc and sparc64 Debian architectures) adds the
required options to enable or disable PIE via gcc specs
files, if needed, depending on whether gcc injects on that
architecture the flags by itself or not. When the setting is
enabled and gcc injects the flags, it adds nothing. When the
setting is enabled and gcc does not inject the flags, it adds
-fPIE (via /usr/local/share/dpkg/pie-compiler.specs) to
CFLAGS, CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS
and FCFLAGS, and -fPIE -pie (via
/usr/local/share/dpkg/pie-link.specs) to LDFLAGS. When the
setting is disabled and gcc injects the flags, it adds
-fno-PIE (via /usr/local/share/dpkg/no-pie-compile.specs) to
CFLAGS, CXXFLAGS, OBJCFLAGS, OBJCXXFLAGS, GCJFLAGS, FFLAGS
and FCFLAGS, and -fno-PIE -no-pie (via
/usr/local/share/dpkg/no-pie-link.specs) 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.
fixfilepath
This setting (enabled by default) adds
-ffile-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 file.
If both fixdebugpath and fixfilepath are set, this option
takes precedence, because it is a superset of the former.
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.
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
/usr/local/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.
DPKG_COLORS
Sets the color mode (since dpkg 1.18.5). The currently
accepted values are: auto (default), always and never.
DPKG_NLS
If set, it will be used to decide whether to activate Native
Language Support, also known as internationalization (or
i18n) support (since dpkg 1.19.0). The accepted values are:
0 and 1 (default).
Configuration files
/usr/local/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/local/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).
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/local/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/local/share/dpkg/buildflags.mk
export CPPFLAGS CFLAGS LDFLAGS
And you can of course pass the flags to commands manually:
include /usr/local/share/dpkg/buildflags.mk
build-arch:
$(CC) -o hello hello.c $(CPPFLAGS) $(CFLAGS) $(LDFLAGS)
This page is part of the dpkg (Debian Package Manager) project.
Information about the project can be found at
⟨https://wiki.debian.org/Teams/Dpkg/⟩. If you have a bug report
for this manual page, see
⟨http://bugs.debian.org/cgi-bin/pkgreport.cgi?src=dpkg⟩. This
page was obtained from the project's upstream Git repository
⟨https://salsa.debian.org/dpkg-team/dpkg.git⟩ on 2022-12-17. (At
that time, the date of the most recent commit that was found in
the repository was 2022-12-14.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
1.21.1-83-gad5dc 2021-12-07 dpkg-buildflags(1)
Pages that refer to this page: dpkg-buildpackage(1), deb-src-rules(5), debhelper(7)
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