cpp(1)
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CPP(1) GNU CPP(1)
NAME
cpp - The C Preprocessor
SYNOPSIS
cpp [-Dmacro[=defn]...] [-Umacro]
[-Idir...] [-iquotedir...]
[-M|-MM] [-MG] [-MF filename]
[-MP] [-MQ target...]
[-MT target...]
infile [[-o] outfile]
Only the most useful options are given above; see below for
a more complete list of preprocessor-specific options. In
addition, cpp accepts most gcc driver options, which are not
listed here. Refer to the GCC documentation for details.
DESCRIPTION
The C preprocessor, often known as cpp, is a macro processor
that is used automatically by the C compiler to transform
your program before compilation. It is called a macro
processor because it allows you to define macros, which are
brief abbreviations for longer constructs.
The C preprocessor is intended to be used only with C, C++,
and Objective-C source code. In the past, it has been
abused as a general text processor. It will choke on input
which does not obey C's lexical rules. For example,
apostrophes will be interpreted as the beginning of
character constants, and cause errors. Also, you cannot
rely on it preserving characteristics of the input which are
not significant to C-family languages. If a Makefile is
preprocessed, all the hard tabs will be removed, and the
Makefile will not work.
Having said that, you can often get away with using cpp on
things which are not C. Other Algol-ish programming
languages are often safe (Pascal, Ada, etc.) So is assembly,
with caution. -traditional-cpp mode preserves more white
space, and is otherwise more permissive. Many of the
problems can be avoided by writing C or C++ style comments
instead of native language comments, and keeping macros
simple.
Wherever possible, you should use a preprocessor geared to
the language you are writing in. Modern versions of the GNU
assembler have macro facilities. Most high level
programming languages have their own conditional compilation
and inclusion mechanism. If all else fails, try a true
general text processor, such as GNU M4.
C preprocessors vary in some details. This manual discusses
the GNU C preprocessor, which provides a small superset of
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the features of ISO Standard C. In its default mode, the
GNU C preprocessor does not do a few things required by the
standard. These are features which are rarely, if ever,
used, and may cause surprising changes to the meaning of a
program which does not expect them. To get strict ISO
Standard C, you should use the -std=c90, -std=c99 or
-std=c11 options, depending on which version of the standard
you want. To get all the mandatory diagnostics, you must
also use -pedantic.
This manual describes the behavior of the ISO preprocessor.
To minimize gratuitous differences, where the ISO
preprocessor's behavior does not conflict with traditional
semantics, the traditional preprocessor should behave the
same way. The various differences that do exist are
detailed in the section Traditional Mode.
For clarity, unless noted otherwise, references to CPP in
this manual refer to GNU CPP.
OPTIONS
The cpp command expects two file names as arguments, infile
and outfile. The preprocessor reads infile together with
any other files it specifies with #include. All the output
generated by the combined input files is written in outfile.
Either infile or outfile may be -, which as infile means to
read from standard input and as outfile means to write to
standard output. If either file is omitted, it means the
same as if - had been specified for that file. You can also
use the -o outfile option to specify the output file.
Unless otherwise noted, or the option ends in =, all options
which take an argument may have that argument appear either
immediately after the option, or with a space between option
and argument: -Ifoo and -I foo have the same effect.
Many options have multi-letter names; therefore multiple
single-letter options may not be grouped: -dM is very
different from -d -M.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed
as if they appeared during translation phase three in a
#define directive. In particular, the definition is
truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or
shell-like program you may need to use the shell's
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quoting syntax to protect characters such as spaces that
have a meaning in the shell syntax.
If you wish to define a function-like macro on the
command line, write its argument list with surrounding
parentheses before the equals sign (if any).
Parentheses are meaningful to most shells, so you should
quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they are
given on the command line. All -imacros file and
-include file options are processed after all -D and -U
options.
-U name
Cancel any previous definition of name, either built in
or provided with a -D option.
-include file
Process file as if "#include "file"" appeared as the
first line of the primary source file. However, the
first directory searched for file is the preprocessor's
working directory instead of the directory containing
the main source file. If not found there, it is
searched for in the remainder of the "#include "...""
search chain as normal.
If multiple -include options are given, the files are
included in the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced
by scanning file is thrown away. Macros it defines
remain defined. This allows you to acquire all the
macros from a header without also processing its
declarations.
All files specified by -imacros are processed before all
files specified by -include.
-undef
Do not predefine any system-specific or GCC-specific
macros. The standard predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX
threads library. You should use this option
consistently for both compilation and linking. This
option is supported on GNU/Linux targets, most other
Unix derivatives, and also on x86 Cygwin and MinGW
targets.
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-M Instead of outputting the result of preprocessing,
output a rule suitable for make describing the
dependencies of the main source file. The preprocessor
outputs one make rule containing the object file name
for that source file, a colon, and the names of all the
included files, including those coming from -include or
-imacros command-line options.
Unless specified explicitly (with -MT or -MQ), the
object file name consists of the name of the source file
with any suffix replaced with object file suffix and
with any leading directory parts removed. If there are
many included files then the rule is split into several
lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug
output, such as -dM. To avoid mixing such debug output
with the dependency rules you should explicitly specify
the dependency output file with -MF, or use an
environment variable like DEPENDENCIES_OUTPUT. Debug
output is still sent to the regular output stream as
normal.
Passing -M to the driver implies -E, and suppresses
warnings with an implicit -w.
-MM Like -M but do not mention header files that are found
in system header directories, nor header files that are
included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double
quotes in an #include directive does not in itself
determine whether that header appears in -MM dependency
output.
-MF file
When used with -M or -MM, specifies a file to write the
dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would
send preprocessed output.
When used with the driver options -MD or -MMD, -MF
overrides the default dependency output file.
-MG In conjunction with an option such as -M requesting
dependency generation, -MG assumes missing header files
are generated files and adds them to the dependency list
without raising an error. The dependency filename is
taken directly from the "#include" directive without
prepending any path. -MG also suppresses preprocessed
output, as a missing header file renders this useless.
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This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each
dependency other than the main file, causing each to
depend on nothing. These dummy rules work around errors
make gives if you remove header files without updating
the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency
generation. By default CPP takes the name of the main
input file, deletes any directory components and any
file suffix such as .c, and appends the platform's usual
object suffix. The result is the target.
An -MT option sets the target to be exactly the string
you specify. If you want multiple targets, you can
specify them as a single argument to -MT, or use
multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are
special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it
were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not
implied. The driver determines file based on whether an
-o option is given. If it is, the driver uses its
argument but with a suffix of .d, otherwise it takes the
name of the input file, removes any directory components
and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is
understood to specify the dependency output file, but if
used without -E, each -o is understood to specify a
target object file.
Since -E is not implied, -MD can be used to generate a
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dependency output file as a side-effect of the
compilation process.
-MMD
Like -MD except mention only user header files, not
system header files.
-fpreprocessed
Indicate to the preprocessor that the input file has
already been preprocessed. This suppresses things like
macro expansion, trigraph conversion, escaped newline
splicing, and processing of most directives. The
preprocessor still recognizes and removes comments, so
that you can pass a file preprocessed with -C to the
compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the
front ends.
-fpreprocessed is implicit if the input file has one of
the extensions .i, .ii or .mi. These are the extensions
that GCC uses for preprocessed files created by
-save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand
macros.
The option's behavior depends on the -E and
-fpreprocessed options.
With -E, preprocessing is limited to the handling of
directives such as "#define", "#ifdef", and "#error".
Other preprocessor operations, such as macro expansion
and trigraph conversion are not performed. In addition,
the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and
most builtin macros is disabled. Macros such as
"__LINE__", which are contextually dependent, are
handled normally. This enables compilation of files
previously preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for
-fpreprocessed take precedence. This enables full
preprocessing of files previously preprocessed with "-E
-fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names in identifiers. This
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option is enabled by default for C99 (and later C
standard versions) and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths
with canonicalization.
-ftabstop=width
Set the distance between tab stops. This helps the
preprocessor report correct column numbers in warnings
or errors, even if tabs appear on the line. If the
value is less than 1 or greater than 100, the option is
ignored. The default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This
allows the compiler to emit diagnostic about the current
macro expansion stack when a compilation error occurs in
a macro expansion. Using this option makes the
preprocessor and the compiler consume more memory. The
level parameter can be used to choose the level of
precision of token location tracking thus decreasing the
memory consumption if necessary. Value 0 of level de-
activates this option. Value 1 tracks tokens locations
in a degraded mode for the sake of minimal memory
overhead. In this mode all tokens resulting from the
expansion of an argument of a function-like macro have
the same location. Value 2 tracks tokens locations
completely. This value is the most memory hungry. When
this option is given no argument, the default parameter
value is 2.
Note that "-ftrack-macro-expansion=2" is activated by
default.
-fexec-charset=charset
Set the execution character set, used for string and
character constants. The default is UTF-8. charset can
be any encoding supported by the system's "iconv"
library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide
string and character constants. The default is UTF-32
or UTF-16, whichever corresponds to the width of
"wchar_t". As with -fexec-charset, charset can be any
encoding supported by the system's "iconv" library
routine; however, you will have problems with encodings
that do not fit exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from
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the character set of the input file to the source
character set used by GCC. If the locale does not
specify, or GCC cannot get this information from the
locale, the default is UTF-8. This can be overridden by
either the locale or this command-line option.
Currently the command-line option takes precedence if
there's a conflict. charset can be any encoding
supported by the system's "iconv" library routine.
-fworking-directory
Enable generation of linemarkers in the preprocessor
output that let the compiler know the current working
directory at the time of preprocessing. When this
option is enabled, the preprocessor emits, after the
initial linemarker, a second linemarker with the current
working directory followed by two slashes. GCC uses
this directory, when it's present in the preprocessed
input, as the directory emitted as the current working
directory in some debugging information formats. This
option is implicitly enabled if debugging information is
enabled, but this can be inhibited with the negated form
-fno-working-directory. If the -P flag is present in
the command line, this option has no effect, since no
"#line" directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and
answer answer. This form is preferred to the older form
-A predicate(answer), which is still supported, because
it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and
answer answer.
-C Do not discard comments. All comments are passed
through to the output file, except for comments in
processed directives, which are deleted along with the
directive.
You should be prepared for side effects when using -C;
it causes the preprocessor to treat comments as tokens
in their own right. For example, comments appearing at
the start of what would be a directive line have the
effect of turning that line into an ordinary source
line, since the first token on the line is no longer a
#.
-CC Do not discard comments, including during macro
expansion. This is like -C, except that comments
contained within macros are also passed through to the
output file where the macro is expanded.
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In addition to the side-effects of the -C option, the
-CC option causes all C++-style comments inside a macro
to be converted to C-style comments. This is to prevent
later use of that macro from inadvertently commenting
out the remainder of the source line.
The -CC option is generally used to support lint
comments.
-P Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the
preprocessor on something that is not C code, and will
be sent to a program which might be confused by the
linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C
preprocessors, as opposed to ISO C preprocessors.
Note that GCC does not otherwise attempt to emulate a
pre-standard C compiler, and these options are only
supported with the -E switch, or when invoking CPP
explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character
sequences, all starting with ??, that are defined by ISO
C to stand for single characters. For example, ??/
stands for \, so '??/n' is a character constant for a
newline.
By default, GCC ignores trigraphs, but in standard-
conforming modes it converts them. See the -std and
-ansi options.
-remap
Enable special code to work around file systems which
only permit very short file names, such as MS-DOS.
-H Print the name of each header file used, in addition to
other normal activities. Each name is indented to show
how deep in the #include stack it is. Precompiled
header files are also printed, even if they are found to
be invalid; an invalid precompiled header file is
printed with ...x and a valid one with ...! .
-dletters
Says to make debugging dumps during compilation as
specified by letters. The flags documented here are
those relevant to the preprocessor. Other letters are
interpreted by the compiler proper, or reserved for
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future versions of GCC, and so are silently ignored. If
you specify letters whose behavior conflicts, the result
is undefined.
-dM Instead of the normal output, generate a list of
#define directives for all the macros defined during
the execution of the preprocessor, including
predefined macros. This gives you a way of finding
out what is predefined in your version of the
preprocessor. Assuming you have no file foo.h, the
command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
-dD Like -dM except in two respects: it does not include
the predefined macros, and it outputs both the
#define directives and the result of preprocessing.
Both kinds of output go to the standard output file.
-dN Like -dD, but emit only the macro names, not their
expansions.
-dI Output #include directives in addition to the result
of preprocessing.
-dU Like -dD except that only macros that are expanded,
or whose definedness is tested in preprocessor
directives, are output; the output is delayed until
the use or test of the macro; and #undef directives
are also output for macros tested but undefined at
the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used
from CPP or with -E, it dumps debugging information
about location maps. Every token in the output is
preceded by the dump of the map its location belongs to.
When used from GCC without -E, this option has no
effect.
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be
searched for header files during preprocessing.
If dir begins with =, then the = is replaced by the
sysroot prefix; see --sysroot and -isysroot.
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Directories specified with -iquote apply only to the
quote form of the directive, "#include "file"".
Directories specified with -I, -isystem, or -idirafter
apply to lookup for both the "#include "file"" and
"#include <file>" directives.
You can specify any number or combination of these
options on the command line to search for header files
in several directories. The lookup order is as follows:
1. For the quote form of the include directive, the
directory of the current file is searched first.
2. For the quote form of the include directive, the
directories specified by -iquote options are
searched in left-to-right order, as they appear on
the command line.
3. Directories specified with -I options are scanned in
left-to-right order.
4. Directories specified with -isystem options are
scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are
scanned in left-to-right order.
You can use -I to override a system header file,
substituting your own version, since these directories
are searched before the standard system header file
directories. However, you should not use this option to
add directories that contain vendor-supplied system
header files; use -isystem for that.
The -isystem and -idirafter options also mark the
directory as a system directory, so that it gets the
same special treatment that is applied to the standard
system directories.
If a standard system include directory, or a directory
specified with -isystem, is also specified with -I, the
-I option is ignored. The directory is still searched
but as a system directory at its normal position in the
system include chain. This is to ensure that GCC's
procedure to fix buggy system headers and the ordering
for the "#include_next" directive are not inadvertently
changed. If you really need to change the search order
for system directories, use the -nostdinc and/or
-isystem options.
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-I- Split the include path. This option has been
deprecated. Please use -iquote instead for -I
directories before the -I- and remove the -I- option.
Any directories specified with -I options before -I- are
searched only for headers requested with
"#include "file""; they are not searched for
"#include <file>". If additional directories are
specified with -I options after the -I-, those
directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of
the current file directory as the first search directory
for "#include "file"". There is no way to override this
effect of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix
options. If the prefix represents a directory, you
should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with
-iprefix, and add the resulting directory to the include
search path. -iwithprefixbefore puts it in the same
place -I would; -iwithprefix puts it where -idirafter
would.
-isysroot dir
This option is like the --sysroot option, but applies
only to header files (except for Darwin targets, where
it applies to both header files and libraries). See the
--sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing
target-specific C++ headers.
-nostdinc
Do not search the standard system directories for header
files. Only the directories explicitly specified with
-I, -iquote, -isystem, and/or -idirafter options (and
the directory of the current file, if appropriate) are
searched.
-nostdinc++
Do not search for header files in the C++-specific
standard directories, but do still search the other
standard directories. (This option is used when
building the C++ library.)
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-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a
/* comment, or whenever a backslash-newline appears in a
// comment. This warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change
the meaning of the program. Trigraphs within comments
are not warned about, except those that would form
escaped newlines.
This option is implied by -Wall. If -Wall is not given,
this option is still enabled unless trigraphs are
enabled. To get trigraph conversion without warnings,
but get the other -Wall warnings, use -trigraphs -Wall
-Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if"
directive. Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of
a macro (including the case where the macro is expanded
by an #if directive). Such usage is not portable. This
warning is also enabled by -Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are
unused. A macro is used if it is expanded or tested for
existence at least once. The preprocessor also warns if
the macro has not been used at the time it is redefined
or undefined.
Built-in macros, macros defined on the command line, and
macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in
skipped conditional blocks, then the preprocessor
reports it as unused. To avoid the warning in such a
case, you might improve the scope of the macro's
definition by, for example, moving it into the first
skipped block. Alternatively, you could provide a dummy
use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are
followed by text. This sometimes happens in older
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programs with code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This
warning is on by default.
ENVIRONMENT
This section describes the environment variables that affect
how CPP operates. You can use them to specify directories
or prefixes to use when searching for include files, or to
control dependency output.
Note that you can also specify places to search using
options such as -I, and control dependency output with
options like -M. These take precedence over environment
variables, which in turn take precedence over the
configuration of GCC.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated
by a special character, much like PATH, in which to look
for header files. The special character,
"PATH_SEPARATOR", is target-dependent and determined at
GCC build time. For Microsoft Windows-based targets it
is a semicolon, and for almost all other targets it is a
colon.
CPATH specifies a list of directories to be searched as
if specified with -I, but after any paths given with -I
options on the command line. This environment variable
is used regardless of which language is being
preprocessed.
The remaining environment variables apply only when
preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if
specified with -isystem, but after any paths given with
-isystem options on the command line.
In all these variables, an empty element instructs the
compiler to search its current working directory. Empty
elements can appear at the beginning or end of a path.
For instance, if the value of CPATH is
":/special/include", that has the same effect as
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-I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to
output dependencies for Make based on the non-system
header files processed by the compiler. System header
files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file
name, in which case the Make rules are written to that
file, guessing the target name from the source file
name. Or the value can have the form file target, in
which case the rules are written to file file using
target as the target name.
In other words, this environment variable is equivalent
to combining the options -MM and -MF, with an optional
-MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see
above), except that system header files are not ignored,
so it implies -M rather than -MM. However, the
dependence on the main input file is omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX
timestamp to be used in replacement of the current date
and time in the "__DATE__" and "__TIME__" macros, so
that the embedded timestamps become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp,
defined as the number of seconds (excluding leap
seconds) since 01 Jan 1970 00:00:00 represented in
ASCII; identical to the output of @command{date +%s} on
GNU/Linux and other systems that support the %s
extension in the "date" command.
The value should be a known timestamp such as the last
modification time of the source or package and it should
be set by the build process.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), gcc(1), and the Info
entries for cpp and gcc.
COPYRIGHT
Copyright (c) 1987-2017 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation
License, Version 1.3 or any later version published by the
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Free Software Foundation. A copy of the license is included
in the man page gfdl(7). This manual contains no Invariant
Sections. The Front-Cover Texts are (a) (see below), and
the Back-Cover Texts are (b) (see below).
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
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