|The C Preprocessor||www.imodulo.com · 2003-11-10|
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Most often when you use the C preprocessor you will not have to invoke it explicitly: the C compiler will do so automatically. However, the preprocessor is sometimes useful on its own. All the options listed here are also acceptable to the C compiler and have the same meaning, except that the C compiler has different rules for specifying the output file.
Note: Whether you use the preprocessor by way of
cpp, the compiler driver is run first. This program's purpose is to translate your command into invocations of the programs that do the actual work. Their command line interfaces are similar but not identical to the documented interface, and may change without notice.
The C preprocessor 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. Also, if either file is omitted, it means the same as if
- had been specified for that 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:
-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
Predefine name as a macro, with definition
Predefine name as a macro, with definition definition. There are no restrictions on the contents of definition, but if you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's 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 will need to quote the option. With
-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
Cancel any previous definition of name, either built in or provided with a
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined. Standard Predefined Macros.
Add the directory dir to the list of directories to be searched for header files. Search Path. Directories named by
-I are searched before the standard system include directories. If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated (System Headers) .
Write output to file. This is the same as specifying file as the second non-option argument to
gcc has a different interpretation of a second non-option argument, so you must use
-o to specify the output file.
Turns on all optional warnings which are desirable for normal code. At present this is
-Wtrigraphs. Note that many of the preprocessor's warnings are on by default and have no options to control them.
Warn whenever a comment-start sequence
/* appears in a
/* comment, or whenever a backslash-newline appears in a
// comment. (Both forms have the same effect.)
Warn if any trigraphs are encountered. This option used to take effect only if
-trigraphs was also specified, but now works independently. Warnings are not given for trigraphs within comments, as they do not affect the meaning of the program.
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and problematic constructs which should be avoided. Traditional Mode.
Warn the first time
#import is used.
Warn whenever an identifier which is not a macro is encountered in an
#if directive, outside of
defined. Such identifiers are replaced with zero.
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 will also warn 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 CPP will report 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
Warn whenever an
#else or an
#endif are followed by text. This usually happens in code of the form
#if FOO ... #else FOO ... #endif FOO
The second and third
FOO should be in comments, but often are not in older programs. This warning is on by default.
Make all warnings into hard errors. Source code which triggers warnings will be rejected.
Issue warnings for code in system headers. These are normally unhelpful in finding bugs in your own code, therefore suppressed. If you are responsible for the system library, you may want to see them.
Suppress all warnings, including those which GNU CPP issues by default.
Issue all the mandatory diagnostics listed in the C standard. Some of them are left out by default, since they trigger frequently on harmless code.
Issue all the mandatory diagnostics, and make all mandatory diagnostics into errors. This includes mandatory diagnostics that GCC issues without
-pedantic but treats as warnings.
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
-imacros command line options.
Unless specified explicitly (with
-MQ), the object file name consists of the basename of the source file with any suffix replaced with object file suffix. 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 (Environment Variables). Debug output will still be sent to the regular output stream as normal.
-M to the driver implies
-E, and suppresses warnings with an implicit
-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 will appear in
-MM dependency output. This is a slight change in semantics from GCC versions 3.0 and earlier.
When used with
-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 have sent preprocessed output.
When used with the driver options
-MF overrides the default dependency output file.
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.
This feature is used in automatic updating of makefiles.
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:
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, including any path, deletes any file suffix such as
.c, and appends the platform's usual object suffix. The result is the target.
-MT option will set 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 '$(objpfx)foo.o' might give
-MT, but it quotes any characters which are special to Make.
-MQ '$(objpfx)foo.o' gives
The default target is automatically quoted, as if it were given with
-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 take the basename of the input file and applies a
-MD is used in conjunction with
-o switch is understood to specify the dependency output file (but -MF), but if used without
-o is understood to specify a target object file.
-E is not implied,
-MD can be used to generate a dependency output file as a side-effect of the compilation process.
-MD except mention only user header files, not system -header files.
Specify the source language: C, C++, Objective-C, or assembly. This has nothing to do with standards conformance or extensions; it merely selects which base syntax to expect. If you give none of these options, cpp will deduce the language from the extension of the source file:
.S. Some other common extensions for C++ and assembly are also recognized. If cpp does not recognize the extension, it will treat the file as C; this is the most generic mode.
Note: Previous versions of cpp accepted a
-lang option which selected both the language and the standards conformance level. This option has been removed, because it conflicts with the
Specify the standard to which the code should conform. Currently CPP knows about C and C++ standards; others may be added in the future.
standard may be one of:
The ISO C standard from 1990.
c89 is the customary shorthand for this version of the standard.
-ansi option is equivalent to
The 1990 C standard, as amended in 1994.
The revised ISO C standard, published in December 1999. Before publication, this was known as C9X.
The 1990 C standard plus GNU extensions. This is the default.
The 1999 C standard plus GNU extensions.
The 1998 ISO C++ standard plus amendments.
The same as
-std=c++98 plus GNU extensions. This is the default for C++ code.
Split the include path. 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
-I- inhibits the use of the directory of the current file directory as the first search directory for
#include "file". Search Path.
Do not search the standard system directories for header files. Only the directories you have specified with
-I options (and the directory of the current file, if appropriate) are searched.
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.)
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.
-include options are given, the files are included in the order they appear on the command line.
-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
Search dir for header files, but do it after all directories specified with
-I and the standard system directories have been exhausted. dir is treated as a system include directory.
Specify prefix as the prefix for subsequent
-iwithprefix options. If the prefix represents a directory, you should include the final
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
-iwithprefix puts it where
Use of these options is discouraged.
Search dir for header files, after all directories specified by
-I but before the standard system directories. Mark it as a system directory, so that it gets the same special treatment as is applied to the standard system directories. System Headers.
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
.mi. These are the extensions that GCC uses for preprocessed files created by
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.
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as
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. Assertions.
Cancel an assertion with the predicate predicate and answer answer.
CHARS is a sequence of one or more of the following characters, and must not be preceded by a space. Other characters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify characters whose behavior conflicts, the result is undefined.
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
will show all the predefined macros.
M 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.
D, but emit only the macro names, not their expansions.
#include directives in addition to the result of preprocessing.
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. Preprocessor Output.
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
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.
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.
-CC option is generally used to support lint comments.
Try to imitate the behavior of old-fashioned C preprocessors, as opposed to ISO C preprocessors. Traditional Mode.
Process trigraph sequences. Initial processing.
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
Print text describing all the command line options instead of preprocessing anything.
Verbose mode. Print out GNU CPP's version number at the beginning of execution, and report the final form of the include path.
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.
Print out GNU CPP's version number. With one dash, proceed to preprocess as normal. With two dashes, exit immediately.
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