3.4 Options Controlling C Dialect

The following options control the dialect of C (or languages derived from C, such as C++ and Objective C) that the compiler accepts:

-ansi

In C mode, support all ISO C89 programs. In C++ mode, remove GNU extensions that conflict with ISO C++.

This turns off certain features of GCC that are incompatible with ISO C (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. It also enables the undesirable and rarely used ISO trigraph feature. For the C compiler, it disables recognition of C++ style `//' comments as well as the inline keyword.

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite `-ansi'. You would not want to use them in an ISO C program, of course, but it is useful to put them in header files that might be included in compilations done with `-ansi'. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without `-ansi'.

The `-ansi' option does not cause non-ISO programs to be rejected gratuitously. For that, `-pedantic' is required in addition to `-ansi'. See section 3.8 Options to Request or Suppress Warnings.

The macro __STRICT_ANSI__ is predefined when the `-ansi' option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

Functions which would normally be built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions with `-ansi' is used. See section Other built-in functions provided by GNU CC, for details of the functions affected.

-std=

Determine the language standard. A value for this option must be provided; possible values are

`iso9899:1990'

Same as `-ansi'

`iso9899:199409'

ISO C as modified in amend. 1

`iso9899:1999'

ISO C99. Note that this standard is not yet fully supported; see http://gcc.gnu.org/c99status.html for more information.

`c89'

same as `-std=iso9899:1990'

`c99'

same as `-std=iso9899:1999'

`gnu89'

default, iso9899:1990 + gnu extensions

`gnu99'

iso9899:1999 + gnu extensions

`iso9899:199x'

same as `-std=iso9899:1999', deprecated

`c9x'

same as `-std=iso9899:1999', deprecated

`gnu9x'

same as `-std=gnu99', deprecated

Even when this option is not specified, you can still use some of the features of newer standards in so far as they do not conflict with previous C standards. For example, you may use __restrict__ even when `-std=c99' is not specified.

The `-std' options specifying some version of ISO C have the same effects as `-ansi', except that features that were not in ISO C89 but are in the specified version (for example, `//' comments and the inline keyword in ISO C99) are not disabled.

See section Language Standards Supported by GCC, for details of these standard versions.

-aux-info filename

Output to the given filename prototyped declarations for all functions declared and/or defined in a translation unit, including those in header files. This option is silently ignored in any language other than C.

Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line), whether the declaration was implicit, prototyped or unprototyped (`I', `N' for new or `O' for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (`C' or `F', respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration.

-fno-asm

Do not recognize asm, inline or typeof as a keyword, so that code can use these words as identifiers. You can use the keywords __asm__, __inline__ and __typeof__ instead. `-ansi' implies `-fno-asm'.

In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the `-fno-gnu-keywords' flag instead, which has the same effect. In C99 mode (`-std=c99' or `-std=gnu99'), this switch only affects the asm and typeof keywords, since inline is a standard keyword in ISO C99.

-fno-builtin

Don't recognize built-in functions that do not begin with `__builtin_' as prefix. See section Other built-in functions provided by GNU CC, for details of the functions affected, including those which are not built-in functions when `-ansi' or `-std' options for strict ISO C conformance are used because they do not have an ISO standard meaning.

GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to alloca may become single instructions that adjust the stack directly, and calls to memcpy may become inline copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library.

In C++, `-fno-builtin' is always in effect. The `-fbuiltin' option has no effect. Therefore, in C++, the only way to get the optimization benefits of built-in functions is to call the function using the `__builtin_' prefix. The GNU C++ Standard Library uses built-in functions to implement many functions (like std::strchr), so that you automatically get efficient code.

-fhosted

Assert that compilation takes place in a hosted environment. This implies `-fbuiltin'. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to `-fno-freestanding'.

-ffreestanding

Assert that compilation takes place in a freestanding environment. This implies `-fno-builtin'. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to `-fno-hosted'.

See section Language Standards Supported by GCC, for details of freestanding and hosted environments.

-trigraphs

Support ISO C trigraphs. The `-ansi' option (and `-std' options for strict ISO C conformance) implies `-trigraphs'.

-traditional

Attempt to support some aspects of traditional C compilers. Specifically:

• All extern declarations take effect globally even if they are written inside of a function definition. This includes implicit declarations of functions.

• The newer keywords typeof, inline, signed, const and volatile are not recognized. (You can still use the alternative keywords such as __typeof__, __inline__, and so on.)

• Comparisons between pointers and integers are always allowed.

• Integer types unsigned short and unsigned char promote to unsigned int.

• Out-of-range floating point literals are not an error.

• Certain constructs which ISO regards as a single invalid preprocessing number, such as `0xe-0xd', are treated as expressions instead.

• String "constants" are not necessarily constant; they are stored in writable space, and identical looking constants are allocated separately. (This is the same as the effect of `-fwritable-strings'.)

• All automatic variables not declared register are preserved by longjmp. Ordinarily, GNU C follows ISO C: automatic variables not declared volatile may be clobbered.

• The character escape sequences `\x' and `\a' evaluate as the literal characters `x' and `a' respectively. Without `-traditional', `\x' is a prefix for the hexadecimal representation of a character, and `\a' produces a bell.

You may wish to use `-fno-builtin' as well as `-traditional' if your program uses names that are normally GNU C built-in functions for other purposes of its own.

You cannot use `-traditional' if you include any header files that rely on ISO C features. Some vendors are starting to ship systems with ISO C header files and you cannot use `-traditional' on such systems to compile files that include any system headers.

The `-traditional' option also enables `-traditional-cpp', which is described next.

-traditional-cpp

Attempt to support some aspects of traditional C preprocessors. Specifically:

• Comments convert to nothing at all, rather than to a space. This allows traditional token concatenation.

• In a preprocessing directive, the `#' symbol must appear as the first character of a line.

• Macro arguments are recognized within string constants in a macro definition (and their values are stringified, though without additional quote marks, when they appear in such a context). The preprocessor always considers a string constant to end at a newline.

• The predefined macro __STDC__ is not defined when you use `-traditional', but __GNUC__ is (since the GNU extensions which __GNUC__ indicates are not affected by `-traditional'). If you need to write header files that work differently depending on whether `-traditional' is in use, by testing both of these predefined macros you can distinguish four situations: GNU C, traditional GNU C, other ISO C compilers, and other old C compilers. The predefined macro __STDC_VERSION__ is also not defined when you use `-traditional'. See section `Standard Predefined Macros' in The C Preprocessor, for more discussion of these and other predefined macros.

• The preprocessor considers a string constant to end at a newline (unless the newline is escaped with `\'). (Without `-traditional', string constants can contain the newline character as typed.)

-fcond-mismatch

Allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.

-funsigned-char

Let the type char be unsigned, like unsigned char.

Each kind of machine has a default for what char should be. It is either like unsigned char by default or like signed char by default.

Ideally, a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But many programs have been written to use plain char and expect it to be signed, or expect it to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such a program work with the opposite default.

The type char is always a distinct type from each of signed char or unsigned char, even though its behavior is always just like one of those two.

-fsigned-char

Let the type char be signed, like signed char.

Note that this is equivalent to `-fno-unsigned-char', which is the negative form of `-funsigned-char'. Likewise, the option `-fno-signed-char' is equivalent to `-funsigned-char'.

-fsigned-bitfields

-funsigned-bitfields

-fno-signed-bitfields

-fno-unsigned-bitfields

These options control whether a bit-field is signed or unsigned, when the declaration does not use either signed or unsigned. By default, such a bit-field is signed, because this is consistent: the basic integer types such as int are signed types.

However, when `-traditional' is used, bit-fields are all unsigned no matter what.

-fwritable-strings

Store string constants in the writable data segment and don't uniquize them. This is for compatibility with old programs which assume they can write into string constants. The option `-traditional' also has this effect.

Writing into string constants is a very bad idea; "constants" should be constant.

-fallow-single-precision

Do not promote single precision math operations to double precision, even when compiling with `-traditional'.

Traditional K&R C promotes all floating point operations to double precision, regardless of the sizes of the operands. On the architecture for which you are compiling, single precision may be faster than double precision. If you must use `-traditional', but want to use single precision operations when the operands are single precision, use this option. This option has no effect when compiling with ISO or GNU C conventions (the default).

-fshort-wchar

Override the underlying type for `wchar_t' to be `short unsigned int' instead of the default for the target. This option is useful for building programs to run under WINE.