Next: C++ Dialect Options, Previous: Invoking G++, Up: Invoking GCC
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansiThis turns off certain features of GCC that are incompatible with ISO
C90 (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 Warning Options.
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 Other built-in functions provided by GCC, for details of the functions
affected.
-std=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 C90
but are in the specified version (for example, `//' comments and
the inline keyword in ISO C99) are not disabled.
See Language Standards Supported by GCC, for details of
these standard versions.
-fgnu89-inlineinline functions when in C99 mode.
See An Inline Function is As Fast As a Macro. Using this
option is roughly equivalent to adding the gnu_inline function
attribute to all inline functions (see Function Attributes).
This option is accepted by GCC versions 4.1.3 and up. In GCC versions /* APPLE LOCAL extern inline */ prior to 4.3 (4.2 for Apple's gcc), C99 inline semantics are not supported, and thus this option is effectively assumed to be present regardless of whether or not it is specified; the only effect of specifying it explicitly is to disable warnings about using inline functions in C99 mode. Likewise, the option -fno-gnu89-inline is not supported in versions of /* APPLE LOCAL extern inline */ GCC before 4.3 (4.2 for Apple's gcc). It is supported only in C99 or gnu99 mode, not in C89 or gnu89 mode.
The preprocesor macros __GNUC_GNU_INLINE__ and
__GNUC_STDC_INLINE__ may be used to check which semantics are
in effect for inline functions. See Common Predefined Macros.
-aux-info filenameBesides 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.
-faltivec-fasm-blocks-fno-asmasm, 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-fno-builtin-functionGCC 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 addition,
when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to
that function, or to generate more efficient code, even if the
resulting code still contains calls to that function. For example,
warnings are given with -Wformat for bad calls to
printf, when printf is built in, and strlen is
known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with `__builtin_'. If a function is named this is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhostedmain has a return
type of int. Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestandingmain. The most obvious example is an OS kernel.
This is equivalent to -fno-hosted.
See Language Standards Supported by GCC, for details of
freestanding and hosted environments.
-fopenmp#pragma omp in C/C++ and
!$omp in Fortran. When -fopenmp is specified, the
compiler generates parallel code according to the OpenMP Application
Program Interface v2.5 http://www.openmp.org/.
-fms-extensionsSome cases of unnamed fields in structures and unions are only
accepted with this option. See Unnamed struct/union fields within structs/unions, for details.
-trigraphs-no-integrated-cppThe semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional-traditional-cpp-fcond-mismatch-fno-nested-functions-fpch-preprocess-fnon-lvalue-assign float *p, q, r;
((int *)p)++;
(cond ? q : r) = 3.0;
As a transitional measure, the Apple version of GCC 4.0 allows casts and conditional expressions to be used as lvalues in certain situations. This is accomplished via the -fnon-lvalue-assign switch, which is on by default. Whenever an lvalue cast or an lvalue conditional expression is encountered, the compiler will issue a deprecation warning and then rewrite the expression as follows:
(type)expr ---becomes---> *(type *)&expr
cond ? expr1 : expr2 ---becomes---> *(cond ? &expr1 : &expr2)
To disallow lvalue casts and lvalue conditional expressions altogether,
specify -fno-non-lvalue-assign; lvalue casts and lvalue conditional
expressions will be disallowed in future versions of Apple's GCC.
-funsigned-charchar 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-charchar 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-bitfieldssigned or unsigned. By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as int are signed types.
-fconstant-cfstrings__builtin__CFStringMakeConstantString
is called on a literal string. (APPLE ONLY)
-Wnonportable-cfstrings-fwritable-stringsWriting into string constants is a very bad idea; “constants” should be constant.
This option is deprecated.