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:
-ansi
In C mode, this is equivalent to -std=c89. In C++ mode, it is equivalent to -std=c++98.
This 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 that would normally be built in but do not have semantics
defined by ISO C (such as alloca and ffs) are not built-in
functions when -ansi is used. See Other built-in functions provided by GCC, for details of the functions
affected.
-std=
Determine the language standard. See Language Standards Supported by GCC, for details of these standard versions. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c89 or
c++98, and GNU dialects of those standards, such as
gnu89 or gnu++98. By specifing a base standard, the
compiler will accept all programs following that standard and those
using GNU extensions that do not contradict it. For example,
-std=c89 turns off certain features of GCC that are
incompatible with ISO C90, such as the asm and typeof
keywords, but not other GNU extensions that do not have a meaning in
ISO C90, such as omitting the middle term of a ?:
expression. On the other hand, by specifing a GNU dialect of a
standard, all features the compiler support are enabled, even when
those features change the meaning of the base standard and some
strict-conforming programs may be rejected. The particular standard
is used by -pedantic to identify which features are GNU
extensions given that version of the standard. For example
-std=gnu89 -pedantic would warn about C++ style //
comments, while -std=gnu99 -pedantic would not.
A value for this option must be provided; possible values are
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional
GNU semantics for inline functions when in C99 mode.
See An Inline Function is As Fast As a Macro. This option
is accepted and ignored by GCC versions 4.1.3 up to but not including
4.3. In GCC versions 4.3 and later it changes the behavior of GCC in
C99 mode. Using this option is roughly equivalent to adding the
gnu_inline function attribute to all inline functions
(see Function Attributes).
The option -fno-gnu89-inline explicitly tells GCC to use the
C99 semantics for inline when in C99 or gnu99 mode (i.e., it
specifies the default behavior). This option was first supported in
GCC 4.3. This option is not supported in C89 or gnu89 mode.
The preprocessor 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 (The C Preprocessor).
-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
-fno-builtin-
function
Don't recognize built-in functions that do not begin with __builtin_ as prefix. See Other built-in functions provided by GCC, 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 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))
-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 Language Standards Supported by GCC, for details of
freestanding and hosted environments.
-fopenmp
Enable handling of OpenMP directives #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/
. This option
implies -pthread, and thus is only supported on targets that
have support for -pthread.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
Some 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
Support ISO C trigraphs. The -ansi option (and -std
options for strict ISO C conformance) implies -trigraphs.
-no-integrated-cpp
Performs a compilation in two passes: preprocessing and compiling. This option allows a user supplied "cc1", "cc1plus", or "cc1obj" via the -B option. The user supplied compilation step can then add in an additional preprocessing step after normal preprocessing but before compiling. The default is to use the integrated cpp (internal cpp)
The semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional
-traditional-cpp
Formerly, these options caused GCC to attempt to emulate a pre-standard
C compiler. They are now only supported with the -E switch.
The preprocessor continues to support a pre-standard mode. See the GNU
CPP manual for details.
-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++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of
elements and/or incompatible element types. This option should not be
used for new code.
-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.