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, -Wpedantic 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 are normally 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=The compiler can accept several base standards, such as ‘c90’ or
‘c++98’, and GNU dialects of those standards, such as
‘gnu90’ or ‘gnu++98’. When a base standard is specified, the
compiler accepts all programs following that standard plus those
using GNU extensions that do not contradict it. For example,
-std=c90 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, when a GNU dialect of a standard is
specified, all features supported by the compiler are enabled, even when
those features change the meaning of the base standard. As a result, some
strict-conforming programs may be rejected. The particular standard
is used by -Wpedantic to identify which features are GNU
extensions given that version of the standard. For example
-std=gnu90 -Wpedantic warns about C++ style ‘//’
comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
__STDC_VERSION__, and so is supported to the same extent as C11.
-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).
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 is not supported in -std=c90 or
-std=gnu90 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.
-fpermitted-flt-eval-methods=styleFLT_EVAL_METHOD that indicate that operations and constants with
a semantic type that is an interchange or extended format should be
evaluated to the precision and range of that type. These new values are
a superset of those permitted under C99/C11, which does not specify the
meaning of other positive values of FLT_EVAL_METHOD. As such, code
conforming to C11 may not have been written expecting the possibility of
the new values.
-fpermitted-flt-eval-methods specifies whether the compiler
should allow only the values of FLT_EVAL_METHOD specified in C99/C11,
or the extended set of values specified in ISO/IEC TS 18661-3.
style is either c11 or ts-18661-3 as appropriate.
The default when in a standards compliant mode (-std=c11 or similar)
is -fpermitted-flt-eval-methods=c11. The default when in a GNU
dialect (-std=gnu11 or similar) is
-fpermitted-flt-eval-methods=ts-18661-3.
-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.
-fallow-parameterless-variadic-functionsAlthough it is possible to define such a function, this is not very
useful as it is not possible to read the arguments. This is only
supported for C as this construct is allowed by C++.
-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 which 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 that 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))
-fgimple__GIMPLE.
This is an experimental feature that allows unit testing of GIMPLE
passes.
-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.
-fopenacc#pragma acc in C/C++ and
!$acc in Fortran. When -fopenacc is specified, the
compiler generates accelerated code according to the OpenACC Application
Programming Interface v2.0 https://www.openacc.org. This option
implies -pthread, and thus is only supported on targets that
have support for -pthread.
-fopenacc-dim=geom-fopenmp#pragma omp in C/C++ and
!$omp in Fortran. When -fopenmp is specified, the
compiler generates parallel code according to the OpenMP Application
Program Interface v4.5 http://www.openmp.org/. This option
implies -pthread, and thus is only supported on targets that
have support for -pthread. -fopenmp implies
-fopenmp-simd.
-fopenmp-simd#pragma omp
in C/C++ and !$omp in Fortran. Other OpenMP directives
are ignored.
-fgnu-tmFor more information on GCC's support for transactional memory, See The GNU Transactional Memory Library.
Note that the transactional memory feature is not supported with
non-call exceptions (-fnon-call-exceptions).
-fms-extensionsIn C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
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.
Note that this option is off for all targets but x86
targets using ms-abi.
-fplan9-extensionsThis enables -fms-extensions, permits passing pointers to
structures with anonymous fields to functions that expect pointers to
elements of the type of the field, and permits referring to anonymous
fields declared using a typedef. See Unnamed struct/union fields within structs/unions, for details. This is only
supported for C, not C++.
-fcond-mismatch-flax-vector-conversions-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.
-fsso-struct=endiannessWarning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target.