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=c90. 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, -Wpedantic is required in addition to -ansi. See 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 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=
¶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 ‘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
Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
ISO C90 as modified in amendment 1.
ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See https://gcc.gnu.org/c99status.html for more information. The names ‘c9x’ and ‘iso9899:199x’ are deprecated.
ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name ‘c1x’ is deprecated.
ISO C17, the 2017 revision of the ISO C standard
(published in 2018). This standard is
same as C11 except for corrections of defects (all of which are also
applied with -std=c11) and a new value of
__STDC_VERSION__
, and so is supported to the same extent as C11.
ISO C23, the 2023 revision of the ISO C standard (expected to be published in 2024). The support for this version is experimental and incomplete. The name ‘c2x’ is deprecated.
GNU dialect of ISO C90 (including some C99 features).
GNU dialect of ISO C99. The name ‘gnu9x’ is deprecated.
GNU dialect of ISO C11. The name ‘gnu1x’ is deprecated.
GNU dialect of ISO C17. This is the default for C code.
The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The name ‘gnu2x’ is deprecated.
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
GNU dialect of -std=c++98.
The 2011 ISO C++ standard plus amendments. The name ‘c++0x’ is deprecated.
GNU dialect of -std=c++11. The name ‘gnu++0x’ is deprecated.
The 2014 ISO C++ standard plus amendments. The name ‘c++1y’ is deprecated.
GNU dialect of -std=c++14. The name ‘gnu++1y’ is deprecated.
The 2017 ISO C++ standard plus amendments. The name ‘c++1z’ is deprecated.
GNU dialect of -std=c++17. This is the default for C++ code. The name ‘gnu++1z’ is deprecated.
The 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. The name ‘c++2a’ is deprecated.
GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future releases. The name ‘gnu++2a’ is deprecated.
The next revision of the ISO C++ standard, planned for 2023. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
GNU dialect of -std=c++2b. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
The next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
-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. In C, -ansi implies -fno-asm.
In C++, inline
is a standard keyword and is not affected by
this switch. You may want to use the -fno-gnu-keywords flag
instead, which disables typeof
but not asm
and
inline
. 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. In C23 mode
(-std=c23 or -std=gnu23), this switch only affects
the asm
keyword, since typeof
is a standard keyword in
ISO C23.
-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 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))
-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++.
-ffreestanding
¶Assert that compilation targets 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.
-fgimple
¶Enable parsing of function definitions marked with __GIMPLE
.
This is an experimental feature that allows unit testing of GIMPLE
passes.
-fgnu-tm
¶When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel’s current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.
For more information on GCC’s support for transactional memory, See The GNU Transactional Memory Library in GNU Transactional Memory Library.
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-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.
Using this option is roughly equivalent to adding the
gnu_inline
function attribute to all inline functions
(see Declaring Attributes of Functions).
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 in The C Preprocessor.
-fhosted
¶Assert that compilation targets 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.
-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.
-fms-extensions
¶Accept some non-standard constructs used in Microsoft header files.
In 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 except for x86 targets using ms-abi.
-foffload=disable
¶-foffload=default
-foffload=target-list
Specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to -foffload=default, is to generate code for all supported offload targets. The -foffload=disable form generates code only for the host fallback, while -foffload=target-list generates code only for the specified comma-separated list of offload targets.
Offload targets are specified in GCC’s internal target-triplet format. You can
run the compiler with -v to show the list of configured offload targets
under OFFLOAD_TARGET_NAMES
.
-foffload-options=options
¶-foffload-options=target-triplet-list=options
With -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.
Typical command lines are
-foffload-options='-fno-math-errno -ffinite-math-only' -foffload-options=nvptx-none=-latomic -foffload-options=amdgcn-amdhsa=-march=gfx906
-fopenacc
¶Enable handling of OpenACC directives ‘#pragma acc’ in C/C++ and ‘!$acc’ in free-form Fortran and ‘!$acc’, ‘c$acc’ and ‘*$acc’ in fixed-form Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 https://www.openacc.org. This option implies -pthread, and thus is only supported on targets that have support for -pthread.
-fopenacc-dim=geom
¶Specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of ’:’-separated sizes, in order ’gang’, ’worker’ and, ’vector’. A size can be omitted, to use a target-specific default value.
-fopenmp
¶Enable handling of OpenMP directives ‘#pragma omp’, ‘[[omp::directive(...)]]’, ‘[[omp::sequence(...)]]’ and ‘[[omp::decl(...)]]’ in C/C++ and ‘!$omp’ in Fortran. It additionally enables the conditional compilation sentinel ‘!$’ in Fortran. In fixed source form Fortran, the sentinels can also start with ‘c’ or ‘*’. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 https://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
¶Enable handling of OpenMP’s simd
, declare simd
,
declare reduction
, assume
, ordered
, scan
and loop
directive, and of combined or composite directives with
simd
as constituent with #pragma omp
,
[[omp::directive(...)]]
, [[omp::sequence(...)]]
and
[[omp::decl(...)]]
in C/C++ and !$omp
in Fortran. It
additionally enables the conditional compilation sentinel ‘!$’ in
Fortran. In fixed source form Fortran, the sentinels can also start with
‘c’ or ‘*’. Other OpenMP directives are ignored. Unless
-fopenmp is additionally specified, the loop
region binds
to the current task region, independent of the specified bind
clause.
-fopenmp-target-simd-clone
¶-fopenmp-target-simd-clone=device-type
In addition to generating SIMD clones for functions marked with the
declare simd
directive, GCC also generates clones
for functions marked with the OpenMP declare target
directive
that are suitable for vectorization when this option is in effect. The
device-type may be one of none
, host
, nohost
,
and any
, which correspond to keywords for the device_type
clause of the declare target
directive; clones are generated for
the intersection of devices specified.
-fopenmp-target-simd-clone is equivalent to
-fopenmp-target-simd-clone=any and
-fno-openmp-target-simd-clone is equivalent to
-fopenmp-target-simd-clone=none.
At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it is disabled by default.
-fpermitted-flt-eval-methods=style
¶ISO/IEC TS 18661-3 defines new permissible values for
FLT_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.
The ‘-fdeps-*’ options are used to extract structured dependency information for a source. This involves determining what resources provided by other source files will be required to compile the source as well as what resources are provided by the source. This information can be used to add required dependencies between compilation rules of dependent sources based on their contents rather than requiring such information be reflected within the build tools as well.
-fdeps-file=file
¶Where to write structured dependency information.
-fdeps-format=format
¶The format to use for structured dependency information. ‘p1689r5’ is the only supported format right now. Note that when this argument is specified, the output of ‘-MF’ is stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools.
-fdeps-target=file
¶Analogous to -MT but for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources.
-fplan9-extensions
¶Accept some non-standard constructs used in Plan 9 code.
This 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++.
-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.
-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.
-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.
-fstrict-flex-arrays (C and C++ only)
¶-fstrict-flex-arrays=level (C and C++ only)
Control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing the elements of such an array. The value of level controls the level of strictness.
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest; all trailing arrays of structures are treated as flexible array members.
The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case a trailing array is treated as a flexible array member only when it is declared as a flexible array member per C99 standard onwards.
The possible values of level are the same as for the
strict_flex_array
attribute (see Specifying Attributes of Variables).
You can control this behavior for a specific trailing array field of a
structure by using the variable attribute strict_flex_array
attribute
(see Specifying Attributes of Variables).
The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. See Options to Request or Suppress Warnings, for more information.
-fsso-struct=endianness
¶Set the default scalar storage order of structures and unions to the specified endianness. The accepted values are ‘big-endian’, ‘little-endian’ and ‘native’ for the native endianness of the target (the default). This option is not supported for C++.
Warning: 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.