Options Controlling C Dialect

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

c90 c89 iso9899:1990

Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.

iso9899:199409

ISO C90 as modified in amendment 1.

c99 c9x iso9899:1999 iso9899:199x

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.

c11 c1x iso9899:2011

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.

c17 c18 iso9899:2017 iso9899:2018

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.

c2x

The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete.

gnu90 gnu89

GNU dialect of ISO C90 (including some C99 features).

gnu99 gnu9x

GNU dialect of ISO C99. The name gnu9x is deprecated.

gnu11 gnu1x

GNU dialect of ISO C11. The name gnu1x is deprecated.

gnu17 gnu18

GNU dialect of ISO C17. This is the default for C code.

gnu2x

The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete.

c++98 c++03

The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.

gnu++98 gnu++03

GNU dialect of -std=c++98.

c++11 c++0x

The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.

gnu++11 gnu++0x

GNU dialect of -std=c++11. The name gnu++0x is deprecated.

c++14 c++1y

The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.

gnu++14 gnu++1y

GNU dialect of -std=c++14. The name gnu++1y is deprecated.

c++17 c++1z

The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.

gnu++17 gnu++1z

GNU dialect of -std=c++17. This is the default for C++ code. The name gnu++1z is deprecated.

c++20 c++2a

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++20 gnu++2a

GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2a is deprecated.

c++2b c++23

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++2b gnu++23

GNU dialect of -std=c++2b. 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 C2X mode (-std=c2x or -std=gnu2x), this switch only affects the asm keyword, since typeof is a standard keyword in ISO C2X.

-fasm

Default setting; overrides -fno-asm.

-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))

-fbuiltin

Default setting; overrides -fno-builtin.

-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 Enabling libitm.

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.

-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 Structure and Union Fields, for details.

Note that this option is off for all targets except for x86 targets using ms-abi.

-foffload=disable

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

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=-lgfortran -foffload-options=-lm -foffload-options="-lgfortran-lm -lm" -foffload-options=nvptx-none=-latomic -foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm

-fopenacc

Enable handling of OpenACC directives #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 v2.6 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 in C/C++, [[omp::directive(...)]] and [[omp::sequence(...)]] in C++ and !$omp in Fortran. 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, loop directives and combined or composite directives with simd as constituent with #pragma omp in C/C++, [[omp::directive(...)]] and [[omp::sequence(...)]] in C++ and !$omp in Fortran. Other OpenMP directives are ignored.

-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.

-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 Structure and Union Fields, 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

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 positive form is equivalent to -fstrict-flex-arrays=3, which is the strictest. 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 negative form is equivalent to -fstrict-flex-arrays=0, which is the least strict. All trailing arrays of structures are treated as flexible array members.

-fno-strict-flex-arrays

Default setting; overrides -fstrict-flex-arrays.

-fstrict-flex-arrays=level

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.

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).

-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.