C Dialect Options (Using the GNU Compiler Collection (GCC))

### 3.4 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` ¶
`--ansi`

  In C mode, this is equivalent to `-std=c90`. In C++ mode, it is equivalent to `-std=c++98`.

`-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/projects/c-status.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.

  ‘`c23`’
  ‘`c2x`’
  ‘`iso9899:2024`’

    ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name ‘`c2x`’ is deprecated.

  ‘`c2y`’

    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.

  ‘`gnu23`’
  ‘`gnu2x`’

    GNU dialect of ISO C23. This is the default for C code. The name ‘`gnu2x`’ is deprecated.

  ‘`gnu2y`’

    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.

  ‘`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`. The name ‘`gnu++1z`’ is deprecated.

  ‘`c++20`’
  ‘`c++2a`’

    The 2020 ISO C++ standard plus amendments. C++20 modules support is still experimental and needs to be enabled with `-fmodules` option. The name ‘`c++2a`’ is deprecated.

  ‘`gnu++20`’
  ‘`gnu++2a`’

    GNU dialect of `-std=c++20`. This is the default for C++ code. C++20 modules support is still experimental and needs to be enabled with `-fmodules` option. The name ‘`gnu++2a`’ is deprecated.

  ‘`c++23`’
  ‘`c++2b`’

    The 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in incompatible ways in future releases. The name ‘`c++2b`’ is deprecated.

  ‘`gnu++23`’
  ‘`gnu++2b`’

    GNU dialect of `-std=c++23`. Support is experimental, and could change in incompatible ways in future releases. The name ‘`gnu++2b`’ is deprecated.

  ‘`c++26`’
  ‘`c++2c`’

    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++26`’
  ‘`gnu++2c`’

    GNU dialect of `-std=c++26`. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

  ‘`c++29`’
  ‘`c++2d`’

    The upcoming revision of the ISO C++ standard, planned for 2029. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.

  ‘`gnu++29`’
  ‘`gnu++2d`’

    GNU dialect of `-std=c++29`. 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 Builtins for C Library Functions, 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 Common 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 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.

`-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` ¶
`-funsigned-char`

  `-fsigned-char` lets the type `char` be signed, like `signed char`, while `-funsigned-char` lets the type `char` be unsigned, like `unsigned char`.

  Note that `-fsigned-char` 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`.

  Each target supported by GCC has a default for what `char` should be, which is typically specified in the processor-specific ABI document for that target. 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. These options 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; 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 `-fno-strict-flex-arrays` is equivalent to `-fstrict-flex-arrays=0`, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members.

  There are two more levels in between 0 and 3, which are provided to support older code that uses the GCC zero-length array extension (‘`[0]`’) or one-element array as flexible array members (‘`[1]`’). When level is 1, the trailing array is treated as a flexible array member when it is declared as either ‘`[]`’, ‘`[0]`’, or ‘`[1]`’. When level is 2, the trailing array is treated as a flexible array member when it is declared as either ‘`[]`’, or ‘`[0]`’.

  You can control this behavior for a specific trailing array field of a structure by using the variable attribute `strict_flex_array` attribute (see Common Attributes).

  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.