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 .

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.

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.

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++.

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 .

Enable parsing of function definitions marked with __GIMPLE . This is an experimental feature that allows unit testing of GIMPLE passes.

When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intels 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.

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 .

Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code.

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;
};

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 GCCs internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES .

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

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.

Enable handling of OpenMPs 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.

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.

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.

Where to write structured dependency information.

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.

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.

Accept some non-standard constructs used in Plan 9 code.

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.

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 .

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.

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.