Extensions for GNU C compatibility

The following features are enabled by default at all language levels. For details about supported GCC features, see the GNU Compiler Collection online documentation at http://gcc.gnu.org/onlinedocs/.

Table 1. Default IBM® XL C and C++ extensions for GNU C compatibility
Language feature	Discussed in:
`__alignof__` operator	GCC online documentation
`__attribute__` keyword	Variable attributes, Function attributes
`__complex__` keyword	GCC online documentation
`__extension__` keyword	GCC online documentation
`__imag__` and `__real__` complex type operators	GCC online documentation
`__int128` and `unsigned __int128` types	GCC online documentation
`__restrict__` keyword	GCC online documentation
`__thread` storage class specifier	The __thread storage class specifier
`__typeof__` keyword	The typeof keyword
`#include_next` preprocessor directive	The #include_next directive
`#warning` preprocessor directive	GCC online documentation
Alternative keywords	GCC online documentation
`asm` inline assembly-language statements	Inline assembly statements
`asm` labels	GCC online documentation
Complex literal suffixes	GCC online documentation
Computed `goto` statements	GCC online documentation
Dollar signs in identifiers	Characters in identifiers
FORTIFY_SOURCE	GCC online documentation
Function attributes	Function attributes
Initialization of static variables by compound literals	Compound literal expressions
Labels as values	GCC online documentation
Postfix and unary operators on complex types (increment, decrement, and complex conjugation)	GCC online documentation
Statements and declarations in expressions (statement expressions)	GCC online documentation
Static initialization of flexible array members of aggregates	Flexible array members of structures
Structures with flexible array members being members of another structure	Flexible array members of structures
Type attributes	Type attributes
Variable attributes	Variable attributes
Variadic macro extensions	Variadic macros
Zero-extent arrays	GCC online documentation

C only begins The following features are enabled by default when you compile with the xlc invocation command or the -qlanglvl=extc99 | extc89 | extc1x | extended options. C only ends

C++ only begins The following features are enabled by default at all C++ language levels: C++ only ends

Some features can also be enabled or disabled by specific compiler options, which are listed in the below table:

Table 2. IBM XL C and C++ extensions for GNU C compatibility with individual option controls
Language feature	Discussed in:	Individual option controls
`typeof` keyword	The typeof keyword	GCC online documentation
Visibility function attribute¹	visibility	-fvisibility (-qvisibility)
Visibility variable attribute¹	The visibility variable attribute	-fvisibility (-qvisibility)
Note: You can use the -fvisibility (-qvisibility) option to specify visibility attributes for variables and functions if they do not get visibility attributes from pragma directives, explicitly specified attributes, or propagation rules. This option cannot be used to disable visibility attributes for variables or functions. For details about supported GCC features, see the GNU Compiler Collection online documentation at http://gcc.gnu.org/onlinedocs/.