Extensions for GNU C compatibility

The following feature is enabled by default at all language levels:

Table 1. Default IBM XL C extensions for GNU C compatibility
Language feature	Discussed in:
`#include_next` preprocessor directive	The #include_next directive (IBM extension)

The following features are enabled by default when you compile with any of the following commands:

the xlc invocation command
the -qlanglvl=extc99 | extc89 | extc1x | extended options

Table 2. Default IBM XL C extensions for GNU C compatibility
Language feature	Discussed in:
`__alignof` and `__alignof__` keywords	The __alignof__ operator (IBM extension), Keywords for language extensions (IBM extension)
`__align` type qualifier	The __align type qualifier (IBM extension)
`__alignof__` operator	The __alignof__ operator (IBM extension)
`__attribute` and `__attribute__` keywords	Variable attributes (IBM extension), Function attributes (IBM extension), Keywords for language extensions (IBM extension)
`__complex__` keyword	Keywords for language extensions (IBM extension)
`__extension__` keyword	Keywords for language extensions (IBM extension)
`__imag__` and`__real__` keywords	The __real__ and __imag__ operators (IBM extension)Keywords for language extensions (IBM extension)
`__imag__` and `__real__` complex type operators	The __real__ and __imag__ operators (IBM extension)
`__inline__` keyword	The inline function specifier, Keywords for language extensions (IBM extension)
`__label__` keyword	Nested functions (C only) (IBM extension), Locally declared labels (IBM extension), Keywords for language extensions (IBM extension)
`__restrict` and `__restrict__` keywords	The restrict type qualifier, Keywords for language extensions (IBM extension)
`__thread` keyword	The __thread storage class specifier (IBM extension), Keywords for language extensions (IBM extension)
`__typeof__` keyword	The typeof operator (IBM extension), Keywords for language extensions (IBM extension)
`__typeof__` operator	The typeof operator (IBM extension)
`#assert`, `#unassert` preprocessor directives	Assertion directives (IBM extension)
`#warning` preprocessor directive	The #warning directive (IBM extension)
Alternate keywords	Keywords for language extensions (IBM extension)
`asm` labels	Assembly labels (IBM extension)
Cast to a union type	Cast to union type (C only) (IBM extension)
Complex literal suffixes	Complex literals
Computed `goto` statements	Computed goto statement (IBM extension)
Function attributes	Function attributes (IBM extension)
Generalized lvalues	Lvalues and rvalues
Global register variables	Variables in specified registers (IBM extension)
Initialization of static variables by compound literals	Compound literal expressions
Labels as values	Labels as values (IBM extension)
Linkage of inline functions	Linkage of inline functions
Locally declared labels	Locally declared labels (IBM extension)
Nested functions	Nested functions (C only) (IBM extension)
Placement of flexible array members anywhere in structure or union	Flexible array members
Postfix and unary operators on complex types (increment, decrement, and complex conjugation)	Increment operator ++, Decrement operator --, Bitwise negation operator ~
Statements and declarations in expressions (statement expressions)	Statement expressions (IBM extension)
Static initialization of flexible array members of aggregates	Flexible array members
Structures with flexible array members being members of another structure	Flexible array members
Type attributes	Type attributes (IBM extension)
Variable attributes	Variable attributes (IBM extension)
Variadic macro extensions	Variadic macro extensions(IBM extension)
Zero-extent arrays	Zero-extent array members (IBM extension)

The following features are enabled by default when you compile with any of the following commands:

the xlc invocation command
the -qlanglvl=extc99 | extc89 | extc1x | extended options

They are also enabled or disabled by specific compiler options, which are listed in the below table:

Table 3. IBM XL C extensions for GNU C compatibility with individual option controls
Language feature	Discussed in:	Individual option controls
`asm`, and `__asm` keywords	Assembly labels (IBM extension), Inline assembly statements (IBM extension), Keywords for language extensions (IBM extension)	-q[no]keyword=asm, -qasm
`asm` inline assembly-language statements	Inline assembly statements (IBM extension)	-qasm
`typeof` keyword	The typeof operator (IBM extension), Keywords for language extensions (IBM extension)	-q[no]keyword=typeof
`typeof` operator	The typeof operator (IBM extension)	-q[no]keyword=typeof
Visibility function attribute	visibility (IBM extension)	-fvisibility (-qvisibility)
Visibility variable attribute	The visibility variable attribute (IBM extension)	-fvisibility (-qvisibility)
Note: You can use the -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.

The following features require compilation with the use of an additional option:

Table 4. IBM XL C extensions for GNU C compatibility, requiring additional compiler options
Language feature	Discussed in:	Required compilation option
`__thread` storage class specifier	The __thread storage class specifier (IBM extension)	-qtls
Dollar signs in identifiers	Characters in identifiers	-qdollar

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