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 | extended options

Table 2. Default IBM XL C extensions for GNU C compatibility
Language feature	Discussed in:
`__alignof__` operator	The __alignof__ operator (IBM extension)
`__extension__` keyword	Keywords for language extensions (IBM extension)
`__imag__` and `__real__` complex type operators	The __real__ and __imag__ operators (IBM extension)
`__inline__` function specifier	The inline function specifier
`__typeof__` operator	The typeof operator (IBM extension)
`#assert`, `#unassert` preprocessor directives	Assertion directives (IBM extension)
`#warning` preprocessor directive	The #warning directive (IBM extension)
`asm` labels	Assembly labels (IBM extension)
Alternate keywords	Keywords for language extensions (IBM extension)
Cast to a union type	Cast to union type (C only) (IBM extension)
Complex literal suffixes	Complex literals
Complex type arguments to unary operators	Unary expressions
Computed `goto` statements	Computed goto statement (IBM extension)
Function attributes	Function attributes (IBM extension)
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)
Locally declared labels	Locally declared labels (IBM extension)
Generalized lvalues	Lvalues and rvalues
Nested functions	Nested functions (IBM extension)
Placement of flexible array members anywhere in structure or union	Flexible array members
Statements and declarations in expressions	Statement expressions (IBM extension)
Static initialization of flexible array members of aggregates	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 | 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)	-qkeyword=asm, -qasm
`asm` inline assembly-language statements	Inline assembly statements (IBM extension)	-qasm
Visibility function attribute	visibility (IBM extension)	-qvisibility
Visibility variable attribute	The visibility variable attribute (IBM extension)	-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
The `__thread` storage class specifier	The __thread storage class specifier (IBM extension)	-qtls
Dollar signs in identifiers	Characters in identifiers	-qdollar
The `typeof` keyword	The typeof operator (IBM extension)	-qkeyword=typeof