Extensions for GNU C/C++ compatibility

The following subset of the GNU C/C++ language extension is enabled with the LANGLVL(EXTENDED) option, which is the default language level.

Table 1. Default IBM XL C/C++ extensions for compatibility with GNU C/C++
Language feature	Discussed in:
Placement of flexible array members anywhere in structure or union	Flexible array members (C only)
Static initialization of flexible array members of aggregates	Flexible array members (C only)
`__alignof__` operator	The __alignof__ operator (IBM extension)
`__typeof__` operator	The typeof operator (IBM extension)
Generalized lvalues	Lvalues and rvalues
Complex type arguments to unary operators	Unary expressions
`__imag__` and `__real__` complex type operators	The __real__ and __imag__ operators (IBM extension)
Function attributes	Function attributes (IBM extension)

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

Table 2. Default IBM XL C/C++ extensions for compatibility with GNU C/C++, with individual option controls
Language feature	Discussed in:	Individual option controls
Complex literal suffixes	Complex literals	LANGLVL([NO]GNU_SUFFIXIJ)
Zero-extent arrays	Zero-extent array members (IBM extension)	LANGLVL([NO]ZEROEXTARRAY)
Labels as values	Labels as values (IBM extension)	LANGLVL([NO]GNU_LABELVALUE)
Computed `goto` statements	Computed goto statement (IBM extension)	LANGLVL([NO]GNU_COMPUTEDGOTO)
inline assembly-language statements	Inline assembly statements (IBM extension)	ASM \| NOASM