GNU COMPATIBILITY

$ allowed in identifiers

Macros with variable number of parameters

If the last macro parameter is specified with a "..." after it, then that parameter represents a list of any number of arguments supplied to the macro.

e.g.

#define TEST(a,b...) printf (a, 5, b)

TEST ("5=%d, 6=%d, 7=%d\n", 6, 7);

Compound expressions

You can write a compound statement within an expression. The value of the last statement is the value of the expression.

e.g.

int a = ({printf ("assigning a\n"); 5;});

Finding the address of a local label

The && unary operator can be used to give the address of a local label.

e.g.

{
void *addr;
label:
    addr = &&label;
}

Constant structure constructors in expressions

You can specify a structured constant within an expression by placing the structured constant after a structure cast.

e.g.

struct TEST a;

a = (struct TEST) {3, 4, 5};

Missing the middle argument of a conditional expression

If the middle argument of a conditional expression is missing, it is assumed to be the value of the condition.

e.g.

a = b ?: c;

If b is non-zero, the value of the expression is b, otherwise the value is c.

Conditional expressions can be LValues

The result of a conditional expression can be used on the left hand side of an assignment.

Ranges in case statements

Cases in switch statements can use "..." to specify a range of values.

Goto expression

Gotos can take a variable value if a "*" is put before the expression.

Non-constant initializers

Elements of structured intializers can be non-constant expressions, if the variable being assigned is local and not static.

Array of length 0

Arrays can have a length of 0.

Typedef from an expression

The type in a typedef declaration can be taken as the type of an expression

e.g.

int a;

typedef TYPE = a;

Forward declaration of a label

__typeof__

Anywhere where a type is required - e.g. a declaration or cast - the __typeof__ keyword can be used to return the type of an expression.

e.g.

a = (__typeof__ (a)) 4;

__attribute__

Special attributes of identifiers can be set with the __attribute__ keyword. This is similar to the __declspec keyword.

__attribute__ ((__stdcall)) or __attribute__ ((stdcall)) or __attribute ((__stdcall__))

__attribute__ ((__pascal))

__attribute__ ((__fastcall))

__attribute__ ((__syscall))

__attribute__ ((dllexport))

__attribute__ ((dllimport))

__attribute__ ((wcall))

__attribute__ ((naked))

__attribute__ ((thread)) or __attribute__ ((__thread__))

__attribute__ ((alignedvalue (n))) or __attribute__ ((__alignedvalue__ (n)))

__attribute__ ((aligned (n))) or __attribute__ ((__aligned__ (n)))

__attribute__ ((packed)) or __attribute__ ((__packed__))

__attribute__ ((noreturn)) or __attribute__ ((__noreturn__))

__attribute__ ((const)) or __attribute__ ((__const__))