AN3350 datasheet(99/185 Pages) STMICROELECTRONICS

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AN3350

C compiler

Doc ID 018512 Rev 2

99/185

#define POW2(n) ({ int i,r; for(r=1,i=n; i>0; --i) r *= 2; r;})

int main()

{

return POW2(4);

}

12.3.5

Redefining macros

When the GCC extensions setting is on, macros may be redefined with the #define

directive without first undefining them with the #undef directive.Example 41 shows an

example.

Example 41 Redefining a macro without undefining first

#define SOCK_MAXCOLOR 100

#undef SOCK_MAXCOLOR

#define SOCK_MAXCOLOR 200 /* OK: this macro is previously undefined.

#define SOCK_MAXCOLOR 300

12.3.6

The typeof() operator

When the GCC extensions setting is on, the compiler recognizes the typeof() operator.

This compile-time operator returns the type of an expression. You may use the value

returned by this operator in any statement or expression where the compiler expects you to

specify a type. The compiler evaluates this operator at compile time. The __typeof()__

operator is the same as this operator.Example 42 shows an example.

Example 42 Using the typeof() operator

int *ip;

/* Variables iptr and jptr have the same type. */

typeof(ip) iptr;

int *jptr;

/* Variables i and j have the same type. */

typeof(*ip) i;

int j;

12.3.7

Void and function pointer arithmetic

The ISO/IEC 9899-1990 C Standard does not accept arithmetic expressions that use

pointers to void or functions. With GCC extensions on, the compiler accepts arithmetic

manipulation of pointers to void and functions.

AN3350 Datasheet(PDF) 99 Page - STMicroelectronics