A summary of GCC extensions to C

There is a lot of code in the Linux kernel that uses many of the extended features of GCC, and this article focuses on a small summary of these features, all from the GCC Handbook

1. A block of code that is enclosed by a pair of curly braces can be used as an expression, using a looping statement (while, a for), a switch statement (if, switch), a local variable, and so on. The code is as follows:

1234	({ inty = foo (); int z;    if (y > 0) z = y;    elsez = - y;    z; })

The last statement must be an expression and end with a semicolon, which is used for the return value of the code block in other statements or expressions

12	#define maxint(a,b) \({int_a = (a), _b = (b); _a > _b ? _a : _b; })

This macro definition is much more secure than the following code.

1	#define max(a,b) ((a) > (b) ? (a) : (b))

2. Affirm local label (locally label)

GCC allows the declaration of any local tag in a block of code, but only in the declared code block, as follows

1	__label__ label1, label2, /* . . . */;

This statement only declares two label names, but does not define the label itself, but also need to use the label signature followed by a colon (label:) to define the tag, the local label is generally used in a complex macro definition is more effective, if a macro definition in a function is expanded more than once (by multiple references), Then a common label will be duplicated, and the local tag will be good to avoid the problem, such as the following code:

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#define SEARCH(array, target) \({ \    __label__ found; \    typeof (target) _SEARCH_target = (target); \    typeof (*(array)) *_SEARCH_array = (array); \    int i, j; \    int value; \    for (i = 0; i < max; i++) \        for (j = 0; j < max; j++) \            if (_SEARCH_array[i][j] == _SEARCH_target) \                { value = i; goto found; } \    value = -1; \  found: \    value; \})

3. Get the value of a label

You can get the value of a label (address), the label's worth type is void *, the worthwhile type is a constant, can be used in any place where the type needs to be changed, using the following syntax:

1234	void*ptr;/* . . . */ptr = &&foo;//foo为一个标签goto*ptr;

You can also use this:

123	staticconst int array[] = { &&foo - &&foo, &&bar - &&foo,                               &&hack - &&foo };goto*(&&foo + array[i]);

4. Inline functions

The inline function can access any visible variable where it is defined, and for the moment we call the function containing the inline function as containing_function, if we have access to the local variable of containing_function in the inline function, It is very likely that we call inline functions outside the containing_function to cause confusion and the consequences are expected. Conversely, it is safe to call this inline function internally in containing_function, the sample code is as follows:

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hack ( int *array, int size) {      void store ( int index, int value)          {array[index] = value;}      intermediate (store, size); }

You can also use tags defined in containing_function in inline functions:

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bar (int *array, int offset, int size){    __label__ failure;    int access (int *array, int index)    {        if (index > size)        goto failure;        return array[index + offset];    }    int i;    /* . . . */    for (i = 0; i < size; i++)        /* . . . */ access (array, i) /* . . . */        /* . . . */    return 0;    /* Control comes here from access    if it detects an error. */  failure:    return -1;}

5. Constructors

There are many built-in functions in the GCC compiler these functions have been __buildin_ started and are not described in detail.

6.typeof

If we need a type, we can use TypeOf, which returns the type of a variable. If you want your program to be compatible with iOS C programs, then you can use __typeof__, the sample code is as follows:

1234	#define max(a,b) \    ({ typeof (a) _a = (a); \    typeof (b) _b = (b); \    _a > _b ? _a : _b; })

More usage is as follows:

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typeof (*x) y;typeof (*x) y;typeof (typeof (char*)[4]) y;//等同于char *y[4];#define pointer(T) typeof(T *)#define array(T, N) typeof(T [N])array (pointer (char), 4) y;

__auto_type can be used in GNU C + +

7. Omit the operand in the conditional expression

If the conditional expression (? :) omits the first operand, then if the condition is true then the value of the expression is both the value of the condition, as follows:

1	x ? : y//如果x不为0，那么该表达式的值为x

8.0 Long arrays

0 long arrays are useful in the last member of a struct, as follows:

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structf1 {    int x; int y[];} f1 = { 1, { 2, 3, 4 } }; struct f2 {    struct f1 f1; intdata[3];} f2 = { { 1 }, { 2, 3, 4 } };

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struct foo { int X; int y[]; struct bar { struct foo Z;}; struct foo a = {1, {2, 3, 4}}; //Legal struct bar B = {{1, {2, 3, 4}}}; //illegal struct bar C = {{1, {}}}; //Legal struct foo d[1] = {{1, {2, 3, 4}}; //illegal

9. Non-fixed initialization

The set initialization of automatic variables does not require constant expressions:

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foo (floatf, float g){    floatbeat_freqs[2] = { f-g, f+g };    /* . . . */}

10. Compound Text Amount

12	structfoo {int a; char b[2];} structure;structure = ((structfoo) {x + y, ’a’, 0});

The above code is equivalent to:

1234	{    structfoo temp = {x + y, ’a’, 0};    structure = temp;}

Again for example:

char **foo = ( char *[]) { " x " }; TD class= "Code" >

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123	staticstruct foo x = (struct foo) {1, ’a’, ’b’};static int y[] = (int []) {1, 2, 3};static int z[] = (int[3]) {1};

The above code is equivalent to:

123	staticstruct foo x = {1, ’a’, ’b’};static int y[] = {1, 2, 3};static intz[] = {1, 0, 0};

11. Specify initialization

This feature is a frequently used feature:

1	inta[6] = { [4] = 29, [2] = 15 };

Equivalent to:

1	inta[6] = { 0, 0, 15, 0, 29, 0 };

Specify the scope and member designation of the struct:

1	intwidths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };

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structpoint { int x, y; };struct point p = { .y = yvalue, .x = xvalue };struct point p = { y: yvalue, x: xvalue }; union foo { int i; double d; };union foo f = { .d = 4 }; structpoint ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };

To be Continued ...

From for notes (Wiz)

A summary of GCC extensions to C