User manual REDHAT ENTERPRISE LINUX 4

[. . . ] Red Hat Enterprise Linux 4 Red Hat Enterprise Linux 4 Copyright © 1987, 1989, 1991-2004 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1. 2 or any later version published by the Free Software Foundation; with the Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled "GNU Free Documentation License". (a) The FSF's Front-Cover Text is: A GNU Manual (b) The FSF's Back-Cover Text is: You have freedom to copy and modify this GNU Manual, like GNU software. [. . . ] A local label declaration looks like this: __label__ label; or __label__ label1, label2, /* . . . */; Local label declarations must come at the beginning of the block, before any ordinary declarations or statements. Chapter 6. Extensions to the C Language Family 133 The label declaration defines the label name, but does not define the label itself. You must do this in the usual way, with label:, within the statements of the statement expression. The local label feature is useful for complex macros. If a macro contains nested loops, a goto can be useful for breaking out of them. However, an ordinary label whose scope is the whole function cannot be used: if the macro can be expanded several times in one function, the label will be multiply defined in that function. For example: #define SEARCH(value, array, target) do { __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:; } while (0) \ \ \ \ \ \ \ \ \ \ \ \ \ This could also be written using a statement-expression: #define SEARCH(array, target) ({ __label__ found; typeof (target) _SEARCH_target = (target); typeof (*(array)) *_SEARCH_array = (array); int i, j; int value; max; i++) for (i = 0; i for (j = 0; j max; j++) if (_SEARCH_array[i][j] == _SEARCH_target) { value = i; goto found; } value = -1; found: value; }) \ \ \ \ \ \ \ \ \ \ \ \ \ \ Local label declarations also make the labels they declare visible to nested functions, if there are any. Section 6. 4 Nested Functions, for details. 6. 3. Labels as Values You can get the address of a label defined in the current function (or a containing function) with the unary operator &&. This value is a constant and can be used wherever a constant of that type is valid. */ ptr = &&foo; 2 2 2 2 134 Chapter 6. Extensions to the C Language Family To use these values, you need to be able to jump to one. This is done with the computed goto statement1, goto *exp;. For example, goto *ptr; Any expression of type void * is allowed. One way of using these constants is in initializing a static array that will serve as a jump table: static void *array[] = { &&foo, &&bar, &&hack }; Then you can select a label with indexing, like this: goto *array[i]; Note that this does not check whether the subscript is in bounds--array indexing in C never does that. Such an array of label values serves a purpose much like that of the switch statement. The switch statement is cleaner, so use that rather than an array unless the problem does not fit a switch statement very well. Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching. You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. [. . . ] Even a precise fraction "of the profits from this disk" is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all. Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. [. . . ]