gcc_extensions
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gcc_extensions [2019/07/25 12:53] rpjday [Case ranges] |
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| ==== Statements and declarations in expressions ==== | ==== Statements and declarations in expressions ==== | ||
| + | |||
| + | A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression. | ||
| <code> | <code> | ||
| Line 11: | Line 13: | ||
| ({int _a = (a), _b = (b); _a > _b ? _a : _b; }) | ({int _a = (a), _b = (b); _a > _b ? _a : _b; }) | ||
| </code> | </code> | ||
| + | |||
| + | ==== Local labels ==== | ||
| + | |||
| + | GCC allows you to declare local labels in any nested block scope. A local label is just like an ordinary label, but you can only reference it (with a goto statement, or by taking its address) within the block in which it is declared. | ||
| + | |||
| + | <code> | ||
| + | __label__ label; | ||
| + | </code> | ||
| + | |||
| + | ==== Labels as values ==== | ||
| + | |||
| + | You can get the address of a label defined in the current function (or a containing function) with the unary operator ‘&&’. The value has type void *. This value is a constant and can be used wherever a constant of that type is valid. | ||
| + | |||
| + | <code> | ||
| + | void *ptr; | ||
| + | /* … */ | ||
| + | ptr = &&foo; | ||
| + | ... | ||
| + | goto *ptr; | ||
| + | </code> | ||
| + | |||
| + | ==== Nested functions ==== | ||
| + | |||
| + | <code> | ||
| + | foo (double a, double b) | ||
| + | { | ||
| + | double square (double z) { return z * z; } | ||
| + | |||
| + | return square (a) + square (b); | ||
| + | } | ||
| + | </code> | ||
| + | |||
| + | See "trampolines." | ||
| + | |||
| + | ==== Nonlocal gotos ==== | ||
| + | |||
| + | Use ''%%__builtin_setjmp%%'' and ''%%__builtin_longjmp%%''. | ||
| + | |||
| + | ==== Constructing function calls ==== | ||
| + | |||
| + | ==== typeof ==== | ||
| + | |||
| + | <code> | ||
| + | #define max(a,b) \ | ||
| + | ({ typeof (a) _a = (a); \ | ||
| + | typeof (b) _b = (b); \ | ||
| + | _a > _b ? _a : _b; }) | ||
| + | </code> | ||
| + | |||
| + | ==== Conditionals ==== | ||
| + | |||
| + | Equivalent: | ||
| + | |||
| + | <code> | ||
| + | x ? x : y | ||
| + | x ? : y | ||
| + | </code> | ||
| + | |||
| + | ==== Non-constant initializers ==== | ||
| + | |||
| + | ==== Designated initializers ==== | ||
| + | |||
| + | ==== Case ranges ==== | ||
| + | |||
| + | <code> | ||
| + | case low ... high: | ||
| + | </code> | ||
| + | |||
| + | Don't forget spaces. | ||
| + | |||
| + | |||
| + | |||
gcc_extensions.txt · Last modified: 2019/07/25 12:53 by rpjday
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