GCC: Preprocessing statements--#if, #elif, #else, #endif和 #ifdef, #ifndef

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Author: Qing Lin, blog name: Flying Air and static crossing

#if语句

#if语句会计算它后面的表达式, and checks the result of the expression, if the result is true (true), the subsequent statement is compiled, and if False (false), the subsequent statement is not compiled.

For example:

#if COUNT
Char *desc = "The Count is Non-zero";
#endif

Compile only if Count is not 0

Char *desc = "The Count is Non-zero";

Statement.

Here are some of the syntax rules for an expression that follows # if:

1) expressions can include shaping constants and macros, which must be assigned (the difference is defined)

2) You can use parentheses to specify the order of the expressions.

3) expressions can contain +,-,*,/,<< and >> arithmetic operators in the C language. These arithmetic operations use the value of the maximum representation of the integer type of the current platform, typically a 64-bit integer value.

4) Expressions can also include <,>,<=,>= and = = comparison operators.

5) expressions can include && and | | The logical operator.

6) The non-(!) operator reverses the result of an expression, such as: #if! (Limxp > 12)

If LIMXP > 12, then the result of the expression is false

7) #if也可以和defined语句一起使用 so that you can check whether a macro has been defined (or you can use #ifdef directly). Such as:

#if defined (MINXP)

This means that if MINXP is defined, then the expression is true. You can also precede the result with a non-(!) operator, such as:

#if!defined (MINXP)

This means that if MINXP is not defined, then the expression is true.

8) If an identifier is not defined, its value is always the 0,-WUNDEF option that can be used to generate this warning message.

9) Macros with parameters (GCC: Preprocessing statements--#define, #error和 #warning) are also 0 values in the expression, and the-WUNDEF option can be used to generate this warning message.

) #else提供另外的选择, such as:

#if mintxt <= 5
#define Mintlog 11
#else
#define Mintlog 14
#endif
One) #elif可以提供一个或多个的选择表达式, such as:

#if mintxt <= 5
#define Mintlog 11
#elif Mintxt = = 6
#define Mintlog 12
#elif Mintxt = = 7
#define Mintlog 13
#else
#define Mintlog 14
#endif

#ifdef

If the macro that follows the same line as the #ifdef is defined, then the statement block after the #ifdef is compiled, and the block ends with #endif. For example:

#ifdef Mintarray
int xarray[20];
#endif/* Mintarray */

#endif后面的注释语句并不是必须的/* Mintarray * *, but it's a good way to do it, so you know which macro you defined is the end position.

and #ifdef the idea of the opposite is #ifndef.

Please refer to the GCC manual for more information.