The _Pragma Operator, Pragma Directives (#pragma)

#pragma directive : Specifies implementation-defined instructions to the compiler.

C90 does not permit a #pragma directive to be produced as the result of a macro expansion.
However, the C99 _Pragma operator enables you to embed a preprocessor macro in a pragma directive,
and _Pragma is permitted in C90 if --strict is not specified. For example:

# define RWDATA(X) PRAGMA(arm section rwdata=#X)
# define PRAGMA(X) _Pragma(#X)
RWDATA(foo)  // same as #pragma arm section rwdata="foo"
int y = 1;   // y is placed in section "foo"

_Pragma("directive") 　其中directive是要满打满算的编译指令。_Pragma运算符允许编译指令参与宏替换。

_Pragma操作符

C99增加了_Pragma操作符，以便更灵活地利用pragma工具。在宏展开之后，下面这种形式的操作符表达式

_Pragma ("字符串常量") 

的处理方式是把字符串常量的内容（在删除两边的双引号，并把字符串常量内部的\"替换为"，把\\替换为\之后）看成是#pragma指令中所出现的预处理器标记。

#pragma指令本身必须出现在同一行中，而且它的预处理器标记并不会执行宏展开，但_Pragma可以被其他表达式所包围，并且可以通过宏展开而产生。

_Pragma("STDC FENV_ACCESS ON") --> #pragma STDC FENV_ACCESS ON

_Pragma ( "align(power)" ) --> #pragma align(power)

General Purpose Pragmas

 

#pragma	Description
#pragma align	Aligns data items within structures.
#pragma alloca	Provides an inline version of the functionalloca(size_t size).
#pragma altivec_vrsave	This pragma is accepted and ignored.
#pragma chars	Sets the sign type of character data.
#pragma comment	Places a comment into the object file.
#pragma complexgcc	Instructs the compiler how to pass parameters when calling complex math functions.
#pragma define	Forces the definition of a template class without actually defining an object of the class.
#pragma disjoint	Lists the identifiers that are not aliased to each other within the scope of their use.
#pragma enum	Specifies the size of enum variables that follow.
#pragma execution_frequency	Lets you mark program source code that you expect will be either very frequently or very infrequently executed.
#pragma hashome	Informs the compiler that the specified class has a home module that will be specified by the IsHomepragma.
#pragma ibm snapshot	Sets a debugging breakpoint at the point of the pragma, and defines a list of variables to examine when program execution reaches that point.
#pragma implementation	Tells the compiler the name of the file containing the function-template definitions that correspond to the template declarations in the include file which contains the pragma.
#pragma info	Controls the diagnostic messages generated by theinfo(...) compiler options.
#pragma ishome	Informs the compiler that the specified class's home module is the current compilation unit.
#pragma isolated_call	Lists functions that do not alter data objects visible at the time of the function call.
#pragma langlvl	Selects the C or C++ language level for compilation.
#pragma leaves	Takes a function name and specifies that the function never returns to the instruction after the function call.
#pragma map	Tells the compiler that all references to an identifier are to be converted to a new name.
#pragma mc_func	Lets you define a function containing a short sequence of machine instructions.
#pragma options	Specifies options to the compiler in your source program.
#pragma option_override	Specifies alternate optimization options for specific functions.
#pragma pack	Modifies the current alignment rule for members of structures that follow this pragma.
#pragma priority	Specifies the order in which static objects are to be initialized at run time.
#pragma reachable	Declares that the point after the call to a routine marked reachable can be the target of a branch from some unknown location.
#pragma reg_killed_by	Specifies those registers which value will be corrupted by the specified function. It must be used together with #pragma mc_func.
#pragma report	Controls the generation of specific messages.
#pragma strings	Sets storage type for strings.
#pragma unroll	Unrolls inner loops in the program. This can help improve program performance.