Pragma是什么? Pragma是什么?
翻譯:SkyJacker
后附英文原文。


(譯者注:
一句話，pragma就是為了讓編譯器編譯出的C或C++程序與機器硬件和操作系統保持完全兼容而定義的宏擴展，
#pragma是和特定編譯器相關的。)
兩部分:
1.Pragma說明;
2.Pragma的語法。

一、Pragma說明(Directives)

C和C++程序的每次執行都支持其所在的主機或操作系統所具有的一些獨特的特點。
一些程序,例如，需要精確控制數據存放的內存區域或控制某個函數接收的參數。
#pragma指示為編譯器提供了一種在不同機器和操作系統上編譯以保持C和C++完全兼容的方法。?
Pragmas是由機器和相關的操作系統定義的，通常對每個編譯器來說是不同的。


二、語法(Syntax)

#pragma token-string(特征字符串)

特征字符串是一連串的字符，就是要給一個特定編譯器提供說明和編譯意見。

符號(#)必須是pragma所在那一行的第一個非空格字符;
#號和pragma之間可以有任意個空格符。
在#pragma之后，是可以被編譯器解析的預處理特征字符。
一般認為,#pragma屬于宏擴展。
如果編譯器發現不認識的pragma,會提出警告，但繼續編譯下去。

Pragmas可以用在條件聲明上，提供最新的功能性的預處理程序，或者提供給編譯器定義執行的信息。
C和C++編譯器認可如下pragmas:
alloc_text
comment //注釋
init_seg1?
optimize? //最優化
auto_inline
component? //組成部件
inline_depth
pack?????? //包
bss_seg
data_seg
inline_recursion? //內嵌遞歸
pointers_to_members1
check_stack
function???
intrinsic? //內在的
setlocale
code_seg
hdrstop
message?
vtordisp1
const_seg
include_alias
once
warning


這是MSDN的一篇文章，原作者曾經想使用
#pragma pack(1) // 用GCC在MIPS平臺上將結構體成員結合到一塊連續的內存塊，但是沒有做到。

在linux環境下使用intel-based GCC,#pragma pack(1)可以工作。

建議參考具體編譯器的文檔，在里面應該有pragma的說明。

---------------------------------------原文-------------------------------------------

Pragma Directives
Each implementation of C and C++ supports some features unique to its host machine or operating system.

Some programs, for instance, need to exercise precise control over the memory areas where data is placed or

to control the way certain functions receive parameters.
The #pragma directives offer a way for each compiler
to offer machine- and operating-system-specific features
while retaining overall compatibility with the C and C++
languages. Pragmas are machine- or operating-system-specific by definition,
and are usually different for every compiler.

Syntax

#pragma token-string

The token-string is a series of characters that gives a specific compiler instruction and arguments,
if any.

The number sign (#) must be the first non-white-space character on the line containing the pragma;

white-space characters can separate the number sign and the word pragma.
Following #pragma, write any text that the translator can parse as preprocessing tokens.
The argument to #pragma is subject to macro expansion.

If the compiler finds a pragma it does not recognize, it issues a warning, but compilation continues.

Pragmas can be used in conditional statements, to provide new preprocessor functionality,
or to provide implementation-defined information to the compiler.
The C and C++ compilers recognize the following pragmas:

alloc_text comment init_seg1 optimize
auto_inline component inline_depth pack
bss_seg data_seg inline_recursion pointers_to_members1
check_stack function intrinsic setlocale
code_seg hdrstop message vtordisp1
const_seg include_alias once warning


This is an article from MSDN, I ever wanted to use
#pragma pack(1) //which can combine structure members to one continuous memory block
on MIPS platform using GCC, but it doesn't work.


See the compiler's document, it should be illustrated there.

Under linux env using intel-based GCC, it works.

總結

以上是生活随笔為你收集整理的Pragma是什么?的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。