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閱讀ffmpeg源碼是 發(fā)現(xiàn)一些函數(shù)前面加了 attribute_deprecated 屬性;如:attribute_deprecated int url_fopen( AVIOContext **s, const char *url, int flags);在libavutil/attributes.h 中有如下定義:
__attribute__ 語法為GNU C 的特性,__attribute__可以設(shè)置函數(shù)屬性(Function Attribute)、變量屬性(Variable Attribute)和類型屬性(Type Attribute)。__attribute__語法格式為:__attribute__ ((attribute))需要注意的是: 使用__attribute__的時候,只能函數(shù)的聲明處使用__attribute__,并且在“;“前。
在開發(fā)一些庫的時候,API的接口可能會過時,為了提醒開發(fā)者這個函數(shù)已經(jīng)過時。只要函數(shù)被使用,在編譯是都會產(chǎn)生警告,警告信息中包含過時接口的名稱及代碼中的引用位置。下面是GNU 網(wǎng)站(http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Function-Attributes.html)上對這個屬性的解釋:deprecatedThe deprecated attribute results in a warning if the function is used anywhere in the source file. This is useful when identifying functions that are expected to be removed in a future version of a program. The warning also includes the location of the declaration of the deprecated function, to enable users to easily find further information about why the function is deprecated, or what they should do instead. Note that the warnings only occurs for uses: int old_fn () __attribute__ ((deprecated)); int old_fn (); int (*fn_ptr)() = old_fn; results in a warning on line 3 but not line 2.下面是一個列子:root@wang:/work/wanghuan/gnu# cat gnu.c
root@wang:/work/wanghuan/gnu# gcc gnu.c -o gnu gnu.c: In function ‘main’:gnu.c:12: warning: ‘attribute’ is deprecated (declared at gnu.c:5) //編譯警告root@wang:/work/wanghuan/gnu# ./gnu GNU attribute
關(guān)于__attribute__屬性,有多種類型,由于ARM編譯器支持GNU語法,在ARM的網(wǎng)站http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0348bc/Caccahah.html 有這些特性的詳細介紹。
posted on 2013-01-29 16:05 楊粼波 閱讀(795) 評論(0) 編輯 收藏 引用
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