C语言宏的使用

使用条件宏进行条件编译

譬如,对于同一份代码,我想编译出两个不同的版本,在其中一个版本中去掉某一部分功能,

这时可以通过条件宏判断是否编译,例:



如果不使用条件宏进行控制,想编译两个不同版本的程序,就需要保存两份源代码。

条件编译的语法和if else语法类似,必须以#endif结尾例如:

#if 常量表达式
 //代码1
#elif 常量表达式
 //代码2
#elif 常量表达式
 //代码3
#else
 //代码4
#endif

条件编译时将会根据常量表达式来求值,如果常量表达式的值为假,那么对应的代码将不会被 编译,还可以判断某个宏是否定义,例如:

#ifdef(NameOfMacro)
     //代码1
#else
     //代码2
#endif

如果NameOfMacro被定义,那么代码1将会参与编译,否则代码2参与编译,如下写法也是等价的:

#if defined(NameOfMacro)
     //代码1
#else
     //代码2
#endif

#if !defined(NameOfMacro)
     //代码2
#else
     //代码1
#endif

宏一般都定义在头文件中,要想使用改宏则使用#include包含该头文件即可,也可以定义在源文件中,但是这种情况比较少,一定要确保条件判断中的宏的定义位置位于条件判断之前,否则不会生效;宏的作用范围从定义它的位置开始到源文件结束,在其他源文件中该宏不可见,如果想使用某个宏,则必须使用#include指令包含对应的头文件。

除了在头文件和源文件中定义宏,还可以在定义预处理宏,

例如:在VS项目属性中



预处理宏只能是空的宏,但预处理宏作用于整个工程,对该工程中任意源文件可见,优先于任何定义在源码中的宏.

使用空宏进行说明

可以使用空宏对函数参数进行说明,微软的API中就使用了空宏,例如:

   WINBASEAPI
   BOOL
   WINAPI
   WriteFileEx(
             _In_ HANDLE hFile,
             _In_reads_bytes_opt_(nNumberOfBytesToWrite) LPCVOID lpBuffer,
             _In_ DWORD nNumberOfBytesToWrite,
             _Inout_ LPOVERLAPPED lpOverlapped,
             _In_ LPOVERLAPPED_COMPLETION_ROUTINE lpCompletionRoutine
             );

In,_Inout_定义如下:



_In_表示该参数为传入参数,Inout_表改参数为传入传出参数,为了防止_In,_Inout_被

别人定了,所以先使用#ifdef判断该宏是否被定义,如果被定义则使用#undef取消定义,然后

在重新定义该宏。

#和##运算符

除了#define 标识符 记号序列 这种形式定义宏,还可以定义带有参数的宏,形式如下:

#define 标识符(标识符列表(可选)) 记号序列,

在定义带有形参的宏定义中,#和##将会影响宏替换的过程,如果记号序列中某一个参数前

面有一个#,那么替换时将会在这个参数前后插入",将其变成一个字符串,即使该参数是

另一个带参宏,也不会在进行宏展开替换;如果该参数为一个字符串常量,那么预处理器会

分别为字符串前后的"号进行转义,将"变成字符,然后在前后各插入一个";

例:

 #define TEST0(p1) #p1"333"
 #define TEST(p1,p2,p3) #p1#p2 #p3
 int main()
 {
   int nTest1;
   int nTest2;
   int nTest3;
   printf("%s", TEST(TEST0(0),"333",nTest3));
   return 0;
 }

运行结果:

从上面的例子可以看出#运算符主要作用就是将宏定义中的参数变成字符串,而##运算符的

作用就是将宏定义中相邻的两个的参数连接在一起组成一个标识符,如果该标识符不符合C

语言表示符的组成规则,那么编译时会报错；

例如:

对于使用了##和#的宏定义,预处理器需要进行反复扫描分析记号序列,以此来查找已定义的

标识符进行替换,如果一个表示符在某次扫描中被替换后,再次扫描遇到此标识符时,则不在

进行替换。

例:

 #define TEST(p1,p2) p1##p2
 int main()
 {
   int nTest1;
   int nTest2;
   int nTest3;
   int TEST(TEST(nTest1, nTest2), nTest3) = 3;
   printf("%d", TEST(TEST(nTest1, nTest2), nTest3));
   return 0;
 }

使用/P命令查看编译预处理后的结果:

 int main()
 {
   int nTest1;
   int nTest2;
   int nTest3;
   int TEST(nTest1, nTest2)nTest3 = 3;
   printf("%d", TEST(nTest1, nTest2)nTest3);
   return 0;
 }

可以看出TEST(TEST(nTest1, nTest2), nTest3)第一次展开后为TEST(nTest1, nTest2)nTest3,

然后又遇到了TEST宏定义,则不在进行展开,直接将TEST(nTest1, nTest2)nTest3作为最终结果,但是

TEST(nTest1, nTest2)nTest3显然不符合C语言表示符的定义,所以编译时会报错。

注意:/P命令使用完成后,必须从命令行中删除它,否则编译的时候会报找不到xxxxx.obj错误

#和##运算符是C语言宏定义中两个比较有创造力的运算符,可以创造出许多"黑魔法"宏定义

预定义的宏

C语言中有些预定义的宏,这些宏不能取消定义和重定义:

code[class*="language-"], pre[class*="language-"] { color: rgba(51, 51, 51, 1); font-family: Consolas, "Liberation Mono", Menlo, Courier, monospace; text-align: left; white-space: pre; word-spacing: normal; word-break: normal; word-wrap: normal; line-height: 1.4; -moz-tab-size: 8; -o-tab-size: 8; tab-size: 8; -webkit-hyphens: none; -moz-hyphens: none; -ms-hyphens: none; hyphens: none }
pre[class*="language-"] { padding: 0.8em; overflow: auto; border-radius: 3px; background: rgba(245, 245, 245, 1) }
:not(pre)>code[class*="language-"] { padding: 0.1em; border-radius: 0.3em; white-space: normal; background: rgba(245, 245, 245, 1) }
.token.comment, .token.blockquote { color: rgba(150, 152, 150, 1) }
.token.cdata { color: rgba(24, 54, 145, 1) }
.token.doctype, .token.punctuation, .token.variable, .token.macro.property { color: rgba(51, 51, 51, 1) }
.token.operator, .token.important, .token.keyword, .token.rule, .token.builtin { color: rgba(167, 29, 93, 1) }
.token.string, .token.url, .token.regex, .token.attr-value { color: rgba(24, 54, 145, 1) }
.token.property, .token.number, .token.boolean, .token.entity, .token.atrule, .token.constant, .token.symbol, .token.command, .token.code { color: rgba(0, 134, 179, 1) }
.token.tag, .token.selector, .token.prolog { color: rgba(99, 163, 92, 1) }
.token.function, .token.namespace, .token.pseudo-element, .token.class, .token.class-name, .token.pseudo-class, .token.id, .token.url-reference .token.variable, .token.attr-name { color: rgba(121, 93, 163, 1) }
.token.entity { cursor: help }
.token.title, .token.title .token.punctuation { font-weight: bold; color: rgba(29, 62, 129, 1) }
.token.list { color: rgba(237, 106, 67, 1) }
.token.inserted { background-color: rgba(234, 255, 234, 1); color: rgba(85, 165, 50, 1) }
.token.deleted { background-color: rgba(255, 236, 236, 1); color: rgba(189, 44, 0, 1) }
.token.bold { font-weight: bold }
.token.italic { font-style: italic }
.language-json .token.property { color: rgba(24, 54, 145, 1) }
.language-markup .token.tag .token.punctuation { color: rgba(51, 51, 51, 1) }
code.language-css, .language-css .token.function { color: rgba(0, 134, 179, 1) }
.language-yaml .token.atrule { color: rgba(99, 163, 92, 1) }
code.language-yaml { color: rgba(24, 54, 145, 1) }
.language-ruby .token.function { color: rgba(51, 51, 51, 1) }
.language-markdown .token.url { color: rgba(121, 93, 163, 1) }
.language-makefile .token.symbol { color: rgba(121, 93, 163, 1) }
.language-makefile .token.variable { color: rgba(24, 54, 145, 1) }
.language-makefile .token.builtin { color: rgba(0, 134, 179, 1) }
.language-bash .token.keyword { color: rgba(0, 134, 179, 1) }
pre[data-line] { position: relative; padding: 1em 0 1em 3em }
pre[data-line] .line-highlight-wrapper { position: absolute; top: 0; left: 0; background-color: rgba(0, 0, 0, 0); display: block; width: 100% }
pre[data-line] .line-highlight { position: absolute; left: 0; right: 0; margin-top: 1em; background: linear-gradient(90deg, rgba(153, 122, 102, 0.1) 70%, rgba(153, 122, 102, 0)); pointer-events: none; line-height: inherit; white-space: pre }
pre[data-line] .line-highlight:before, pre[data-line] .line-highlight[data-end]:after { content: attr(data-start); position: absolute; top: 0.4em; left: 0.6em; min-width: 1em; padding: 0 0.5em; background-color: rgba(153, 122, 102, 0.4); color: rgba(245, 242, 240, 1); font: bold 65% / 1.5 sans-serif; text-align: center; vertical-align: 0.3em; border-radius: 999px; text-shadow: none; box-shadow: 0 1px rgba(255, 255, 255, 1) }
pre[data-line] .line-highlight[data-end]:after { content: attr(data-end); top: auto; bottom: 0.4em }
html body { font-family: "Helvetica Neue", Helvetica, "Segoe UI", Arial, freesans, sans-serif; font-size: 16px; line-height: 1.6; color: rgba(51, 51, 51, 1); background-color: rgba(255, 255, 255, 1); overflow: initial; box-sizing: border-box; word-wrap: break-word }
html body>:first-child { margin-top: 0 }
html body h1, html body h2, html body h3, html body h4, html body h5, html body h6 { line-height: 1.2; margin-top: 1em; margin-bottom: 16px; color: rgba(0, 0, 0, 1) }
html body h1 { font-size: 2.25em; font-weight: 300; padding-bottom: 0.3em }
html body h2 { font-size: 1.75em; font-weight: 400; padding-bottom: 0.3em }
html body h3 { font-size: 1.5em; font-weight: 500 }
html body h4 { font-size: 1.25em; font-weight: 600 }
html body h5 { font-size: 1.1em; font-weight: 600 }
html body h6 { font-size: 1em; font-weight: 600 }
html body h1, html body h2, html body h3, html body h4, html body h5 { font-weight: 600 }
html body h5 { font-size: 1em }
html body h6 { color: rgba(92, 92, 92, 1) }
html body strong { color: rgba(0, 0, 0, 1) }
html body del { color: rgba(92, 92, 92, 1) }
html body a:not([href]) { color: inherit; text-decoration: none }
html body a { color: rgba(0, 136, 204, 1); text-decoration: none }
html body a:hover { color: rgba(0, 163, 245, 1); text-decoration: none }
html body img { max-width: 100% }
html body>p { margin-top: 0; margin-bottom: 16px; word-wrap: break-word }
html body>ul, html body>ol { margin-bottom: 16px }
html body ul, html body ol { padding-left: 2em }
html body ul.no-list, html body ol.no-list { padding: 0; list-style-type: none }
html body ul ul, html body ul ol, html body ol ol, html body ol ul { margin-top: 0; margin-bottom: 0 }
html body li { margin-bottom: 0 }
html body li.task-list-item { list-style: none }
html body li>p { margin-top: 0; margin-bottom: 0 }
html body .task-list-item-checkbox { margin: 0 0.2em 0.25em -1.8em; vertical-align: middle }
html body .task-list-item-checkbox:hover { cursor: pointer }
html body blockquote { margin: 16px 0; font-size: inherit; padding: 0 15px; color: rgba(92, 92, 92, 1); border-left: 4px solid rgba(214, 214, 214, 1) }
html body blockquote>:first-child { margin-top: 0 }
html body blockquote>:last-child { margin-bottom: 0 }
html body hr { height: 4px; margin: 32px 0; background-color: rgba(214, 214, 214, 1); border: 0 none }
html body table { margin: 10px 0 15px; border-collapse: collapse; border-spacing: 0; display: block; width: 100%; overflow: auto; word-break: keep-all }
html body table th { font-weight: bold; color: rgba(0, 0, 0, 1) }
html body table td, html body table th { border: 1px solid rgba(214, 214, 214, 1); padding: 6px 13px }
html body dl { padding: 0 }
html body dl dt { padding: 0; margin-top: 16px; font-size: 1em; font-style: italic; font-weight: bold }
html body dl dd { padding: 0 16px; margin-bottom: 16px }
html body code { font-family: Menlo, Monaco, Consolas, "Courier New", monospace; font-size: 0.85em !important; color: rgba(0, 0, 0, 1); background-color: rgba(240, 240, 240, 1); border-radius: 3px; padding: 0.2em 0 }
html body code::before, html body code::after { letter-spacing: -0.2em; content: " " }
html body pre>code { padding: 0; margin: 0; font-size: 0.85em !important; word-break: normal; white-space: pre; background: rgba(0, 0, 0, 0); border: 0 }
html body .highlight { margin-bottom: 16px }
html body .highlight pre, html body pre { padding: 1em; overflow: auto; font-size: 0.85em !important; line-height: 1.45; border: rgba(214, 214, 214, 1); border-radius: 3px }
html body .highlight pre { margin-bottom: 0; word-break: normal }
html body pre code, html body pre tt { display: inline; max-width: initial; padding: 0; margin: 0; overflow: initial; line-height: inherit; word-wrap: normal; background-color: rgba(0, 0, 0, 0); border: 0 }
html body pre code:before, html body pre tt:before, html body pre code:after, html body pre tt:after { content: normal }
html body p, html body blockquote, html body ul, html body ol, html body dl, html body pre { margin-top: 0; margin-bottom: 16px }
html body kbd { color: rgba(0, 0, 0, 1); border-top: 1px solid rgba(214, 214, 214, 1); border-right: 1px solid rgba(214, 214, 214, 1); border-bottom: 2px solid rgba(199, 199, 199, 1); border-left: 1px solid rgba(214, 214, 214, 1); padding: 2px 4px; background-color: rgba(240, 240, 240, 1); border-radius: 3px }
@media print { html body { background-color: rgba(255, 255, 255, 1) } html body h1, html body h2, html body h3, html body h4, html body h5, html body h6 { color: rgba(0, 0, 0, 1); page-break-after: avoid } html body blockquote { color: rgba(92, 92, 92, 1) } html body pre { page-break-inside: avoid } html body table { display: table } html body img { display: block; max-width: 100%; max-height: 100% } html body pre, html body code { word-wrap: break-word; white-space: pre } }
.markdown-preview { width: 100%; height: 100%; box-sizing: border-box }
.markdown-preview .pagebreak, .markdown-preview .newpage { page-break-before: always }
.markdown-preview pre.line-numbers { position: relative; padding-left: 3.8em; counter-reset: linenumber 0 }
.markdown-preview pre.line-numbers>code { position: relative }
.markdown-preview pre.line-numbers .line-numbers-rows { position: absolute; pointer-events: none; top: 1em; font-size: 100%; left: 0; width: 3em; letter-spacing: -1px; border-right: 1px solid rgba(153, 153, 153, 1); -webkit-user-select: none; -moz-user-select: none; -ms-user-select: none; user-select: none }
.markdown-preview pre.line-numbers .line-numbers-rows>span { pointer-events: none; display: block; counter-increment: linenumber 1 }
.markdown-preview pre.line-numbers .line-numbers-rows>span:before { content: counter(linenumber); color: rgba(153, 153, 153, 1); display: block; padding-right: 0.8em; text-align: right }
.markdown-preview .mathjax-exps .MathJax_Display { text-align: center !important }
.markdown-preview:not([for="preview"]) .code-chunk .btn-group { display: none }
.markdown-preview:not([for="preview"]) .code-chunk .status { display: none }
.markdown-preview:not([for="preview"]) .code-chunk .output-div { margin-bottom: 16px }
{ width: 8px }
{ border-radius: 10px; background-color: rgba(0, 0, 0, 0) }
{ border-radius: 5px; background-color: rgba(150, 150, 150, 0.66); border: 4px solid rgba(150, 150, 150, 0.66); background-clip: content-box }
html body[for="html-export"]:not([data-presentation-mode]) { position: relative; width: 100%; height: 100%; top: 0; left: 0; margin: 0; padding: 0; overflow: auto }
html body[for="html-export"]:not([data-presentation-mode]) .markdown-preview { position: relative; top: 0 }
@media screen and (min-width: 914px) { html body[for="html-export"]:not([data-presentation-mode]) .markdown-preview { padding: 2em calc(50% - 457px + 2em) } }
@media screen and (max-width: 914px) { html body[for="html-export"]:not([data-presentation-mode]) .markdown-preview { padding: 2em } }
@media screen and (max-width: 450px) { html body[for="html-export"]:not([data-presentation-mode]) .markdown-preview { font-size: 14px !important; padding: 1em } }
@media print { html body[for="html-export"]:not([data-presentation-mode]) #sidebar-toc-btn { display: none } }
html body[for="html-export"]:not([data-presentation-mode]) #sidebar-toc-btn { position: fixed; bottom: 8px; left: 8px; font-size: 28px; cursor: pointer; color: inherit; z-index: 99; width: 32px; text-align: center; opacity: 0.4 }
html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] #sidebar-toc-btn { opacity: 1 }
html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .md-sidebar-toc { position: fixed; top: 0; left: 0; width: 300px; height: 100%; padding: 32px 0 48px; font-size: 14px; box-shadow: 0 0 4px rgba(150, 150, 150, 0.33); box-sizing: border-box; overflow: auto; background-color: inherit }
{ width: 8px }
{ border-radius: 10px; background-color: rgba(0, 0, 0, 0) }
{ border-radius: 5px; background-color: rgba(150, 150, 150, 0.66); border: 4px solid rgba(150, 150, 150, 0.66); background-clip: content-box }
html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .md-sidebar-toc a { text-decoration: none }
html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .md-sidebar-toc ul { padding: 0 1.6em; margin-top: 0.8em }
html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .md-sidebar-toc li { margin-bottom: 0.8em }
html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .md-sidebar-toc ul { list-style-type: none }
html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .markdown-preview { left: 300px; width: calc(100% - 300px); padding: 2em calc(50% - 457px - 150px); margin: 0; box-sizing: border-box }
@media screen and (max-width: 1274px) { html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .markdown-preview { padding: 2em } }
@media screen and (max-width: 450px) { html body[for="html-export"]:not([data-presentation-mode])[html-show-sidebar-toc] .markdown-preview { width: 100% } }
html body[for="html-export"]:not([data-presentation-mode]):not([html-show-sidebar-toc]) .markdown-preview { left: 50%; transform: translateX(-50%) }
html body[for="html-export"]:not([data-presentation-mode]):not([html-show-sidebar-toc]) .md-sidebar-toc { display: none }