blocks|key|173419|text|复制case常量是一个确保在所有C编译器中工作的技巧，无论每个编译器如何报告错误。对于可视化C%2B%2B，它很简单：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|173420|struct+X+{
++++int+a,b;
++++int+c[10];
};
int+_tmain(int+argc,+_TCHAR*+argv[])
{
++++int+dummy;

++++switch+(dummy)+{
++++case+sizeof(X):
++++case+sizeof(X):
++++++++break;
++++}
++++return+0;
}|code-block|syntax|javascript|173421|编译结果：|173422|+------+Build+started:+Project:+cpptest,+Configuration:+Debug+Win32+------
+cpptest.cpp+c:\work\cpptest\cpptest\cpptest.cpp(29):+error+C2196:+case+value+'48'+already+used
+==========+Build:+0+succeeded,+1+failed,+0+up-to-date,+0+skipped+==========|173423|因此结构X的大小是48|173424|编辑(2020年6月3日)：对于gcc或其他只打印“重复大小写值”的编译器，我使用这个技巧来缩小值的范围：|173425|1)添加案例值1==2+(表示false)|173426|2)通过反复试验，缩小值范围，例如，我尝试猜测sizeof(X)+>16：|173427|#include+<stdio.h>
typedef+struct+_X+{
++++int+a;
++++char+b[10];
}+X;
int+main()
{
++++printf("Hello+World");

++++int+dummy=0+++;
++++switch+(dummy)+{
++++case++1==2:
++++case+sizeof(+X)>16:
++++//case+16:
++++break;
++++}
++++return+0;
}|173428|结果：|173429|main.c:+In+function+‘main’:
main.c:14:5:+error:+duplicate+case+value
+++++case+sizeof(+X)>16:
+++++%5E~~~
main.c:13:5:+error:+previously+used+here
+++++case++1==2:|173430|所以它是假的，即sizeof(X)<=16。|173431|3)对其他一些合理的值进行重复。例如，试着猜测它是16，即sizeof(X)==16。如果它不抱怨重复的case值。则表达式为真。|173432|4)可选地添加一个case+16来验证它，例如|173433|#include+<stdio.h>
typedef+struct+_X+{
++++int+a;
++++char+b[10];
}+X;
int+main()
{
++++printf("Hello+World");

++++int+dummy=0+++;
++++switch+(dummy)+{
+++//+case++1==2:
++++case+sizeof(+X):
++++case+16:
++++break;
++++}
++++return+0;
}|173434|结果|173435|main.c:+In+function+‘main’:
main.c:15:5:+error:+duplicate+case+value
+++++case+16:
+++++%5E~~~
main.c:14:5:+error:+previously+used+here
+++++case+sizeof(+X):|173436|确认sizeof(X)为16。|173437|或者，可以观察到gcc可以报告多个重复，因此此技巧可以在单次传递中进行多个猜测：|173438|#include+<stdio.h>
typedef+struct+_X+{
++++int+a;
++++char+b[10];
}+X;
int+main()
{
++++printf("Hello+World");

++++int+dummy=0+++;
++++switch+(dummy)+{
++++case++1==2:+//represents+false
++++case+1==1:+//represents+true
++++case+sizeof(+X)>10:
++++case+sizeof(+X)>12:
++++case+sizeof(+X)>14:
++++case+sizeof(+X)>16:
++++case++sizeof(+X)==16:
++++//case+16:
++++break;
++++}
++++return+0;
}|173439|173440|main.c:+In+function+‘main’:
main.c:14:5:+error:+duplicate+case+value
+++++case+sizeof(+X)>10:
+++++%5E~~~
main.c:13:5:+error:+previously+used+here
+++++case+1==1:
+++++%5E~~~
main.c:15:5:+error:+duplicate+case+value
+++++case+sizeof(+X)>12:
+++++%5E~~~
main.c:13:5:+error:+previously+used+here
+++++case+1==1:
+++++%5E~~~
main.c:16:5:+error:+duplicate+case+value
+++++case+sizeof(+X)>14:
+++++%5E~~~
main.c:13:5:+error:+previously+used+here
+++++case+1==1:
+++++%5E~~~
main.c:17:5:+error:+duplicate+case+value
+++++case+sizeof(+X)>16:
+++++%5E~~~
main.c:12:5:+error:+previously+used+here
+++++case++1==2:
+++++%5E~~~
main.c:18:5:+error:+duplicate+case+value
+++++case++sizeof(+X)==16:
+++++%5E~~~
main.c:13:5:+error:+previously+used+here
+++++case+1==1:
+++++%5E~~~|173441|建议sizeof(X)+>10，>12，>14，但不是>16。==16被添加为最终猜测。|173442|entityMap^0|2|4|0|0|0|0|0|0|0|N|9|0|0|0|0|0|T|D|0|9|7|0|0|0|0|0|0|0|0|0|2|9|0^^$0|@$1|2|3|4|5|6|7|1R|8|@$9|1S|A|1T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|1U|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|1V|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|1W|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|1X|8|@]|D|@]|E|$]]|$1|Q|3|R|5|6|7|1Y|8|@]|D|@]|E|$]]|$1|S|3|T|5|6|7|1Z|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|20|8|@$9|21|A|22|B|C]]|D|@]|E|$]]|$1|W|3|X|5|H|7|23|8|@]|D|@]|E|$I|J]]|$1|Y|3|Z|5|6|7|24|8|@]|D|@]|E|$]]|$1|10|3|11|5|H|7|25|8|@]|D|@]|E|$I|J]]|$1|12|3|13|5|6|7|26|8|@]|D|@]|E|$]]|$1|14|3|15|5|6|7|27|8|@$9|28|A|29|B|C]]|D|@]|E|$]]|$1|16|3|17|5|6|7|2A|8|@$9|2B|A|2C|B|C]]|D|@]|E|$]]|$1|18|3|19|5|H|7|2D|8|@]|D|@]|E|$I|J]]|$1|1A|3|1B|5|6|7|2E|8|@]|D|@]|E|$]]|$1|1C|3|1D|5|H|7|2F|8|@]|D|@]|E|$I|J]]|$1|1E|3|1F|5|6|7|2G|8|@]|D|@]|E|$]]|$1|1G|3|1H|5|6|7|2H|8|@]|D|@]|E|$]]|$1|1I|3|1J|5|H|7|2I|8|@]|D|@]|E|$I|J]]|$1|1K|3|1B|5|6|7|2J|8|@]|D|@]|E|$]]|$1|1L|3|1M|5|H|7|2K|8|@]|D|@]|E|$I|J]]|$1|1N|3|1O|5|6|7|2L|8|@$9|2M|A|2N|B|C]]|D|@]|E|$]]|$1|1P|3|-4|5|6|7|2O|8|@]|D|@]|E|$]]]|1Q|$]]

Duplicate <code>case</code> constant is a trick that is guaranteed to work IN ALL C COMPILERS regardless of how each of them reports error. For Visual C++, it is simple:

<pre><code>struct X {
 int a,b;
 int c[10];
};
int _tmain(int argc, _TCHAR* argv[])
{
 int dummy;

 switch (dummy) {
 case sizeof(X):
 case sizeof(X):
 break;
 }
 return 0;
}
</code></pre>

Compilation result:

<pre><code> ------ Build started: Project: cpptest, Configuration: Debug Win32 ------
 cpptest.cpp c:\work\cpptest\cpptest\cpptest.cpp(29): error C2196: case value '48' already used
 ========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========
</code></pre>

So sizeof the struct X is 48

EDITED (3jun2020):
For gcc or any other compilers that only print "duplicate case value", I use this trick to narrow down the value:

1) add a case value 1==2 (to represent false)

2) by trial and error, narrow down the value e.g. I try to guess the <code>sizeof(X)</code> is >16:

<pre><code>#include &lt;stdio.h&gt;
typedef struct _X {
 int a;
 char b[10];
} X;
int main()
{
 printf("Hello World");

 int dummy=0 ;
 switch (dummy) {
 case 1==2:
 case sizeof( X)&gt;16:
 //case 16:
 break;
 }
 return 0;
}
</code></pre>

result:

<pre><code>main.c: In function ‘main’:
main.c:14:5: error: duplicate case value
 case sizeof( X)&gt;16:
 ^~~~
main.c:13:5: error: previously used here
 case 1==2:
</code></pre>

so it is false, i.e. sizeof(X)&lt;=16. 

3) repeat with some other sensible values. e.g. try to guess that it is 16 i.e. <code>sizeof(X)==16</code>. If it doesn't complain about duplicate case value. Then the expression is true.

4) optionally add a <code>case 16</code> to verify it e.g.

<pre><code>#include &lt;stdio.h&gt;
typedef struct _X {
 int a;
 char b[10];
} X;
int main()
{
 printf("Hello World");

 int dummy=0 ;
 switch (dummy) {
 // case 1==2:
 case sizeof( X):
 case 16:
 break;
 }
 return 0;
}
</code></pre>

result

<pre><code>main.c: In function ‘main’:
main.c:15:5: error: duplicate case value
 case 16:
 ^~~~
main.c:14:5: error: previously used here
 case sizeof( X):
</code></pre>

confirming that sizeof(X) is 16.

Alternatively, it is observed that gcc can report multiple duplicates, so this trick is possible for making multiple guesses on a single pass:

<pre><code>#include &lt;stdio.h&gt;
typedef struct _X {
 int a;
 char b[10];
} X;
int main()
{
 printf("Hello World");

 int dummy=0 ;
 switch (dummy) {
 case 1==2: //represents false
 case 1==1: //represents true
 case sizeof( X)&gt;10:
 case sizeof( X)&gt;12:
 case sizeof( X)&gt;14:
 case sizeof( X)&gt;16:
 case sizeof( X)==16:
 //case 16:
 break;
 }
 return 0;
}
</code></pre>

result 

<pre><code>main.c: In function ‘main’:
main.c:14:5: error: duplicate case value
 case sizeof( X)&gt;10:
 ^~~~
main.c:13:5: error: previously used here
 case 1==1:
 ^~~~
main.c:15:5: error: duplicate case value
 case sizeof( X)&gt;12:
 ^~~~
main.c:13:5: error: previously used here
 case 1==1:
 ^~~~
main.c:16:5: error: duplicate case value
 case sizeof( X)&gt;14:
 ^~~~
main.c:13:5: error: previously used here
 case 1==1:
 ^~~~
main.c:17:5: error: duplicate case value
 case sizeof( X)&gt;16:
 ^~~~
main.c:12:5: error: previously used here
 case 1==2:
 ^~~~
main.c:18:5: error: duplicate case value
 case sizeof( X)==16:
 ^~~~
main.c:13:5: error: previously used here
 case 1==1:
 ^~~~
</code></pre>

suggesting the <code>sizeof(X)</code> is >10, >12, >14 but is not >16. The ==16 is added as a final guess.

blocks|key|173481|text|我偶然发现了一个类似于Bakhazard的great+solution的解决方案，这个解决方案产生的警告要少得多，所以您可能会发现它很有用：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|173482|char+(*__fail)(void)[sizeof(uint64_t)]+=+1;|code-block|syntax|javascript|173483|这将产生错误消息|173484|Function+cannot+return+array+type+'char+[8]'|173485|这是用最新版本的clang(1)测试的。|style|CODE|173486|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/users/5896643/bakhazard|1|https://stackoverflow.com/a/35261673/7903589^0|B|9|0|L|E|1|0|0|0|0|8|8|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@$A|11|B|12|1|13]|$A|14|B|15|1|16]]|C|$]]|$1|D|3|E|5|F|7|17|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|18|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|19|8|@]|9|@]|C|$G|H]]|$1|M|3|N|5|6|7|1A|8|@$A|1B|B|1C|O|P]]|9|@]|C|$]]|$1|Q|3|-4|5|6|7|1D|8|@]|9|@]|C|$]]]|R|$S|$5|T|U|V|C|$W|X]]|Y|$5|T|U|V|C|$W|Z]]]]

I stumbled upon a solution similar to <a href="https://stackoverflow.com/users/5896643/bakhazard">Bakhazard</a>'s <a href="https://stackoverflow.com/a/35261673/7903589">great solution</a>, and this one produces a much less verbose warning, so you may find it useful:

<pre><code>char (*__fail)(void)[sizeof(uint64_t)] = 1;
</code></pre>

This produces the error message

<pre><code>Function cannot return array type 'char [8]'
</code></pre>

This was tested with the latest version of <code>clang(1)</code>.

blocks|key|4679346|text|我的gcc+C编译器拒绝使用上述任何一种解决方案打印该大小。我反转了逻辑以注入编译器警告，说明它不是什么大小。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4679347|enum+e
{
++++X+=+sizeof(struct+mystruct)
};

void+foo()
{
++++static+enum+e+ev;

++++switch+(ev)
++++{
++++case+0:
++++case+4:
++++case+8:
++++case+12:
++++case+16:
++++case+20:
++++++++break;
++++}
}|code-block|syntax|javascript|4679348|然后，我必须查看遗漏的数字的警告。|4679349|warning:+case+value+'0'+not+in+enumerated+type+'e'+[-Wswitch]
warning:+case+value+'4'+not+in+enumerated+type+'e'+[-Wswitch]
warning:+case+value+'12'+not+in+enumerated+type+'e'+[-Wswitch]
warning:+case+value+'16'+not+in+enumerated+type+'e'+[-Wswitch]
warning:+case+value+'20'+not+in+enumerated+type+'e'+[-Wswitch]|4679350|所以我的struct大小是8。|4679351|我的包装是4。|4679352|嗯..。这是一种选择。|4679353|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|6|7|Y|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

My gcc C compiler refuses to print the size using any of the above solutions. I inverted the logic to inject compiler warnings for what size it is not.

<pre><code>enum e
{
 X = sizeof(struct mystruct)
};

void foo()
{
 static enum e ev;

 switch (ev)
 {
 case 0:
 case 4:
 case 8:
 case 12:
 case 16:
 case 20:
 break;
 }
}
</code></pre>

Then I have to look through the warnings for the missing number.

<pre><code>warning: case value '0' not in enumerated type 'e' [-Wswitch]
warning: case value '4' not in enumerated type 'e' [-Wswitch]
warning: case value '12' not in enumerated type 'e' [-Wswitch]
warning: case value '16' not in enumerated type 'e' [-Wswitch]
warning: case value '20' not in enumerated type 'e' [-Wswitch]
</code></pre>

So then my struct size is 8.

My packing is 4.

Meh... it's an option.

blocks|key|4679393|text|另一种方法(实际有效)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4679394|char+__foo[sizeof(MyStruct)+%2B+1]+=+{[sizeof(MyStruct)]+=+""};|code-block|syntax|javascript|4679395|与老的gcc+5.x一起工作。会产生如下错误：|4679396|a.c:8:54:+error:+initializer+element+is+not+computable+at+load+time
a.c:8:54:+note:+(near+initialization+for+'a[8]')|4679397|附注：显然，这一条是(非常)针对gcc的。所有其他方法对我都不起作用。|4679398|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

One more way (that actually works):

<pre><code>char __foo[sizeof(MyStruct) + 1] = {[sizeof(MyStruct)] = ""};
</code></pre>

Works with old'ish gcc 5.x. Yields an error like this:

<pre><code>a.c:8:54: error: initializer element is not computable at load time
a.c:8:54: note: (near initialization for 'a[8]')
</code></pre>

p.s. obviously, this one is (very) gcc specific. All other methods weren't working for me.

blocks|key|173590|text|@jws好主意！然而，sizeof(xxx)是一个常量表达式(除了VLA，https://en.cppreference.com/w/c/language/sizeof)，因此sizeof运算符即使在选择的情况下也应该有效：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|173591|enum+e1+{dummy=-1};
enum+e1+ev;
switch+(ev)+{
++++case+sizeof(myType):;
++++break;
++++default:;
}|code-block|syntax|javascript|173592|。。它在我的GCC中工作："..\WinThreads.c:18:9:+warning:+case值'4‘不在枚举类型'enum+e1’-Wswitch中“|173593|entityMap|0|LINK|mutability|MUTABLE|url|https://en.cppreference.com/w/c/language/sizeof^0|11|1B|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

@jws
nice idea!. Howewer, sizeof(xxx) is a constant expression (except VLA, <a href="https://en.cppreference.com/w/c/language/sizeof" rel="nofollow noreferrer">https://en.cppreference.com/w/c/language/sizeof</a>), so the sizeof operator should work even in the case selection:

<pre><code>enum e1 {dummy=-1};
enum e1 ev;
switch (ev) {
 case sizeof(myType):;
 break;
 default:;
}
</code></pre>

.. it work in my GCC: "..\WinThreads.c:18:9: warning: case value '4' not in enumerated type 'enum e1' [-Wswitch] "

blocks|key|4679490|text|//main.cpp
#include+<cstddef>
template+<std::size_t+x>
struct+show_size;

void+foo()
{
++++show_size<sizeof(my_type)>();//!!please+change+`my_type`+to+your+expected
}

int+main()
{
++++return+0;
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|4679491|你可以编译这个非常简单的代码，在它的预编译阶段，编译器会给出错误，在这个阶段sizeof(my_type)会给出具体的值。例如：|unstyled|offset|length|style|CODE|4679492|g%2B%2B+main.cpp|4679493|entityMap^0|0|12|F|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|P|8|@$G|Q|H|R|I|J]]|9|@]|A|$]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|M|3|-4|5|F|7|T|8|@]|9|@]|A|$]]]|N|$]]

<pre><code>//main.cpp
#include &lt;cstddef&gt;
template &lt;std::size_t x&gt;
struct show_size;

void foo()
{
 show_size&lt;sizeof(my_type)&gt;();//!!please change `my_type` to your expected
}

int main()
{
 return 0;
}
</code></pre>
You can compile this pretty simple code, and during its pre-compilation stage, the compiler will give error, in which the <code>sizeof(my_type)</code> will give concrete value. e.g.:
<pre class="lang-sh prettyprint-override"><code>g++ main.cpp
</code></pre>

blocks|key|4679528|text|对我有效的快速而简单的解决方案(GCC)：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4679529|(char[sizeof(long+long)])"bla";|code-block|syntax|javascript|4679530|这将导致显示long+long大小的错误消息|offset|length|style|CODE|4679531|ISO+C%2B%2B+forbids+casting+to+an+array+type+'char+[8]'|4679532|entityMap^0|0|0|6|9|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

Quick and simple solution that worked for me (GCC):
<pre class="lang-cpp prettyprint-override"><code>(char[sizeof(long long)])&quot;bla&quot;;
</code></pre>
This results in an error message that reveals the size of <code>long long</code>:
<pre><code>ISO C++ forbids casting to an array type 'char [8]'
</code></pre>

How can I print the result of sizeof() at compile time in C?

For now I am using a static assert (home brewed based on other web resources) to compare the sizeof() result to various constants. While this works... it is far from elegant or fast. I can also create an instance of the variable/struct and look in the map file but this is also less elegant and fast than a direct call/command/operator. Further, this is an embedded project using multiple cross-compilers... so building and loading a sample program to the target and then reading out a value is even more of a hassle than either of the above.

In my case (old GCC), <code>#warning sizeof(MyStruct)</code> does not actually interpret sizeof() before printing the warning.

How can I print the result of sizeof() at compile time in C?

编译器

如何在C中编译时打印sizeof()的结果？现在，我使用静态断言(基于其他web资源自制)来比较sizeof()结果与各种常量。在这个过程中..。它远不是优雅或快速的。我还可以创建一个变量/结构的实例并查看映射文件，但这也不如直接调用/命令/操作符那么优雅和快速。此外，这是一个使用多个交叉编译器的嵌入式项目...因此，...

问如何在C中编译时打印sizeof()的结果？
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问如何在C中编译时打印sizeof()的结果？
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