mac10.15编译swoole失败，出现“no member named 'signbit' in the global namespace”错误信息的解决方案

在mac 10.15.16系统上，编译swoole 4.4.18的时候，在执行make命令时，出现错误提示：`no member named 'signbit' in the global namespace
using ::signbit;`，使得编译总是失败。

对于想在mac使用swoole的开发者而言，多少算是一种挫败。本来兴致冲冲想在mac身上研究一下swoole，却在编译swoole的php插件时栽了这么一个跟头。为了解决这个拦路虎，和朋友问了一圈，无解，又在在网上搜了很多文章，始终未找到解答。

不过，只要真心想解决问题，并且为之努力，机遇还是会到来的。

具体出错信息如下：

In file included from /usr/local/Cellar/php/7.4.7/include/php/main/php.h:33:
In file included from /usr/local/Cellar/php/7.4.7/include/php/Zend/zend.h:27:
In file included from /usr/local/Cellar/php/7.4.7/include/php/Zend/zend_types.h:25:
In file included from /usr/local/Cellar/php/7.4.7/include/php/Zend/zend_portability.h:43:
In file included from /usr/local/Cellar/php/7.4.7/include/php/Zend/zend_config.h:1:
In file included from /usr/local/Cellar/php/7.4.7/include/php/main/../main/php_config.h:2336:
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/cmath:317:9: error: 
      no member named 'signbit' in the global namespace
using ::signbit;
      ~~^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/cmath:318:9: error: 
      no member named 'fpclassify' in the global namespace
using ::fpclassify;
      ~~^
/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/cmath:319:9: error: 
      no member named 'isfinite' in the global namespace; did you mean 'finite'?
using ::isfinite;
      ~~^
/usr/local/include/math.h:749:12: note: 'finite' declared here
extern int finite(double)
           ^

...

/Library/Developer/CommandLineTools/usr/bin/../include/c++/v1/cmath:649:25: error: no template named 'numeric_limits'
  return _FloatBigger ? numeric_limits<_IntT>::max() :  (numeric_limits<_IntT>::max() >> _Bits << _Bits);
                        ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
177 warnings and 20 errors generated.
make: *** [php_swoole_cxx.lo] Error 1

在网上还找到一篇文章，描述的现象而言，和我遇到的问题是一致的：《Mac 安装 Swoole 报错： error: no member named 'isless' in the global namespace using ::isless;》

最终对我起到帮助的一篇文章是：Catalina C++: Using headers yield error: no member named 'signbit' in the global namespace，其中Niloy Datta的回答帮我解决了难题。

下面引用一下他的发言内容：

Using the command:（使用命令查看include的路径）

gcc -Wp,-v -E -

my #include <...> search sequence:（我的#include查询顺序是）

/usr/local/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/clang/10.0.1/include
 /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/usr/include
 /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk/System/Library/Frameworks (framework directory)

The reason of #include error is described below:(#include出现错误原因如下)

• \#include resides in /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1
• It includes < math.h \>.
• It searches /usr/local/include directory as this is the first directory to search. There is a math.h in "/usr/local/include/c++/9.3.0/" directory
• It tries to use this.
• But expectation was to use the math.h of the same directory /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1
• The math.h of /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1 include math.h of /usr/local/include using #include_next<math.h>
• As wrong math.h is included/linked with /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/cmath, the compilation error happens

The fix:(解决方案)

1. If we can alter the search order of #include<...> to search /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1 at first, it can be fixed.
2. Using \#include</Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/math.h> instead of <math.h> in /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/include/c++/v1/cmath

再用中文说明一下

归根结底，其实就是和#include的检索顺序有关系。首先可以通过命令找到自己系统的#include的顺序。在我的系统里边按照 gcc -Wp,-v -E -，结果如下：

clang -cc1 version 11.0.3 (clang-1103.0.32.62) default target x86_64-apple-darwin19.6.0
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/local/include"
ignoring nonexistent directory "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/Library/Frameworks"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /Library/Developer/CommandLineTools/usr/lib/clang/11.0.3/include
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include
 /Library/Developer/CommandLineTools/usr/include
 /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks (framework directory)
End of search list.

而能找到math.h的第一个路径，则是/usr/local/include/math.h这个文件，这和预期是不一致的，预期要使用的math.h是其同目录的math.h。因此，我们可以对原来的cmath代码进行调整，将<math.h>改成"math.h"即可。

修改“/Library/Developer/CommandLineTools/usr/include/c++/v1/cmath”中的#include <math.h>为#include "math.h"。

这就是最终的解决方案，按此方法修改后，make命令可以正常进行，swoole可以正常在mac下编译通过了。