[C++知识库]::malloc has not been defined ::realloc has not been defined

编译这个工程 的时候，遇到了这样的错误：

$ make
make  all-am
make[1]: Entering directory `/home/tiina/clearwater/sprout/modules/libmemcached'
  CXX      libhashkit/libhashkit_libhashkit_la-aes.lo
In file included from /opt/rh/devtoolset-9/root/usr/include/c++/9/stdlib.h:36,
                 from ./libhashkit/common.h:45,
                 from libhashkit/aes.cc:38:
/opt/rh/devtoolset-9/root/usr/include/c++/9/cstdlib:151:11: error: '::malloc' has not been declared
  151 |   using ::malloc;
      |           ^~~~~~
/opt/rh/devtoolset-9/root/usr/include/c++/9/cstdlib:164:11: error: '::realloc' has not been declared
  164 |   using ::realloc;
      |           ^~~~~~~
In file included from ./libhashkit/common.h:45,
                 from libhashkit/aes.cc:38:
/opt/rh/devtoolset-9/root/usr/include/c++/9/stdlib.h:65:12: error: 'std::malloc' has not been declared
   65 | using std::malloc;
      |            ^~~~~~
/opt/rh/devtoolset-9/root/usr/include/c++/9/stdlib.h:73:12: error: 'std::realloc' has not been declared
   73 | using std::realloc;
      |            ^~~~~~~
make[1]: *** [libhashkit/libhashkit_libhashkit_la-aes.lo] Error 1
make[1]: Leaving directory `/home/tiina/clearwater/sprout/modules/libmemcached'

难道是C++标准库出了问题？
NO
问题出在configure.ac文件里的
AC_FUNC_MALLOC
configure.ac里的这句话，会在config.h文件里增加一句：
#define malloc rpl_malloc
而/opt/rh/devtoolset-9/root/usr/include/c++/9/cstdlib里面又有#undef malloc
所以程序运行起来类似这样：

#define malloc rpl_malloc
#include <stdlib.h>
#undef malloc 
namespace std {
  using ::malloc;
}
// Your program:
int main()
{
  int* i = static_cast(std::malloc(sizeof(int)));
  return 0;
}

（
虽然上文是应用中写的，但实际上#undef malloc是在stdlib.h的第135行通过#include<cstdlib>，从cstdlib的头文件里引入的）。
使用g++ -P -E test.cc可以看到：

468 extern void *rpl_malloc (size_t __size) throw () __attribute__ ((__malloc__)) ;

因此，报的错误也是在cstdlib的using ::malloc 和using std::malloc
）

configure.ac的AC_FUNC_MALLOC

autoconf工具会在运行过程中执行一系列的test，其中就有malloc(0)是否返回NULL，如果是的话就认为是用的rpl_malloc——一个需要你自己定义的malloc。但是会让这个test返回NULL的可能性太多了，比如lib库的路径写错了，它只会有提示，而不是终止编译，这种虽和malloc无关，但也会返回NULL。为了不执行这种test，把configure.ac的AC_FUNC_MALLOC去掉，就能通过编译了。

感谢：
https://nerdland.net/unstumping-the-internet/malloc-has-not-been-declared/#rootcause