在下面的声明中_Nullable是什么意思？
void foo(int *_Nullable ptr)
版本11.4的GCC没有编译该代码，因为它将_Nullable关键字视为参数名.然而，我在MAN 7页上看到了这个关键词: https://man7.org/linux/man-pages/man2/epoll_ctl.2.html个 https://man7.org/linux/man-pages/man2/timer_gettime.2.html
在下面的声明中_Nullable是什么意思？
void foo(int *_Nullable ptr)
版本11.4的GCC没有编译该代码，因为它将_Nullable关键字视为参数名.然而，我在MAN 7页上看到了这个关键词: https://man7.org/linux/man-pages/man2/epoll_ctl.2.html个 https://man7.org/linux/man-pages/man2/timer_gettime.2.html
_Nullable simply means that the pointer passed to the function is allowed to be NULL. It is however not a keyword in C (and doesn't occur even once in the draft for the C23 standard) but an extension supported only by some compilers, for example clang and icx.
Linux man页使用_Nullable来向读者指示它们被允许传递NULL指针，但头文件中的实际声明不是_Nullable.
希望向编译器提供静态分析和优化提示_Nullable和_Nonnull(指示指针not可以是NULL)的库将视所使用的平台/编译器而 Select 性地将指针声明为_Nullable和_Nonnull.
示例:在Linux上的zipconf.h(针对libzip的特定于平台的包含文件)中，您将看到
#define _Nullable
#define _Nonnull
and all the zip functions will have both _Nullable and _Nonnull in their declarations. On Linux these will however be replaced with nothing thanks to the above #defines. On a platform dominated by clang or icx these definitions will not exist and the implementation specific keywords will then be passed on to the compiler as-is.