What does _Nullable mean in following declaration?
void foo(int *_Nullable ptr)
gcc of version 11.4 doesn’t compile that code, because it treats _Nullable keyword as an argument name. Yet I see this keyword in man 7 pages:
https://man7.org/linux/man-pages/man2/epoll_ctl.2.html
https://man7.org/linux/man-pages/man2/timer_gettime.2.html
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_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.
The Linux man pages uses _Nullable to indicate to the readers that they are allowed to pass in a NULL pointer, but the actual declarations in the header files are free from _Nullable.
Libraries who wishes to give the compiler the static analysis and optimizing hints _Nullable and _Nonnull (indicating that the pointer may not be NULL) will optionally declare pointers as _Nullable and _Nonnull depending on which platform/compiler that is used.
Example: In zipconf.h (a platform specific include file for libzip) on Linux, you would see
#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.
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It is used by warning system. If it is nullable and you do not check for NULL then emit warning.
– 0___________9 hours ago
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@0___________ I was just thinking, "should I add something about static analysis?" when you commented 🙂 Done!
– Ted Lyngmo9 hours ago