MAP_HUGE_1GB and MAP_HUGE_2MB missing?

Question

I am on CentOS 7 with kernel version 3.10.0-1160.15.2.el7.x86_64.

When I tried to use MAP_HUGE_1GB and MAP_HUGE_2MB flags, g++-9.3.1 complained:

error: ‘MAP_HUGE_1GB’ was not declared in this scope; did you mean ‘MAP_HUGETLB’?

Then, I found this post, the answer suggested that I need to "#define _GNU_SOURCE before #include <sys/mman.h> because this is a nonstandard flag".

However, defining _GNU_SOURCE causes warning: "_GNU_SOURCE" redefined but g++ still complains those 2 flags aren't declared.

Then, I found this post, and it suggests that _GNU_SOURCE defined as libstdc++ is used.

Now, I am confused how to use MAP_HUGE_1GB and MAP_HUGE_2MB flags.

Thanks!

Answer 1

Glibc doesn't define either MAP_HUGE_1GB or MAP_HUGE_2MB in any of its headers. If you have the kernel-headers package installed, you can get those constants by doing #include <linux/mman.h>. This isn't really ideal, though, so I'm not sure why glibc doesn't give you a better way to get them.