Strace on a tree structure C program

When I tried to access 'unallocated node value' , as usual 'Segmentation fault' came as output :-)

But i am not fully clear on how mmap() system call allocate the memory...

I also came to know that mprotect() system call sets the protection,

If the calling process tries to access memory in a manner that violates the protection, then the kernel generates a SIGSEGV signal for the process.

Program and Strace details: