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Full Discussion: C++ program is crashing on re-assigning const static member variable using an int pointer
Top Forums Programming C++ program is crashing on re-assigning const static member variable using an int pointer Post 302489053 by Corona688 on Wednesday 19th of January 2011 10:56:08 AM
Old 01-19-2011
Global constants get hardware write-protection on lots of OSes. The program literally cannot write to them, memory map settings prevent it, the CPU itself knows you shouldn't and throws an exception if you try.

A static variable is as good as a global variable in most respects, this included.

Trying to get around this with trickery isn't a good idea. Even if you alter page protection to let you write, the program might ignore it when you do! The compiler, knowing the variable is a constant, will think "hmm, this variable will never change -- I can save 2 instructions per go by hardcoding it everywhere instead of reading it from memory..."
 

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LEARN ABOUT MINIX

subpage_prot

SUBPAGE_PROT(2) 					     Linux Programmer's Manual						   SUBPAGE_PROT(2)

NAME

       subpage_prot - define a subpage protection for an address range

SYNOPSIS

       long subpage_prot(unsigned long addr, unsigned long len,
			 uint32_t *map);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

       The PowerPC-specific subpage_prot() system call provides the facility to control the access permissions on individual 4 kB subpages on sys-
       tems configured with a page size of 64 kB.

       The protection map is applied to the memory pages in the region starting at addr and continuing for len bytes.	Both  of  these  arguments
       must be aligned to a 64-kB boundary.

       The  protection map is specified in the buffer pointed to by map.  The map has 2 bits per 4 kB subpage; thus each 32-bit word specifies the
       protections of 16 4 kB subpages inside a 64 kB page (so, the number of 32-bit words pointed to by map should equate to the number of  64-kB
       pages specified by len).  Each 2-bit field in the protection map is either 0 to allow any access, 1 to prevent writes, or 2 or 3 to prevent
       all accesses.

RETURN VALUE

       On success, subpage_prot() returns 0.  Otherwise, one of the error codes specified below is returned.

ERRORS

       EFAULT The buffer referred to by map is not accessible.

       EINVAL The addr or len arguments are incorrect.	Both of these arguments must be aligned to a multiple of the system page  size,  and  they
	      must not refer to a region outside of the address space of the process or to a region that consists of huge pages.

       ENOMEM Out of memory.

VERSIONS

       This  system call is provided on the PowerPC architecture since Linux 2.6.25.  The system call is provided only if the kernel is configured
       with CONFIG_PPC_64K_PAGES.  No library support is provided.

CONFORMING TO

       This system call is Linux-specific.

NOTES

       Glibc does not provide a wrapper for this system call; call it using syscall(2).

       Normal page protections (at the 64-kB page level) also apply; the subpage protection mechanism is an additional constraint, so putting 0 in
       a 2-bit field won't allow writes to a page that is otherwise write-protected.

   Rationale
       This system call is provided to assist writing emulators that operate using 64-kB pages on PowerPC systems.  When emulating systems such as
       x86, which uses a smaller page size, the emulator can no longer use the memory-management unit (MMU) and normal system calls  for  control-
       ling  page protections.	(The emulator could emulate the MMU by checking and possibly remapping the address for each memory access in soft-
       ware, but that is slow.)  The idea is that the emulator supplies an array of protection masks to apply to  a  specified	range  of  virtual
       addresses.   These  masks are applied at the level where hardware page-table entries (PTEs) are inserted into the hardware page table based
       on the Linux PTEs, so the Linux PTEs are not affected.  Implicit in this is that the regions of the address space that  are  protected  are
       switched to use 4-kB hardware pages rather than 64-kB hardware pages (on machines with hardware 64-kB page support).

SEE ALSO

       mprotect(2), syscall(2)

       Documentation/vm/hugetlbpage.txt in the Linux kernel source tree

COLOPHON

       This  page is part of release 4.15 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the
       latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Linux								    2017-09-15							   SUBPAGE_PROT(2)
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