mremap - re-map a virtual memory address


     #include <unistd.h>
     #include <sys/mman.h>

     void * mremap(void * old_address, size_t old_size  ,  size_t
     new_size, unsigned long flags));


     mremap expands (or  shrinks)  an  existing  memory  mapping,
     potentially  moving  it  at the same time (controlled by the
     flags argument and the available virtual address space).

     old_address is the old address of the virtual  memory  block
     that  you want to expand (or shrink).  Note that old_address
     has to be page aligned. old_size is the old size of the vir-
     tual  memory  block.   new_size is the requested size of the
     virtual memory block after the resize.

     The flags argument is a bitmap of flags.

     In Linux the memory is divided into pages.  A  user  process
     has  (one  or) several linear virtual memory segments.  Each
     virtual memory segment has one  or  more  mappings  to  real
     memory  pages (in the page table).  Each virtual memory seg-
     ment has its own protection (access rights), which may cause
     a   segmentation   violation   if  the  memory  is  accessed
     incorrectly (e.g., writing to a read-only segment).  Access-
     ing virtual memory outside of the segments will also cause a
     segmentation violation.

     mremap uses the Linux page table scheme.  mremap changes the
     mapping  between  virtual  addresses and memory pages.  This
     can be used to implement a very efficient realloc.


          indicates if the operation should fail, or  change  the
          virtual  address  if  the  resize cannot be done at the
          current virtual address.


     On success mremap returns  a  pointer  to  the  new  virtual
     memory  area.   On  error,  -1 is returned, and errno is set


          An invalid argument was given. Most likely  old_address
          was not page aligned.

          "Segmentation  fault."  Some  address  in   the   range
          old_address  to old_address+old_size is an invalid vir-
          tual memory address for this process.  You can also get
          EFAULT  even  if  there  exist  mappings that cover the
          whole address space requested, but those  mappings  are
          of different types.

          The memory segment is locked and cannot be re-mapped.

          The memory area cannot be expanded at the current  vir-
          tual address, and the MREMAP_MAYMOVE flag is not set in
          flags. Or, there is not enough (virtual) memory  avail-


     This call is Linux-specific, and should not be used in  pro-
     grams intended to be portable.  4.2BSD had a (never actually
     implemented) mremap(2) call with completely different seman-


     getpagesize(2),  realloc(3),  malloc(3),  brk(2),   sbrk(2),

     Your favorite OS text book for  more  information  on  paged
     memory.   (Modern Operating Systems by Andrew S. Tannenbaum,
     Inside Linux by Randolf Bentson,  The  Design  of  the  UNIX
     Operating System by Maurice J. Bach.)