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(libc.info)Query Memory Parameters

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How to get information about the memory subsystem?
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   The page size of the virtual memory the process sees is essential to
know in several situations.  Some programming interface (e.g., `mmap',
Note: Memory-mapped I/O) require the user to provide information
adjusted to the page size.  In the case of `mmap' is it necessary to
provide a length argument which is a multiple of the page size.
Another place where the knowledge about the page size is useful is in
memory allocation.  If one allocates pieces of memory in larger chunks
which are then subdivided by the application code it is useful to
adjust the size of the larger blocks to the page size.  If the total
memory requirement for the block is close (but not larger) to a multiple
of the page size the kernel's memory handling can work more effectively
since it only has to allocate memory pages which are fully used.  (To do
this optimization it is necessary to know a bit about the memory
allocator which will require a bit of memory itself for each block and
this overhead must not push the total size over the page size multiple.

   The page size traditionally was a compile time constant.  But recent
development of processors changed this.  Processors now support
different page sizes and they can possibly even vary among different
processes on the same system.  Therefore the system should be queried at
runtime about the current page size and no assumptions (except about it
being a power of two) should be made.

   The correct interface to query about the page size is `sysconf'
(Note: Sysconf Definition) with the parameter `_SC_PAGESIZE'.  There
is a much older interface available, too.

 - Function: int getpagesize (void)
     The `getpagesize' function returns the page size of the process.
     This value is fixed for the runtime of the process but can vary in
     different runs of the application.

     The function is declared in `unistd.h'.

   Widely available on System V derived systems is a method to get
information about the physical memory the system has.  The call

       sysconf (_SC_PHYS_PAGES)

returns the total number of pages of physical the system has.  This
does not mean all this memory is available.  This information can be
found using

       sysconf (_SC_AVPHYS_PAGES)

   These two values help to optimize applications.  The value returned
for `_SC_AVPHYS_PAGES' is the amount of memory the application can use
without hindering any other process (given that no other process
increases its memory usage).  The value returned for `_SC_PHYS_PAGES'
is more or less a hard limit for the working set.  If all applications
together constantly use more than that amount of memory the system is
in trouble.

   The GNU C library provides in addition to these already described
way to get this information two functions.  They are declared in the
file `sys/sysinfo.h'.  Programmers should prefer to use the `sysconf'
method described above.

 - Function: long int get_phys_pages (void)
     The `get_phys_pages' function returns the total number of pages of
     physical the system has.  To get the amount of memory this number
     has to be multiplied by the page size.

     This function is a GNU extension.

 - Function: long int get_avphys_pages (void)
     The `get_phys_pages' function returns the number of available
     pages of physical the system has.  To get the amount of memory
     this number has to be multiplied by the page size.

     This function is a GNU extension.