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vmmap(1)                  BSD General Commands Manual                 vmmap(1)


NAME

     vmmap -- Display the virtual memory regions allocated in a process


SYNOPSIS

     vmmap [-w] [-v] [-pages] [-interleaved] [-submap] [-allSplitLibs]
           [-noCoalesce] [-summary] pid | partial-executable-name |
           memory-graph-file [address]


DESCRIPTION

     vmmap displays the virtual memory regions allocated in a specified
     process, helping a programmer understand how memory is being used, and
     what the purposes of memory at a given address may be.

     vmmap requires one argument -- either the process ID or the full or par-
     tial executable name of the process to examine, or the pathname of a mem-
     ory graph file generated by leaks or the Xcode Memory Graph Debugger.

     If the optional address is given, information is only shown for the VM
     region containing that address (if any) and the regions around it.


OPTIONS

     -w, -wide      Print wide output, to show full paths of mapped files.

     -v, -verbose   Equivalent to -w -submap -allSplitLibs -noCoalesce

     -pages         Print region sizes in page counts rather than kilobytes.

     -interleaved   Print all regions in ascending order of starting address,
                    rather than printing all non-writable regions followed by
                    all writable regions.

     -submap        Print information about VM submaps.

     -allSplitLibs  Print information about all shared system split libraries,
                    even those not loaded by this process.

     -noCoalesce    Do not coalesce adjacent identical regions.  Default is to
                    coalesce for more concise output.

     -summary       Print only the summary of VM usage, not the individual
                    region detail.


EXPLANATION OF OUTPUT

     For each region, vmmap describes the starting address, ending address,
     size of the region (in kilobytes or pages), read/write permissions for
     the page, sharing mode for the page, and the purpose of the pages.

     The size of the virtual memory region represents the virtual memory pages
     reserved, but not necessarily allocated.  For example, using the vm_allo-
     cate Mach system call reserves the pages, but physical memory won't be
     allocated for the page until the memory is actually touched.  A memory-
     mapped file may have a virtual memory page reserved, but the pages are
     not instantiated until a read or write happens.  Thus, this size may not
     correctly describe the application's true memory usage.

     By default, the sizes are shown in kilobytes or megabytes.  If the -pages
     flag is given, then the sizes are in number of VM pages.

     The protection mode describes if the memory is readable, writable, or
     executable.  Each virtual memory region has a current permission, and a
     maximum permission.  In the line for a virtual memory region, the current
     permission is displayed first, the maximum permission second.  For exam-
     ple, the first page of an application (starting at address 0x00000000)
     permits neither reads, writes, or execution ("---"), ensuring that any
     reads or writes to address 0, or dereferences of a NULL pointer immedi-
     ately cause a bus error.  Pages representing an executable always have
     the execute and read bits set ("r-x").  The current permissions usually
     do not permit writing to the region.  However, the maximum permissions
     allow writing so that the debugger can request write access to a page to
     insert breakpoints.  Permissions for executables appear as "r-x/rwx" to
     indicate these permissions.

     The share mode describes whether pages are shared between processes,and
     what happens when pages are modified.  Private pages (PRV) are pages only
     visible to this process.  They are allocated as they are written to, and
     can be paged out to disk. Copy-on-write (COW) pages are shared by multi-
     ple processes (or shared by a single process in multiple locations).
     When the page is modified, the writing process then receives its own pri-
     vate copy of the page.  Empty (NUL) sharing implies that the page does
     not really exist in physical memory.  Aliased (ALI) and shared (SHM) mem-
     ory is shared between processes.

     The share mode typically describes the general mode controlling the
     region.  For example, as copy-on-write pages are modified, they become
     private to the application.  Even with the private pages, the region is
     still COW until all pages become private.  Once all pages are private,
     then the share mode would change to private.

     The far left column names the purpose of the memory: malloc regions,
     stack, text or data segment, etc.  For regions loaded from binaries, the
     far right shows the library loaded into the memory.

     If the -submap flag is given, then vmmap's output includes descriptions
     of submaps.  A submap is a shared set of virtual memory page descriptions
     that the operating system can reuse between multiple processes.  Submaps
     minimize the operating system's memory usage by representing the virtual
     memory regions only once.  Submaps can either be shared by all processes
     (machine-wide) or local to the process (process-only).  (Understanding
     where submaps are located is irrelevant for most developers, but may be
     interesting for anyone working with low levels of the virtual memory sys-
     tem.)

     For example, one submap contains the read-only portions of the most com-
     mon dynamic libraries.  These libraries are needed by most programs on
     the system, and because they are read-only, they will never be changed.
     As a result, the operating system shares these pages between all the pro-
     cesses, and only needs to create a single data structure to describe how
     this memory is laid out in every process.

     That section of memory is referred to as the "split library region", and
     it is shared system-wide.  So, technically, all of the dynamic libraries
     that have been loaded into that region are in the VM map of every
     process, even though some processes may not be using some of those
     libraries.  By default, vmmap shows only those shared system split
     libraries that have been loaded into the specified target process.  If
     the -allSplitLibs flag is given, information about all shared system
     split libraries will be printed, regardless of whether they've been
     loaded into the specified target process or not.

     If the contents of a machine-wide submap are changed -- for example, the
     debugger makes a section of memory for a dylib writable so it can insert
     debugging traps -- then the submap becomes local, and the kernel will
     allocate memory to store the extra copy.

     % FRAG, fragmentation, in the MALLOC ZONE summary is computed by the fol-
     lowing method:

         % FRAG = 100 - (100 * Allocated / (Dirty + Swapped))

     Dirty and swapped are memory which has been written to by the process.
     Allocated is the number of bytes currently allocated from malloc.


SEE ALSO

     heap(1), leaks(1), malloc_history(1), stringdups(1), lsof(8)

     The heap, leaks, and malloc_history commands can be used to look at vari-
     ous aspects of a process's memory usage.

     The lsof command can be used to get a list of open and mapped files in
     one or more processes, which can help determine why a volume can't be
     unmounted or ejected, for example.

     The Xcode developer tools also include Instruments, a graphical applica-
     tion that can give information similar to that provided by vmmap. The
     Allocations instrument graphically displays dynamic, real-time informa-
     tion about the object and memory use in an application (including VM
     allocations), as well as backtraces of where the allocations occured.
     The VM Tracker instrument in the Allocations template graphically dis-
     plays information about the virtual memory regions in a process.

BSD                             August 24, 2016                            BSD

Mac OS X 10.12.6 - Generated Sun Oct 29 09:58:17 CDT 2017
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