vmmap(1) BSD General Commands Manual vmmap(1)
NAME
vmmap -- Display the virtual memory regions allocated in a process
SYNOPSIS
vmmap [-d seconds] [-w] [-resident] [-pages] [-interleaved] [-submap]
[-allSplitLibs] [-noCoalesce] [-v] pid | partial-executable-name
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. The process can
be specified by process ID or by full or partial executable name.
OPTIONS
-d seconds Take two snapshots of the vm regions of the process, sepa-
rated by the specified time, and print the delta between
those snapshots.
-w, -wide Print wide output.
-resident Show both the virtual and resident sizes for each region,
in the form [ virtual/resident].
-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.
-v, -verbose Equivalent to -w -resident -dirty -purge -submap
-allSplitLibs -noCoalesce
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.
If the -resident flag is given, then both the virtual and physical size
of each region is shown, in the form [virtual/resident]. By default, the
sizes are shown in kilobytes. If the -pages flag is given, then the
sizes are in number of 4KB 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 flags 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.
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 Mar. 16, 2013 BSD
Mac OS X 10.9 - Generated Sun Oct 13 13:44:59 CDT 2013