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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 [-d seconds] [-w] [-resident] [-pages] [-interleaved] [-submap] [-allSplitLibs] pid |
partial-executable-name
DESCRIPTION
vmmap displays the virtual memory regions allocated in a specified process, helping a programmer under-stand understand
stand 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, separated 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/resi-dent]. virtual/resident].
dent].
-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 nonwritable
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.
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_allocate 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 example, 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 immediately 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. Permis-sions Permissions
sions 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 multiple processes
(or shared by a single process in multiple locations). When the page is modified, the writing process
then receives its own private copy of the page. Empty (NUL) sharing implies that the page does not
really exist in physical memory. Aliased (ALI) and shared (SHM) memory is shared between processes.
The share mode typically describes the general mode controlling the region. For example, as copy-on-write copy-onwrite
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, stack, text or data segment (for Mach-O
binaries), PEF binary, 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 system.)
For example, the memory between 0x90000000 and 0x9fffffff is a submap containing the read-only portions
of the most common 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 processes, 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,) lsof(1)
The heap, leaks, and malloc_history commands can be used to look at various aspects of a process's mem-ory memory
ory 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 mach system call vm_region retrieves the information used by vmmap.
BSD March 15, 2007 BSD
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