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leaks(1) General Commands Manual leaks(1)

leaks — Search a process's memory for unreferenced malloc buffers

leaks [options] pid | partial-executable-name | memory-graph-file

leaks [options] -atExit -- command

[-list] [-groupByType] [-nostacks] [-nosources] [-quiet] [-exclude symbol] [-outputGraph path] [-fullContent] [-readonlyContent] [-noContent] [-diffFrom=<memgraph>] [-traceTree address] [-referenceTree] [-autoreleasePools] [-debug=<mode>]

leaks identifies leaked memory -- memory that the application has allocated, but has been lost and cannot be freed. Specifically, leaks examines a specified process's memory for values that may be pointers to malloc-allocated buffers. Any buffer reachable from a pointer in writable global memory (e.g., __DATA segments), a register, or on the stack is assumed to be memory in use. Any buffer reachable from a pointer in a reachable malloc-allocated buffer is also assumed to be in use. The buffers which are not reachable are leaks; the buffers could never be freed because no pointer exists in memory to the buffer, and thus free() could never be called for these buffers. Such buffers waste memory; removing them can reduce swapping and memory usage. Leaks are particularly dangerous for long-running programs, for eventually the leaks could fill memory and cause the application to crash.

leaks requires one argument -- either the process ID or the full or partial executable name of the process to examine, or the pathname of a memory graph file generated by leaks or the Xcode Memory Graph Debugger. (Unless the -atExit -- command argument is given, see below for more details.)

Once the leaked buffers have been identified, leaks analyzes them to find "root leaks" (those which are not referenced by any other buffer) and "root cycles" (cycles of objects which reference or retain each other, but which are not referenced by any other buffer outside the cycle). Then, it identifies the tree of buffers which are referenced by those root leaks and root cycles, if any. leaks then prints each such "leak tree".

If the MallocStackLogging environment variable was set when the application was launched, leaks also prints a stack trace describing where the buffer was allocated.

A memory graph file archives the memory state of a process for further analysis at a later time, on a different machine, or by other people. It includes information about all VM and malloc nodes in the process, and the references between them. Memory graph files can be generated by leaks using the -outputGraph option (and the -fullContent option if desired), or by examining a live process with the Xcode Memory Graph Debugger then using the Export Memory Graph menu item from the File menu. The standard filename suffix for memory graph files is ".memgraph". These files can be used as input to various commands including leaks, heap, stringdups, vmmap, malloc_history, footprint, and the Xcode Memory Graph Debugger.

Print the leaks as a list ("classic"-style) rather than as a tree. Warning: this option may be removed in the future.
When printing a tree of leaked objects, group the children of a node in the tree by type, rather than showing individual instances.
Do not print backtraces of leaked blocks even if the target process has the MallocStackLogging environment variable set.
Do not print sourceFile:lineNumber in backtraces. This can improve performance when examining a process with a huge number of debug symbols.
Do not print process description header or binary image list.
symbol
Exclude leaked blocks whose backtraces include the specified symbol. This option can be repeated for multiple symbols. This allows ignoring leaks that, for example, are allocated in libraries for which you do not have source code.
path
Generate a memory graph file containing information about all VM and malloc nodes, and the references between them. path can be a path to a file, or just a directory name; in the latter case a filename with the ".memgraph" suffix will be generated.

By default (for security) when generating a memory graph file, descriptions of the content of some objects will be included but ONLY if they are backed by read-only memory in Mach-O binary images or the dyld shared cache. To store full content pass the -fullContent flag.

When generating a memory graph file, include descriptions of the content of various objects, as would be shown by heap <pid> -addresses all, and as needed by stringdups <pid>. (Full content is the default when targeting a live process, without generating a memory graph file.)
When running leaks against a live target process, print descriptions of the content of memory only if they are backed by read-only memory. (Read-only content is the default when generating memory graph files.)
Do not print the descriptions of the content of leaked memory, or save descriptions of allocation memory into memory graph files. Although that information can be useful for recognizing the contents of the buffer and understanding why it might be leaked, it could expose confidential information from the process if you, for example, file bug reports with that output included.
Show only the new leaks since the specified memgraph.
address
Print a reverse tree of references, from the given block up to the process 'roots' (e.g., global data, registers, or locations on stacks) to the given block. This is useful for determining what is holding onto a buffer such that it has not been freed, and is similar to the information shown in the Xcode Memory Graph Debugger.
Print a top-down tree of all malloc allocations and dynamically-allocated VM regions in the process. This can be useful for getting an overall sense of how memory is held by the process. The -groupByType argument can also be passed to summarize the data.

In this reference tree mode, each allocation only appears once in the output. Some attempt is made to prioritize which reference to an allocation should be considered as the "owning" allocation to decide where in the tree to show the allocation, but since allocations often have several or numerous references to them (some of which may be false or stale references) and only one can be the "parent" in this reference tree output, sometimes allocations are shown in the wrong place in the tree.

Print the contents of all autorelease pools of all threads of the process, and trees of memory that are only held by those allocations. If the autorelease pool got popped then that additional memory that is only held by autorelease pool entries would get released.
This flag offers several additional more detailed modes of output, intended for debugging and deeper investigations. Use -debug=help to get more information about various debug modes.
-- command
Launches the specified command and runs leaks when that process exits. The -atExit argument should be the last argument, followed by -- and the command to launch. For example: 
$ leaks -quiet -atExit -- /bin/ls -lt /tmp/

Using -atExit will automatically set MallocStackLogging=lite for the specified command so that stack backtraces can be shown for leaked allocations. To use a different setting of that env var, such as YES or NO, you can set the env var prior to running leaks. For example:

$ MallocStackLogging=YES leaks -quiet -atExit -- /bin/ls -lt /tmp/

The leaks command may detect more leaks if the target process is run with the MallocScribble environment variable. If this variable is set then when malloc blocks are deallocated they are filled with 0x55 bytes, thus overwriting any "stale" data such as pointers remaining in those blocks. This reduces the number of false pointers remaining in the process memory.

The leaks command exits with one of the following values:

No leaks were detected.
One or more leaks were detected.
An error occurred.

malloc(3), heap(1), malloc_history(1), stringdups(1), vmmap(1), footprint(1), DevToolsSecurity(1)

The Xcode Memory Graph Debuggger graphically shows malloc blocks and VM regions (both leaked and non-leaked), and the references between them.

The Xcode developer tools also include Instruments, a graphical application that can give information similar to that provided by leaks. The Allocations instrument graphically displays dynamic, real-time information about the object and memory use in an application, including backtraces of where the allocations occurred. The Leaks instrument performs memory leak analysis.

March 15, 2022 macOS