Malloc Debug

Malloc debug is a method of debugging native memory problems. It can help detect memory corruption, memory leaks, and use after free issues.

This documentation describes how to enable this feature on Android N or later versions of the Android OS.

The documentation for malloc debug on older versions of Android is here.

In order to enable malloc debug, you must be able to set special system properties using the setprop command from the shell. This requires the ability to run as root on the device.

When malloc debug is enabled, it works by adding a shim layer that replaces the normal allocation calls. The replaced calls are:

  • malloc
  • free
  • calloc
  • realloc
  • posix_memalign
  • memalign
  • malloc_usable_size

On 32 bit systems, these two deprecated functions are also replaced:

  • pvalloc
  • valloc

Any errors detected by the library are reported in the log.

Controlling Malloc Debug Behavior

Malloc debug is controlled by individual options. Each option can be enabled individually, or in a group of other options. Every single option can be combined with every other option.

Option Descriptions

front_guard[=SIZE_BYTES]

Enables a small buffer placed before the allocated data. This is an attempt to find memory corruption occuring to a region before the original allocation. On first allocation, this front guard is written with a specific pattern (0xaa). When the allocation is freed, the guard is checked to verify it has not been modified. If any part of the front guard is modified, an error will be reported in the log indicating what bytes changed.

If the backtrace option is also enabled, then any error message will include the backtrace of the allocation site.

If SIZE_BYTES is present, it indicates the number of bytes in the guard. The default is 32 bytes, the max bytes is 16384. SIZE_BYTES will be padded so that it is a multiple of 8 bytes on 32 bit systems and 16 bytes on 64 bit systems to make sure that the allocation returned is aligned properly.

This option adds a special header to all allocations that contains the guard and information about the original allocation.

Example error:

04-10 12:00:45.621  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 SIZE 100 HAS A CORRUPTED FRONT GUARD
04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[-32] = 0x00 (expected 0xaa)
04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[-15] = 0x02 (expected 0xaa)

rear_guard[=SIZE_BYTES]

Enables a small buffer placed after the allocated data. This is an attempt to find memory corruption occuring to a region after the original allocation. On first allocation, this rear guard is written with a specific pattern (0xbb). When the allocation is freed, the guard is checked to verify it has not been modified. If any part of the rear guard is modified, an error will be reported in the log indicating what bytes changed.

If SIZE_BYTES is present, it indicates the number of bytes in the guard. The default is 32 bytes, the max bytes is 16384.

This option adds a special header to all allocations that contains information about the original allocation.

Example error:

04-10 12:00:45.621  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 SIZE 100 HAS A CORRUPTED REAR GUARD
04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[130] = 0xbf (expected 0xbb)
04-10 12:00:45.622  7412  7412 E malloc_debug:   allocation[131] = 0x00 (expected 0xbb)

guard[=SIZE_BYTES]

Enables both a front guard and a rear guard on all allocations.

If SIZE_BYTES is present, it indicates the number of bytes in both guards. The default is 32 bytes, the max bytes is 16384.

backtrace[=MAX_FRAMES]

Enable capturing the backtrace of each allocation site. This option will slow down allocations by an order of magnitude. If the system runs too slowly with this option enabled, decreasing the maximum number of frames captured will speed the allocations up.

Note that any backtrace frames that occur within the malloc backtrace library itself are not recorded.

If MAX_FRAMES is present, it indicates the maximum number of frames to capture in a backtrace. The default is 16 frames, the maximumum value this can be set to is 256.

This option adds a special header to all allocations that contains the backtrace and information about the original allocation.

As of P, this option will also enable dumping backtrace heap data to a file when the process receives the signal SIGRTMAX - 17 ( which is 47 on most Android devices). The format of this dumped data is the same format as that dumped when running am dumpheap -n. The default is to dump this data to the file /data/local/tmp/backtrace_heap.PID.txt. This is useful when used with native only executables that run for a while since these processes are not spawned from a zygote process.

Note that when the signal is received, the heap is not dumped until the next malloc/free occurs.

backtrace_enable_on_signal[=MAX_FRAMES]

Enable capturing the backtrace of each allocation site. If the backtrace capture is toggled when the process receives the signal SIGRTMAX - 19 (which is 45 on most Android devices). When this option is used alone, backtrace capture starts out disabled until the signal is received. If both this option and the backtrace option are set, then backtrace capture is enabled until the signal is received.

If MAX_FRAMES is present, it indicates the maximum number of frames to capture in a backtrace. The default is 16 frames, the maximumum value this can be set to is 256.

This option adds a special header to all allocations that contains the backtrace and information about the original allocation.

backtrace_dump_on_exit

As of P, when the backtrace option has been enabled, this causes the backtrace dump heap data to be dumped to a file when the program exits. If the backtrace option has not been enabled, this does nothing. The default is to dump this to the file named /data/local/tmp/backtrace_heap.PID.exit.txt.

The file location can be changed by setting the backtrace_dump_prefix option.

backtrace_dump_prefix

As of P, when the backtrace options has been enabled, this sets the prefix used for dumping files when the signal SIGRTMAX - 17 is received or when the program exits and backtrace_dump_on_exit is set.

The default is /data/local/tmp/backtrace_heap.

When this value is changed from the default, then the filename chosen on the signal will be backtrace_dump_prefix.PID.txt. The filename chosen when the program exits will be backtrace_dump_prefix.PID.exit.txt.

fill_on_alloc[=MAX_FILLED_BYTES]

Any allocation routine, other than calloc, will result in the allocation being filled with the value 0xeb. When doing a realloc to a larger size, the bytes above the original usable size will be set to 0xeb.

If MAX_FILLED_BYTES is present, it will only fill up to the specified number of bytes in the allocation. The default is to fill the entire allocation.

fill_on_free[=MAX_FILLED_BYTES]

When an allocation is freed, fill it with 0xef.

If MAX_FILLED_BYTES is present, it will only fill up to the specified number of bytes in the allocation. The default is to fill the entire allocation.

fill[=MAX_FILLED_BYTES]

This enables both the fill_on_alloc option and the fill_on_free option.

If MAX_FILLED_BYTES is present, it will only fill up to the specified number of bytes in the allocation. The default is to fill the entire allocation.

expand_alloc[=EXPAND_BYTES]

Add an extra amount to allocate for every allocation.

If XX is present, it is the number of bytes to expand the allocation by. The default is 16 bytes, the max bytes is 16384.

free_track[=ALLOCATION_COUNT]

When a pointer is freed, do not free the memory right away, but add it to a list of freed allocations. In addition to being added to the list, the entire allocation is filled with the value 0xef, and the backtrace at the time of the free is recorded. The backtrace recording is completely separate from the backtrace option, and happens automatically if this option is enabled. By default, a maximum of 16 frames will be recorded, but this value can be changed using the free_track_backtrace_num_frames option. It can also be completely disabled by setting the option to zero. See the full description of this option below.

When the list is full, an allocation is removed from the list and is checked to make sure that none of the contents have been modified since being placed on the list. When the program terminates, all of the allocations left on the list are verified.

If ALLOCATION_COUNT is present, it indicates the total number of allocations in the list. The default is to record 100 freed allocations, the max allocations to record is 16384.

This option adds a special header to all allocations that contains information about the original allocation.

Example error:

04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 USED AFTER FREE
04-15 12:00:31.305  7412  7412 E malloc_debug:   allocation[20] = 0xaf (expected 0xef)
04-15 12:00:31.305  7412  7412 E malloc_debug:   allocation[99] = 0x12 (expected 0xef)
04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace at time of free:
04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so

In addition, there is another type of error message that can occur if an allocation has a special header applied, and the header is corrupted before the verification occurs. This is the error message that will be found in the log:

04-15 12:00:31.604  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 HAS CORRUPTED HEADER TAG 0x1cc7dc00 AFTER FREE

free_track_backtrace_num_frames[=MAX_FRAMES]

This option only has meaning if free_track is set. It indicates how many backtrace frames to capture when an allocation is freed.

If MAX_FRAMES is present, it indicates the number of frames to capture. If the value is set to zero, then no backtrace will be captured when the allocation is freed. The default is to record 16 frames, the max number of frames to to record is 256.

leak_track

Track all live allocations. When the program terminates, all of the live allocations will be dumped to the log. If the backtrace option was enabled, then the log will include the backtrace of the leaked allocations. This option is not useful when enabled globally because a lot of programs do not free everything before the program terminates.

This option adds a special header to all allocations that contains information about the original allocation.

Example leak error found in the log:

04-15 12:35:33.304  7412  7412 E malloc_debug: +++ APP leaked block of size 100 at 0x2be3b0b0 (leak 1 of 2)
04-15 12:35:33.304  7412  7412 E malloc_debug: Backtrace at time of allocation:
04-15 12:35:33.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
04-15 12:35:33.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
04-15 12:35:33.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
04-15 12:35:33.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
04-15 12:35:33.305  7412  7412 E malloc_debug: +++ APP leaked block of size 24 at 0x7be32380 (leak 2 of 2)
04-15 12:35:33.305  7412  7412 E malloc_debug: Backtrace at time of allocation:
04-15 12:35:33.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
04-15 12:35:33.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
04-15 12:35:33.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
04-15 12:35:33.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so

record_allocs[=TOTAL_ENTRIES]

Keep track of every allocation/free made on every thread and dump them to a file when the signal SIGRTMAX - 18 (which is 46 on most Android devices) is received.

If TOTAL_ENTRIES is set, then it indicates the total number of allocation/free records that can be retained. If the number of records reaches the TOTAL_ENTRIES value, then any further allocations/frees are not recorded. The default value is 8,000,000 and the maximum value this can be set to is 50,000,000.

Once the signal is received, and the current records are written to the file, all current records are deleted. Any allocations/frees occuring while the data is being dumped to the file are ignored.

NOTE: This option is not available until the O release of Android.

The allocation data is written in a human readable format. Every line begins with the THREAD_ID returned by gettid(), which is the thread that is making the allocation/free. If a new thread is created, no special line is added to the file. However, when a thread completes, a special entry is added to the file indicating this.

The thread complete line is:

THREAD_ID: thread_done 0x0

Example:

187: thread_done 0x0

Below is how each type of allocation/free call ends up in the file dump.

pointer = malloc(size)

THREAD_ID: malloc pointer size

Example:

186: malloc 0xb6038060 20

free(pointer)

THREAD_ID: free pointer

Example:

186: free 0xb6038060

pointer = calloc(nmemb, size)

THREAD_ID: calloc pointer nmemb size

Example:

186: calloc 0xb609f080 32 4

new_pointer = realloc(old_pointer, size)

THREAD_ID: realloc new_pointer old_pointer size

Example:

186: realloc 0xb609f080 0xb603e9a0 12

pointer = memalign(alignment, size)

THREAD_ID: memalign pointer alignment size

posix_memalign(&pointer, alignment, size)

THREAD_ID: memalign pointer alignment size

Example:

186: memalign 0x85423660 16 104

pointer = valloc(size)

THREAD_ID: memalign pointer 4096 size

Example:

186: memalign 0x85423660 4096 112

pointer = pvalloc(size)

THREAD_ID: memalign pointer 4096 SIZE_ROUNDED_UP_TO_4096

Example:

186: memalign 0x85423660 4096 8192

record_allocs_file[=FILE_NAME]

This option only has meaning if record_allocs is set. It indicates the file where the recorded allocations will be found.

If FILE_NAME is set, then it indicates where the record allocation data will be placed.

NOTE: This option is not available until the O release of Android.

Additional Errors

There are a few other error messages that might appear in the log.

Use After Free

04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 USED AFTER FREE (free)
04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace of original free:
04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so
04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace at time of failure:
04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so

This indicates that code is attempting to free an already freed pointer. The name in parenthesis indicates that the application called the function free with the bad pointer.

For example, this message:

04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 USED AFTER FREE (realloc)

Would indicate that the application called the realloc function with an already freed pointer.

Invalid Tag

04-15 12:00:31.304  7412  7412 E malloc_debug: +++ ALLOCATION 0x12345678 HAS INVALID TAG 1ee7d000 (malloc_usable_size)
04-15 12:00:31.305  7412  7412 E malloc_debug: Backtrace at time of failure:
04-15 12:00:31.305  7412  7412 E malloc_debug:           #00  pc 00029310  /system/lib/libc.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #01  pc 00021438  /system/lib/libc.so (newlocale+160)
04-15 12:00:31.305  7412  7412 E malloc_debug:           #02  pc 000a9e38  /system/lib/libc++.so
04-15 12:00:31.305  7412  7412 E malloc_debug:           #03  pc 000a28a8  /system/lib/libc++.so

This indicates that a function (malloc_usable_size) was called with a pointer that is either not allocated memory, or that the memory of the pointer has been corrupted.

As with the other error message, the function in parenthesis is the function that was called with the bad pointer.

Backtrace Heap Dump Format

This section describes the format of the backtrace heap dump. This data is generated by am dumpheap -n or, as of P, by the signal or on exit.

The data has this header:

Android Native Heap Dump v1.0

Total memory: XXXX
Allocation records: YYYY
Backtrace size: ZZZZ

Total memory is the total of all of the currently live allocations. Allocation records is the total number of allocation records. Backtrace size is the maximum number of backtrace frames that can be present.

Following this header are two different sections, the first section is the allocation records, the second section is the map data.

The allocation record data has this format:

z ZYGOTE_CHILD_ALLOC  sz    ALLOCATION_SIZE  num  NUM_ALLOCATIONS bt FRAMES

ZYGOTE_CHILD_ALLOC is either 0 or 1. 0 means this was allocated by the zygote process or in a process not spawned from the zygote. 1 means this was allocated by an application after it forked off from the zygote process.

ALLOCATION_SIZE is the size of the allocation. NUM_ALLOCATIONS is the number of allocations that have this size and have the same backtrace. FRAMES is a list of instruction pointers that represent the backtrace of the allocation.

Example:

z 0  sz      400  num    1  bt 0000a230 0000b500
z 1  sz      500  num    3  bt 0000b000 0000c000

The first allocation record was created by the zygote of size 400 only one with this backtrace/size and a backtrace of 0xa230, 0xb500. The second allocation record was create by an application spawned from the zygote of size 500, where there are three of these allocation with the same backtrace/size and a backtrace of 0xb000, 0xc000.

The final section is the map data for the process:

MAPS
7fe9181000-7fe91a2000 rw-p 00000000 00:00 0                              /system/lib/libc.so
.
.
.
END

The map data is simply the output of /proc/PID/maps. This data can be used to decode the frames in the backtraces.

There is a tool to visualize this data, development/scripts/native_heapdump_viewer.py.

Examples

For platform developers

Enable backtrace tracking of all allocation for all processes:

adb shell stop
adb shell setprop libc.debug.malloc.options backtrace
adb shell start

Enable backtrace tracking for a specific process (ls):

adb shell setprop libc.debug.malloc.options backtrace
adb shell setprop libc.debug.malloc.program ls
adb shell ls

Enable backtrace tracking for the zygote and zygote based processes:

adb shell stop
adb shell setprop libc.debug.malloc.program app_process
adb shell setprop libc.debug.malloc.options backtrace
adb shell start

Enable multiple options (backtrace and guards):

adb shell stop
adb shell setprop libc.debug.malloc.options "\"backtrace guards\""
adb shell start

Note: The two levels of quoting in the adb shell command is necessary. The outer layer of quoting is for the shell on the host, to ensure that the inner layer of quoting is sent to the device, to make ‘backtrace guards’ a single argument.

Enable malloc debug using an environment variable (pre-O Android release):

adb shell
# setprop libc.debug.malloc.env_enabled 1
# setprop libc.debug.malloc.options backtrace
# export LIBC_DEBUG_MALLOC_ENABLE=1
# ls

Enable malloc debug using an environment variable (Android O or later):

adb shell
# export LIBC_DEBUG_MALLOC_OPTIONS=backtrace
# ls

Any process spawned from this shell will run with malloc debug enabled using the backtrace option.

adb shell stop
adb shell setprop libc.debug.malloc.options backtrace
adb shell start
adb shell am dumpheap -n <PID_TO_DUMP> /data/local/tmp/heap.txt

It is possible to use the backtrace_enable_on_signal option as well, but, obviously, it must be enabled through the signal before the file will contain any data.

For app developers

Enable malloc debug for a specific program/application (Android O or later):

adb shell setprop wrap.<APP> '"LIBC_DEBUG_MALLOC_OPTIONS=backtrace logwrapper"'

For example, to enable malloc debug for the google search box (Android O or later):

adb shell setprop wrap.com.google.android.googlequicksearchbox '"LIBC_DEBUG_MALLOC_OPTIONS=backtrace logwrapper"'
adb shell am force-stop com.google.android.googlequicksearchbox

NOTE: On pre-O versions of the Android OS, property names had a length limit of 32. This meant that to create a wrap property with the name of the app, it was necessary to truncate the name to fit. On O, property names can be an order of magnitude larger, so there should be no need to truncate the name at all.

To detect leaks while an app is running:

adb shell dumpsys meminfo --unreachable <PID_OF_APP>

Without also enabling malloc debug, this command will only tell you whether it can detect leaked memory, not where those leaks are occurring. If you enable malloc debug with the backtrace option for your app before running the dumpsys command, you'll get backtraces showing where the memory was allocated.

For backtraces from your app to be useful, you‘ll want to keep the symbols in your app’s shared libraries rather than stripping them. That way you'll see the location of the leak directly without having to use something like the ndk-stack tool.

Analyzing heap dumps

To analyze the data produced by the dumpheap command, run this script:

development/scripts/native_heapdump_viewer.py

In order for the script to properly symbolize the stacks in the file, make sure the script is executed from the tree that built the image.

To collect, transfer, and analyze a dump:

adb shell am dumpheap -n <PID_TO_DUMP> /data/local/tmp/heap.txt
adb shell pull /data/local/tmp/heap.txt .
python development/scripts/native_heapdump_viewer.py --symbols /some/path/to/symbols/ heap.txt > heap_info.txt

At the moment, the script will look for symbols in the given directory, using the path the .so file would have on the device. So if your .so file is at /data/app/.../lib/arm/libx.so on the device, it will need to be at /some/path/to/symbols/data/app/.../lib/arm/libx.so locally given the command line above. That is: you need to mirror the directory structure for the app in the symbols directory.