EM-Explorer is available under the Apache License, version 2.0. Please see the 
LICENSE file for details.

1. Tool Description
EM-Explorer is a proof-of-concept stateless model checker which simulates the 
concurrency behaviour of Android applications which are event-driven multi-threaded 
programs. Given an execution trace of an Android app, containing concurrency 
relevant operations such as read-write, fork-join, posting and dequeuing of events 
etc., EM-Explorer systematically explores all interleaving of operations in 
the input trace permitted under the concurrency semantics of Android apps.
Instead of performing a brute-force exploration of all permissible interleavings 
EM-Explorer employs partial order reduction (POR) technique to prune redundant
interleavings, thus exploring only a subset of interleavings under the guidance
of POR algorithm. We have integrated the following POR algorithms with EM-Explorer:

1. EM-DPOR - The 'master' branch of this repository contains implementation of
   the EM-DPOR algorithm integrated with the stateless model-checker. EM-DPOR
   has been implemented with the help of the following reference papers:
   (i)  Partial Order Reduction for Event-driven Multi-threaded Programs,
        Pallavi Maiya, Rahul Gupta, Aditya Kanade and Rupak Majumdar, TACAS 2016.
   (ii) A Partial Order Reduction Technique for Event-driven Multi-threaded Programs,
        Pallavi Maiya, Rahul Gupta, Aditya Kanade and Rupak Majumdar, 2015.
   The EM-DPOR algorithm can be found in Section 4 of (ii). 

2. DPOR - The implementation of DPOR algorithm can be found in the 'dpor' branch of
   the repository. This is implemented by consulting the DPOR algorithm presented in
   Dynamic Partial-Order Reduction for Model Checking Software, Cormac Flanagan and
   Patrice Godefroid, POPL 2005.

The results reported for EM-DPOR and DPOR in the evaluation section of the EM-DPOR 
related papers are obtained using the implementation in src folder of the master and
dpor branches respectively. The Android app traces on which the two algorithms were 
evaluated can be found in test-traces/tacas-2016 directory.

Note that EM-Explorer is only a prototype model checker developed to test out various 
POR techniques. It does not directly explore a program, and only explores various 
interleavings of an "execution trace" from a program. EM-Explorer has been implemented 
to explore interleavings of execution traces of Android apps obtained by running
Android apps using a tool called DroidRacer (

2. Downloading the repository
Open a terminal and issue the following command to clone the repository. This will
download the master branch of the repository.
$ git clone

To download the dpor branch issue the following command from the em-explorer directory
after performing the above step.
<path-to-em-explorer>$ git fetch && git checkout dpor

The above command checks out the dpor branch and also switches to it. To go back to
the master branch which contains EM-DPOR algorithm perform the following:
<path-to-em-explorer>$ git checkout master

3. Compiling the tool source code
Note that EM-Explorer is a standalone C++ program which only contains C++98 compliant 
features. Any compatible version of g++ can be used to compile EM-Explorer's source.

The Makefile to build the tool can be found in the root of the repository. It takes an 
argument called 'EXE' which indicates the name of the executable to be generated. From 
the em-explorer directory issue:
<path-to-em-explorer>$ make EXE="execname"

The executable execname can then be found in the generated bin folder.

To obtain various debug information such as progress made by EM-Explorer periodically,
interleavings (traces) explored and so on, appropriate flags will have to be set in
src/Headers/Debug.h before compiling the source. Refer to the inline comments in
Debug.h for further information on this. Among all the flags, we have found it useful 
to set the following flags to 1:

4. Running the tool
EM-Explorer takes a file config.cfg as input which indicates the execution trace to be
taken as input for further exploration. You can find sample config.cfg files in various 
folders inside test-traces directory. A config.cfg file should be populated with 
following kind of key value pairs:


1. trace - Indicates the execution trace of an Android app obtained by running the app
   using DroidRacer. DroidRacer outputs this trace as 'trace.txt'. Refer Section 7 of
   this document for information on how to obtain trace.txt of an app using DroidRacer.

2. dependence - Indicates the 'dependence.txt' file corresponding to the trace.txt 
   file generated for an app trace by DroidRacer. Android apps interact heavily with
   their environment resulting in events being posted to the app by the environment. 
   DroidRacer uses an environment model to infer the ordering between a few environment
   events which cannot be established by the application of happens-before rules such as 
   fork happens-before threadinit, post happens-before dequeue etc. The file 
   dependence.txt captures such explicitly established ordering between environement
   events. This is supplied only to prevent EM-Explorer from exploring invalid
   interleavings. Section 7 describes the procedure to obtain dependence.txt.

3. exploredTracesFile - Indicates path of the file to which explored interleavings
   of input trace should be output. Note that the explored traces are output only
   if EXPLORED_FILE_WRITING_ENABLED flag in src/Headers/Debug.h is set to 1.

4. exploredTracesFolder - Instead of writing all traces to a single file you can
   opt each explored trace to be stored as an individual file in the folder 
   indicated against this key. This folder should be created before running EM-Explorer.

5. fileOrFolder - setting this to 1 writes explored traces to file indicated against
   exploredTracesFile key, and setting this to 2 stores traces as files in the folder
   path indicated in exploredTracesFolder.

6. whichConfig - Set this to a value 2 or 3. Earlier 2 indicated EM-Explorer to use
   DPOR algorithm and 3 indicated EM-Explorer to use EM-DPOR algorithm. However, currently
   the POR algorithm to be used is decided by the branch in which the source code resides.
   Hence, the value indicated here is effectively ignored.
7. report - Path of a file 'report.txt' to which stats such as traces explored, 
   transitions (operations) explored and time taken are output. If the flag 
   FREQUENT_REPORTING_ENABLED in src/Headers/Debug.h is set to 1 then the periodic
   exploration reports are written to the file indicated here. 

8. reportingFrequencyInMinutes - The interval in which periodic reports must be emitted.

Assume that the executable compiled to bin directory is named emdpor. Then, EM-Explorer
can be run as follows:
<path-to-em-explorer>$ bin/emdpor <path-to-config.cfg> 

Progress of exploration can be monitored by checking report.txt, explored traces printed to 
file or any debug info printed to console based on Debug flags set in src/Headers/Debug.h.

5. Sample input traces
We have included numerous sample inputs comprising of config.cfg, trace.txt and dependence.txt
in sub-directories of the test-traces folder in the root of em-explorer repository. 
The traces included in test-traces/tacas-2016 folder correspond to those reported in
EM-DPOR related papers. You can reproduce the results reported in our paper by running
em-dpor and dpor versions of EM-Explorer with config.cfg files found in test-traces/tacas-2016 
as input. However, make sure to supply absolute paths wherever required in config.cfg files.

We have also provided many hand-constructed traces to understand various aspects of the
EM-DPOR algorithm. These traces and the purpose served by them can be found in various 
readme.txt files in test-traces/benchmark-traces folder. Make sure to provide absolute
paths in config.cfg files. Also, explore these traces by setting EXPLORED_FILE_WRITING_ENABLED 
flag in src/Headers/Debug.h to 1, so that you can observe how EM-DPOR identifies dependent
operations and uses various systematic strategy to reorder them.

6. EM-DPOR Source code
The EM-DPOR enabled EM-Explorer implementation can be found in src folder of master
branch. From the main() function in Explore.cpp the control flow of the algorithm
implementation can be followed. We have annotated important functions and code blocks 
with detailed comments mapping to the algorithm lines, any implementation deviation from 
the algorithm presented in the paper or any non-trivial aspects of implementation. 
Also, the beginning of header files in src/Headers folder include comments indicating
the contents of the corresponding .h file whenever the contents are not apparent from
the name of the .h file. 

7. Obtaining input trace from an Android app using DroidRacer
Refer readme-droidRacer.txt which can be obtained at
and setup DroidRacer. Cloning the repository at
will only download pldi-2014 branch of DroidRacer. The branch of DroidRacer which generates
trace.txt and dependence.txt taken as input by EM-Explorer is called androidPOR-trace. 
After cloning droidracer repo do the following from droidracer folder to download and switch 
to branch androidPOR-trace.
<path-to-droidracer>$ git fetch && git checkout androidPOR-trace

Follow instructions in readme-droidRacer.txt in root of droidracer for info on how to
run an app using DroidRacer. You will need ModelCheckingServer tool mentioned in 
DroidRacer's readme only if you want to collect multiple execution traces of the same app
in an automated manner. To collect a random execution trace of an app you only need to
click on the app icon in the Android emulator after setting up DroidRacer and following
steps related to setting up the configuration file indicating the app to be tested etc.

You can download trace.txt and dependence.txt generated for an app whose process name 
for example is by using an Android SDK tool called adb (Android SDK
can be obtained at Issue the following
command when the Android emulator on which DroidRacer is installed is running, and after
the app corresponding to is explored by DroidRacer.
$<path-to-adb-folder>/adb pull /data/data/ <path-to-destination-folder>/trace.txt 
$<path-to-adb-folder>/adb pull /data/data/ <path-to-destination-folder>/dependence.txt 

The file trace.txt is a processed version of the original execution trace stored in a 
file called abc_log.txt which can be downloaded from Android emulator as follows:
$<path-to-adb-folder>/adb pull /data/data/ <path-to-destination-folder>/abc_log.txt 

Files abc_log.txt and trace.txt are basically logs of concurrency operations such as FORK,
THREADINIT, NOTIFY, POST etc. executed by the app run using DroidRacer.
Compared to abc_log.txt you will observe that trace.txt has many more threads but far lesser
number of event handlers (demarcated using operations CALL and RET respectively indicating
dequeue and completion of execution of an event handler). This is because EM-Explorer does
not handle events (indicated as msg in abc_log.txt and trace.txt) posted with delay (indicated 
by non zero value of delay field of a line in abc_log.txt corresponding to POST operation).
Instead events with greater than zero delays are modeled as events posted without delays 
(i.e. delay = 0) but posted non-deterministically from a different thread. Hence, in trace.txt 
you will find a thread corresponding to every delay post in abc_log.txt. These threads execute a 
DELAY-POST and exit. 
Also, in abc_log.txt you will see a few threads perform POST operation within blocks called 
NATIVE-ENTRY and NATIVE-EXIT. These operations are performed by system threads due to events 
triggered by the app's environment. In trace.txt posts from system threads are modeled as 
non-deterministic posts called NATIVE-POST from additional threads each created per POST 
by a system thread in abc_log.txt. Such additional threads exit after executing a NATIVE-POST
operation. The orderings indicated in dependence.txt establish happens-before ordering
between a few NATIVE-POST operations. The HB ordering indicated in dependence.txt also order 
special operations called ENABLE and TRIGGER. 
In addition to POST, DELAY-POST and NATIVE-POST you will find a type of post called
UI-POST in trace.txt but not present in abc_log.txt. UI-POST is a modeling of an operation 
posting a UI event to the main thread (tid:1). All the UI-POST operations are executed by a
thread with tid:2 which is a special thread created in trace.txt, and all the UI-POST
operations are totally ordered. 
The number of events (msg) in trace.txt is lesser than the number of events in abc_log.txt
because DroidRacer recursively removes event handlers and their corresponding posts which 
do not perform any concurrency operation such as shared memory access, fork a new thread etc.
We did this preprocessing since DPOR would unnecessarily explore reorderings even between 
such empty event handlers. Note that EM-DPOR however is immune to such event handlers due
to its novel dependence relation (refer EM-DPOR paper).

Operation NOP is a special operation emitted prior to every NATIVE-POST and UI-POST operation
to aid in unblocking the corresponding NATIVE-POST or UI-POST operation 'op' only after all
the operations which are indicated to happen-before 'op' in dependence.txt are executed. 
NOP is executed in trace.txt to overcome a technical glitch related to executing
a NATIVE-POST or UI-POST only after all the operations that happen-before it are executed.

8. Additional processing of trace.txt and dependence.txt downloaded from DroidRacer
Dynamic POR algorithms compute happens-before relation over explored traces. Both 
DPOR and EM-DPOR implementation use vector clocks data structure to compute happens-before
relation over explored traces. The size of the vector allocated for vector clocks are decided
by looking at the highest thread ID and event (msg) ID. Hence, it is essential to allocate
consecutive unique IDs to threads and messages to reduce space allocated for vector clocks.
However, the trace.txt generated by DroidRacer may not have consecutive IDs for threads and
events. We provide a helper tool called renumber-tool which can be found in 

Follow the following steps to renumber threads and events in trace.txt generated by DroidRacer:
1. Copy trace.txt into a file called input_trace.txt.
2. Move input_trace.txt to helper-tools/renumber-thread-task-tool/bin.
3. Run the foll. command:  <path-to-helper-tools/renumber-thread-task-tool/bin>$ ./renumber-tool
4. Output trace can be found in a file called trace.txt in helper-tools/renumber-thread-task-tool/bin. 

The source code and Makefile of renumber-tool can be found in helper-tools/renumber-thread-task-tool.
To compile source issue:
<path-to-helper-tools/renumber-thread-task-tool>$ make EXE=renumber-tool

Since we model delay post and post of UI events as non-deterministic posts from threads
which were not in the original trace (abc_log.txt), the trace.txt file generated by DroidRacer
may get malformed at times. We provide the following helper tools which can be found in
helper-tools/ui-delay-fix-tools/bin folder to make trace.txt well-formed:
-> DelayPostFix
-> UiPostFix

After renumbering threads and events, pass the trace.txt and dependence.txt through these two 
tools before providing them as input to EM-Explorer.

Follow the following steps to correct any ill-formedness due to DELAY-POST and UI-POST:
1. Copy trace.txt into a file called input_trace.txt.
2. Copy dependence.txt into a file called input_dependence.txt.
2. Move input_trace.txt and input_dependence.txt to helper-tools/ui-delay-fix-tools/bin.
3. Run the foll. command: <path-to-helper-tools/ui-delay-fix-tools/bin>$ ./DelayPostFix
4. Output can be found in files called trace.txt and dependence.txt in 
   helper-tools/ui-delay-fix-tools/bin folder.
5. Copy the trace.txt and dependence.txt generated by Step 4 into input_trace.txt and
   input_dependence.txt in the same folder.
6. Run the foll. command: <path-to-helper-tools/ui-delay-fix-tools/bin>$ ./UiPostFix
7. Output can be found in files called trace.txt and dependence.txt in 
   helper-tools/ui-delay-fix-tools/bin folder.

The source code and Makefile of DelayPostFix and UiPostFix tools can be found in 
helper-tools/ui-delay-fix-tools folder. Uncommenting line "if(op->getOpId() == UI_POST)"
and commenting line "if(op->getOpId() == DELAY_POST)" in function fixTrace() of 
helper-tools/ui-delay-fix-tools/src/TraceProcessor.h and compiling using Makefile will
generate UiPostFix, whereas doing vice versa will generate DelayPostFix.
Note that the executable name should be passed as argument to 'EXE' variable of Makefile
in both the cases.

Store the trace.txt and dependence.txt obtained after running renumber-tool, DelayPostFix
and UiPostFix into some folder where you want to run experiments, and indicate this path
against appropriate keys of config.cfg which is passed as input to EM-Explorer.

9. Acknowledgments
This work is supported by Google India through a PhD Fellowship in Programming 
Languages and Compilers awarded to Pallavi Maiya. This research was also funded
in part by the Indo-German Max Planck Center in Computer Science (IMPECS).

10. Contact
In case of any queries write to