Mocktime

Dead-simple, non-fancy gettimeofday-based time mocking library.

In my own projects, I've found that sometimes I want to test code that depends on the passage of time - for example, calculating network round-trip time, or estimating energy usage of an Android phone based on system parameters. Using gettimeofday and usleep for these tests can make my test suite take a long time to run.

The timeofday is just a number, though, so I should be able to mock it. I couldn't find any existing library that would let me do this (at least, not without changing all my code to use something other than gettimeofday), so I wrote this.

How to use

First, build and install the library:

$ make
$ sudo make install

And then in your code:

mocktime_enable_mocking();

struct timeval fake_time = {1445470140, 0}; /* October 21, 2015  04:29pm PDT */
mocktime_settimeofday(&fake_time, NULL);

struct timeval the_present;
mocktime_gettimeofday(&the_present, NULL);
/* 'the_present' now contains the time that you set. */

After you call mocktime_enable_mocking, all future calls to the other mocktime_* functions will operate on the fake timeofday inside the mocktime library. If you later call mocktime_disable_mocking, or if you never call mocktime_enable_mocking, the calls will pass through to their non-mocked counterparts.

For convenience, there's also a mocktime_usleep function that advances the mocked time by the specified number of microseconds, just as you'd expect.

TODO

• Automatically replace gettimeofday and friends with the mocked version using dlsym tricks. I may get to it eventually; feel free to submit a pull request for this.

• Add mocks for std::chrono facilities in C++11, as they're much nicer to use. Should maintain C-only compatibility, though.

Android

I also use this library on Android, so I've included an NDK build setup. First symlink the jni directory into your NDK modules path:

$ ln -s /path/to/mocktime/jni $NDK_MODULE_PATH/edu.umich.mobility/mocktime

then add this line to the end of your Android.mk:

$(call import-module, edu.umich.mobility/mocktime)

You'll also need to add mocktime to your LOCAL_SHARED_LIBRARIES.

Caveats

Mocktime only supports a single, global mocked timeofday and is therefore NOT thread-safe. I haven't thought at all about what it would mean to let one thread mock-sleep while others continue running - i.e. whether it's feasible or sensible at all.