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Fluxcapacitor is a tool for Linux, originally created to speed up SockJS tests.

It is somewhat similar to tools like:

• FreezeGun for Python
• timecop or Delorean for Ruby

Instead of patching time libraries in high level programming language, Fluxcapacitor "patches" low-level syscalls using ptrace on Linux.

This approach has a significant advantage: now you can run more than one independent processes that will be affected by speeding up the time. This is especially useful for testing network protocols.

First, compile:

make

For example, these commands should finish instantly:

./fluxcapacitor --libpath=. -- sleep 1
./fluxcapacitor --libpath=. -- python -c "import time; time.sleep(1000)"

Fluxcapacitor does two things:

1. It forces fluxcapacitor_preload.so to be preloaded for the executed command. This library is responsible for two things:

   • It makes sure that clock_gettime will not be used by fast VDSO mechanism, but by the standard system call. (that gives us the opportunity to replace the return value of the system call later).
   • It replaces various time-related libc functions, like time() and gettimeofday() with replacements using clock_gettime underneath.

2. It runs the command (and its forked children) in a ptrace() sandbox, capturing all the syscalls. Some syscalls - notably clock_gettime will have original results overwritten by faked values. Other syscalls, like select(), poll() and epoll_wait() can be interrupted and its return value (-EINTR) will be rewritten to suggest reaching a timeout.

Fluxcapacitor won't work in a number of cases:

1. If your code is statically compiled and fluxcapacitor_preload.so can't play its role.

2. If your code uses non file descriptors event loops, like: signalfd(), sigwait(), wait(), etc. Or if your program relies heavily on signals and things like alert(), setitimer() or timerfd_create().

Basically, for Fluxcapacitor to work all the time queries need to be done using gettimeofday() or clock_gettime() and all the waiting for timeouts must rely on select(), poll() or epoll_wait().