libfaketime-c

Overview

Per github.com/wolfcw/libfaketime (opens in new window), libfaketime returns a value derived from the FAKETIME environment variable instead of the real clock. Because it hooks libc, it works for any dynamically-linked binary - Go, Rust, Python, not just C/C++.

When to use

• Testing C/C++ code that uses libc time.
• Testing any process where you don't control the source.
• Integration tests where one process's perceived clock matters.
• CI tests for time-sensitive logic without overriding system clock.

How to use

1. Install faketime on the test host (apt / brew / from source).
2. Pick a mode: absolute date, relative offset (-1d, +1y), or advance-rate (@... x<rate>).
3. Wrap the target binary: faketime '<spec>' your_command, or set LD_PRELOAD plus FAKETIME directly.
4. Prefix TZ='<zone>' when the behaviour depends on a local zone, for example DST transitions.
5. Set FAKETIME_NO_CACHE=1 when the code reads time many times per second.
6. Assert on the program's visible output or behaviour from your test runner (libfaketime emits nothing itself).
7. Install faketime in CI before running the time-sensitive suite.

Authoring

Install

# Debian/Ubuntu
sudo apt install faketime

# macOS
brew install libfaketime

# From source
git clone https://github.com/wolfcw/libfaketime
cd libfaketime && make && sudo make install

Basic absolute-date mode

faketime '2026-12-31 23:59:00' your_command

your_command runs as if the wall clock is 2026-12-31 23:59:00.

Alternative via LD_PRELOAD directly:

LD_PRELOAD=/usr/local/lib/faketime/libfaketime.so.1 \
  FAKETIME='2026-12-31 23:59:00' \
  your_command

Relative offset (-1d, +1y), advance-rate (-f '@... x<rate>' for time speed-up), high-resolution mode (FAKETIME_NO_CACHE=1), and the spring-forward / cron time-skip / timezone recipes are in references/faketime-modes-and-recipes.md.

Parsing results

libfaketime doesn't emit output itself - it transparently intercepts time syscalls. Your tests assert on the program's visible behaviour:

import subprocess

def test_cron_fires_at_midnight():
    result = subprocess.run(
        ["faketime", "2026-12-31 23:59:30", "./cron-runner"],
        capture_output=True,
        text=True,
        timeout=5,
    )
    assert "Fired at 2027-01-01 00:00:00" in result.stdout

Worked example

Verify a scheduler's behaviour at the non-existent local time 02:30 on a spring-forward day, without changing the host clock:

1. Pick the transition: US Eastern springs forward at 02:00 on 2026-03-08, so 02:30 local does not exist that day.
2. Run with both the zone and the fake instant: TZ='America/New_York' faketime '2026-03-08 02:30:00' ./my-program.
3. libfaketime returns that wall-clock instant from libc, and TZ makes the program resolve it in Eastern.
4. Assert the program either skips the job or normalises to 03:30 (whichever the spec requires), matching dst-transition-reference.

Result: the spring-forward branch runs deterministically on every CI run, with no dependence on the real date or the host clock.

CI integration

jobs:
  time-tests:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - run: sudo apt-get install -y faketime
      - run: pytest tests/time/

Anti-patterns

Limitations

• Linux + macOS only. Windows uses different time syscalls.
• Statically linked binaries unaffected. Go binaries compiled with CGO_ENABLED=0 don't see libfaketime.
• JVM bypasses libc for some time calls (Instant from monotonic system clock). Use mockclock-jvm.
• Doesn't fake monotonic clocks by default; some clock_gettime flags pass through.
• No leap-second simulation. See leap-second-reference.

References

• libfaketime: github.com/wolfcw/libfaketime (opens in new window).
• Companion catalogs: dst-transition-reference, leap-second-reference, iso-8601-vs-rfc-3339-reference.
• Language-native siblings: sinon-fake-timers-js, jest-fake-timers, freezegun-python, timecop-ruby, mockclock-jvm.
• Test matrix: timezone-test-matrix-builder.