Fix: GameMaker Save File Buffer Corruption

A player reports their 30-hour save is unreadable after a crash during saving. You look at the bytes and the file is truncated halfway through — clearly the game died mid-write. GameMaker’s buffer_save is a single file-overwrite and gives no corruption protection. Safe saves need atomicity.

The Fix: Temp-and-Rename

function save_game_safe(slot) {
    var buf = build_save_buffer();
    var payload_size = buffer_tell(buf);

    // Append SHA-256 footer
    var hash = sha1_string_utf8(buffer_base64_encode(buf, 0, payload_size));
    buffer_write(buf, buffer_text, hash);

    var tmp_path = "save_" + string(slot) + ".dat.tmp";
    var live_path = "save_" + string(slot) + ".dat";
    var bak_path = "save_" + string(slot) + ".dat.bak";

    buffer_save(buf, tmp_path);
    buffer_delete(buf);

    if (file_exists(live_path)) {
        if (file_exists(bak_path)) file_delete(bak_path);
        file_rename(live_path, bak_path);
    }
    file_rename(tmp_path, live_path);
}

The sequence:

1. Write to .tmp with hash footer.
2. Move current live to .bak.
3. Move .tmp to live.

A crash between any two steps leaves a recoverable state — either the old save, the new save, or the temp.

Verifying on Load

function load_game_safe(slot) {
    var path = "save_" + string(slot) + ".dat";
    if (!file_exists(path)) path = "save_" + string(slot) + ".dat.bak";
    var buf = buffer_load(path);
    if (!verify_hash_footer(buf)) {
        // fall back to .bak if live is corrupted
        buffer_delete(buf);
        var bak = "save_" + string(slot) + ".dat.bak";
        if (file_exists(bak)) buf = buffer_load(bak);
    }
    return parse_save_buffer(buf);
}

Async Saves

For larger saves, use buffer_save_async and listen for the completion event in Async - Save/Load. Only perform the rename step when the async event fires success. This prevents the rename from happening before the write physically completes to disk.

Understanding the issue

Save data is forever. Once a save format ships, players will have that format in their files; you can't take it back. Bugs in save serialization compound over releases.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

At the engine level, the behavior comes from a deliberate design decision in GameMaker. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

For GameMaker-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

Within GameMaker, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Every save system that doesn’t use temp-and-rename eventually eats a player’s save. The fix is five lines; the cost of skipping it is a refund.”

Related Issues

For broader save strategy, see GameMaker save best practices. For preventing file corruption in general, see how to prevent save file corruption bugs.

Keep exactly one backup generation (.bak). More backups waste disk, zero backups loses data.