Fix: GameMaker Buffer Write String Encoding Corrupt

Here is how to fix GameMaker buffer-based saves where strings come back as garbled text or completely wrong characters. You write a player name with buffer_write(buf, buffer_string, name), save the buffer to disk, load it later, and read with buffer_text — only to get random Unicode mixed with junk bytes. The buffer functions look interchangeable but they encode strings differently.

The Symptom

You serialize a save with several strings (player name, last save location, current quest). On load, the strings read back as gibberish. Sometimes the first string reads correctly but subsequent reads are offset; sometimes everything is corrupt from byte zero.

What Causes This

Mismatched write/read types. buffer_string writes length + 1 bytes (string + null terminator). buffer_text writes exactly length bytes with no terminator. Reading with the wrong function leaves the buffer cursor in the wrong position for everything that follows.

UTF-8 multi-byte characters. GameMaker buffers are UTF-8. string_length counts code points, not bytes. A 5-character string like café! may be 6 bytes (the é takes 2 bytes). Computing offsets from string_length without accounting for this corrupts the cursor.

buffer_u8 with non-ASCII. Writing a single character via buffer_u8 and a string takes only the first byte. Multi-byte characters lose their tail bytes.

Seek mismatch. If you buffer_seek by an absolute byte offset that does not correspond to a value boundary, subsequent reads desynchronize.

The Fix

Step 1: Always pair buffer_string with buffer_string.

// Save
var _buf = buffer_create(256, buffer_grow, 1);
buffer_write(_buf, buffer_string, player_name);
buffer_write(_buf, buffer_string, last_scene);
buffer_write(_buf, buffer_s32, score);
buffer_save(_buf, "save.dat");
buffer_delete(_buf);

// Load
var _buf = buffer_load("save.dat");
player_name = buffer_read(_buf, buffer_string);
last_scene  = buffer_read(_buf, buffer_string);
score       = buffer_read(_buf, buffer_s32);
buffer_delete(_buf);

Use buffer_string on both sides. The null terminator marks the end so the reader knows when to stop. The cursor automatically advances past the terminator.

Step 2: For length-prefixed text, use buffer_text + explicit length.

// Write
var _bytes = string_byte_length(player_name);
buffer_write(_buf, buffer_u32, _bytes);
buffer_write(_buf, buffer_text, player_name);

// Read
var _bytes = buffer_read(_buf, buffer_u32);
var _start = buffer_tell(_buf);
var _str   = buffer_peek(_buf, _start, buffer_text);
buffer_seek(_buf, buffer_seek_relative, _bytes);

This is more verbose but useful when interfacing with external binary protocols.

Step 3: Validate format with a magic header.

// Always start saves with a magic + version
var MAGIC = 0x_BUG_NET_5;
var VERSION = 2;

buffer_write(_buf, buffer_u32, MAGIC);
buffer_write(_buf, buffer_u16, VERSION);
// ... data ...

if (buffer_read(_buf, buffer_u32) != MAGIC) {
    show_debug_message("Save corrupt or wrong format");
    exit;
}
var ver = buffer_read(_buf, buffer_u16);

Step 4: Avoid buffer_u8 for characters. If you must encode a single character, use buffer_string with a 1-character string. buffer_u8 is for byte values, not text.

Step 5: Use buffer_string_length helper if computing offsets. When you need to know how many bytes a string will occupy including the null terminator:

function buffer_string_size(_s) {
    return string_byte_length(_s) + 1;
}

string_byte_length returns UTF-8 byte count (correctly counts multi-byte characters). The +1 accounts for the null terminator.

Backward-Compatible Save Loading

If you change save format between versions, store the version at the top and branch read logic:

if (ver == 1) {
    player_name = buffer_read(_buf, buffer_string);
    score = buffer_read(_buf, buffer_s32);
} else if (ver == 2) {
    player_name = buffer_read(_buf, buffer_string);
    score = buffer_read(_buf, buffer_s32);
    achievements = buffer_read(_buf, buffer_u64);
}

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In GameMaker Studio, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

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

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

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

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

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

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

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

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

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

Boundary conditions deserve specific testing attention. What happens when the input is zero, maximum, negative, or NaN? What happens at the start of a session vs hours in? What happens at the boundary between two systems handling the same data? These are where bugs hide and where regression tests are most valuable.

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

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

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.

“buffer_string is self-delimiting. buffer_text is not. Pick one and use the same on both ends.”

Related Issues

For save corruption in general, see Save File Buffer Corruption. For DS list save persistence, see DS List Not Persisting.

Pair the type. Mind UTF-8 lengths. Magic header for format checks. Saves load clean.