help needed with custom allocators and buffer too small

[marlowa]

When I used the flatbuffer builder with a custom allocator all is well when the buffer is large enough. But when it isnt I get an assertion failure: Assertion `!ReadScalar<voffset_t>(buf_.data() + field_location->id)' failed. Any ideas?

[Advait251206]

Without seeing the allocator implementation, it's hard to say for certain, but my first suspicion would be that the custom allocator isn't correctly handling buffer growth and relocation.

When the builder runs out of space, it may request a larger buffer and move existing contents. If the allocator:

• Returns an invalid pointer,
• Doesn't preserve the existing contents,
• Doesn't honor the requested size/alignment,
• Frees or overwrites the old buffer too early,
• Or returns overlapping/corrupted memory,

then assertions like the one you're seeing can occur later when FlatBuffers tries to read internal bookkeeping data (voffset_t) from locations it expects to be valid.

A few things I'd check:

1. Does the problem reproduce with the default allocator?

   • If not, that strongly points to the custom allocator implementation.

2. Does your allocator correctly copy all existing bytes when growing the buffer?

   • The failure only occurring when expansion is required is a strong clue.

3. Are alignment requirements being respected?

   • FlatBuffers relies heavily on correct alignment.

4. Are you implementing all allocator methods expected by the FlatBuffers version you're using?

   • There have been allocator-related API changes over time.

5. Can you reproduce the issue with a minimal schema and a deliberately tiny initial buffer size?

   • That often makes it easier to isolate whether growth or alignment is the root cause.

If you can share the allocator implementation (or at least the allocation/reallocation logic), someone will likely be able to spot the issue quickly. The fact that everything works until the first resize strongly suggests the problem is in the buffer growth path rather than the actual serialization logic.