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ERROR_INVALID_USER_BUFFER
The supplied user buffer does not satisfy the I/O operation contract.
ERROR_INVALID_USER_BUFFER is Win32 error 1784 (0x6F8). It often surfaces when an API or driver validates a pointer, length, alignment, access direction, lifetime, or structure layout associated with user-mode I/O. The buffer may be non-null and still be invalid. Because asynchronous operations can outlive the initiating call, a pointer that was valid initially can become unusable before completion.
Common buffer contract violations
- the byte count is smaller than the required header or larger than the allocated region
- a structure has the wrong version, packing, alignment, or architecture-specific size
- an input buffer is passed where writable output storage is required, or vice versa
- memory is freed, unmapped, moved, or reused while overlapped I/O is still pending
- a device control code expects a different buffering method or nested pointer layout
Diagnostic data to capture
Record the API or IOCTL, input and output lengths, structure version and sizeof values, process architecture, synchronous versus overlapped mode, buffer allocation type, access flags, driver and device versions, and completion status. Log addresses only in restricted diagnostics because they are process-specific and can disclose memory layout. Preserve the exact binary header in a secure trace when protocol analysis requires it.
How to locate the mismatch
Read the contract for the exact call and compare every length field with the actual allocation. Validate integer conversions before allocation, especially 32-bit to 64-bit transitions and counts multiplied by element size. Zero-initialize versioned structures and set their documented size fields.
For asynchronous I/O, keep the buffer and OVERLAPPED storage alive and unchanged until completion is observed. Use Application Verifier, PageHeap, driver traces, or a minimal reproducer to detect early free and overwrite. If only one device driver fails, compare the request format with that driver’s supported version.
Corrective action
Allocate storage of the documented size and alignment, pass the correct pointer for the selected operation, and retain ownership until the system finishes. Reject untrusted length fields before they influence allocation or copy. Do not fix the symptom by simply making every buffer very large; that can hide unit and structure errors.
Drivers and wrapper libraries should return the required length when their contract supports it and distinguish insufficient buffer from structurally invalid buffer. Cross-architecture tests should verify packing and pointer-free wire formats.
Difference from insufficient buffer
ERROR_INSUFFICIENT_BUFFER usually means valid output storage is too small and may be retried with a reported size. Error 1784 indicates the buffer itself or its relationship to the request is invalid; blindly increasing its length may not help.
Example
A 64-bit service sends an IOCTL using a structure copied from a 32-bit plug-in header. The nested offset is interpreted as a pointer-sized field and the driver returns 1784. Replacing the ABI-dependent structure with the documented fixed-width layout resolves the request.
References
- Microsoft: System Error Codes (1700–3999)
- Microsoft: DeviceIoControl
- Microsoft: Synchronous and asynchronous I/O
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