What does macOS kernel return 0x10000008 (MACH_SEND_MSG_TOO_SMALL) mean?

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: The declared send size does not contain a complete valid Mach message.

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MACH_SEND_INTERRUPTED	MACH_SEND_INVALID_REPLY

MACH_SEND_MSG_TOO_SMALL

A layout and size contract failure

MACH_SEND_MSG_TOO_SMALL points to a mismatch between the buffer presented to mach_msg, the send_size argument, and the message layout declared by the header and descriptors. It can occur before the kernel starts processing the message when the declared send size is smaller than the minimum header, but it can also identify a trailing item that runs beyond the declared message boundary while descriptors are being processed.

This is why simply adding a few bytes to a stack buffer is not a reliable fix. The message size must cover the header, the complex-message body count, every descriptor using its real ABI size, inline payload, and required natural alignment. On 64-bit builds, assuming all descriptor variants have the same layout is particularly dangerous.

What to verify

Set msgh_size from the completed serialized layout, not from a hand-maintained constant.
Use sizeof for the actual structure and descriptor types compiled for the target architecture.
Validate descriptor count, descriptor type, and each inline payload offset before sending.
For generated MIG interfaces, verify that the request buffer type and routine signature come from the same generated definition.

References

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