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MACH_MSG_VM_SPACE
A receiver address-space shortage
MACH_MSG_VM_SPACE is a special bit attached to a broader Mach message error. It means the receiving task could not obtain virtual address space for an out-of-line memory region. Without MACH_RCV_OVERWRITE, received out-of-line regions are dynamically allocated somewhere in the receiver’s map; a sufficiently fragmented, constrained, or exhausted map can prevent that allocation even when physical memory remains available.
The bit can also appear during the pseudo-receive used to restore a failed send. In that path, the returned message buffer may no longer contain the original addresses. The caller must follow the base return code’s ownership rules before deciding whether the request can be rebuilt or retried.
What to inspect
- Log the base error together with each out-of-line size and whether allocation or overwrite reception was requested.
- Check the receiver’s address-space limits and map pressure, especially in long-running processes that reserve many regions.
- Do not confuse this with
MACH_MSG_VM_KERNEL, which reports a kernel resource shortage. - For overwrite receives, verify that the supplied regions are large enough and writable; size mismatches have dedicated receive errors.
References
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