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# VFD SWMR User's Guide
## Caveats
A few library functions are known to work incorrectly with VFD
SWMR.
* Variable-length data written using VFD SWMR may be inaccessible
or inconsistent to a VFD SWMR reader until the writer closes
the file. Instead of the proper variable-length data, a reader may
read NULL or arbitrary bytes. Inconsistencies may also cause the
reader to crash.
* Applications should take care using HDF5 iteration APIs, especially
when iterating large numbers of objects or using long-running
application callbacks. While the library is in an iteration routine,
it does not track changes made by the writer. If the library spends more than
`max_lag` ticks in the routine, then its view of the HDF5 file will become
stale. Under those circumstances, HDF5 content could be mis-read, or the
library could crash with a diagnostic assertion.
* At the present level of development, the writer cannot invalidate a reader's HDF5 object handles (`hid_t`s). If a reader holds an object open---that is, it has a valid handle (`hid_t`) for the object---while the writer deletes it, then
reading content through the handle may yield corrupted data or the data from some
other object, or the library may crash.
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