offset tracking in posix and stdio module are not thread safe in some situations
This is especially true if threads hold distinct file pointers or file descriptors open to the same file concurrently. In this case Darshan will map them to the same underlying file record and use a shared offset variable to track previous position.
In the POSIX case this would affect DXT tracing, because that shared offset field is used to generate the file offset in the trace.
In the STDIO case the same problem exists, plus the shared offset is also used to calculate access sizes for conventional STDIO counters too, since not all STDIO functions report the number of bytes moved.
Problem identified by @glock .