The uv i/o routines need to know what variables change simultaneously. all variables that have changes simultaneously. Conversely routine uvwrite assumes that all variables that change simultaneously with the variables that it writes, have already been written with routine uvputvr. Hence if the programmer is using uvscan, uvread or uvwrite, then simultaneity is not a concern, as long as variables are read after uvscan/uvread, and written before uvwrite. The routine uvnext provides better control, where this is needed. For an input file, a call to uvnext causes the next set of variables (which change simultaneously) to be read. For an output file, uvnext causes a marker to be written into the data, to indicate that variables written after the call did not change simultaneously with variables written before the call.

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