• Because the length of a variable length array is, in general, not known until after it is read, the library must allocate the memory, and the user must free it with nc_free_vlen().
• VLEN data must be handled in a special struct:

  /* This is the type of arrays of vlens. */
  typedef struct {
      size_t len; /* Length of VL data (in base type units) */
      void *p;    /* Pointer to VL data */
  } nc_vlen_t;
  

• ADVICE: In many cases where VLENs seem useful, one should consider instead fixed array sizes and on-the-fly compression, which will result in efficient data storage, without voilating the netCDF classic data model (and thus breaking compatibility with existing software.)

2008 Unidata NetCDF Workshop for Developers and Data Providers > User Defined Types in NetCDF-4