A variable length array record as one record

[Leg501]

Good afternoon , I'm trying to figure out how to write and read a variable length set in 1 entry. What I mean. I have a pointer to a common data pool array of bytes uint8 and a list of segment sizes from this data pool. I want to save each segment as a single item.

std::vector<uint8_t> pool{0x35,0x34, 0x35, 0x00, 0x00, 0x35,0x36,0x36, 0x36, 0x36, 0x36, 0x00, 0x36, 0x00, 0x36, 0x36, 0x36 };
std::vectorsizeseg{4,5,3,5}

waiting in the file when saving:
0x35 0x34 0x35 0x00
0x00, 0x35,0x36,0x36, 0x36
0x36, 0x36, 0x00
0x36, 0x00, 0x36, 0x36, 0x36

When you read it, you can reconstruct it into a set of vectors if it is possible
std::vector<vector<uint8_t>>

[DavidAce]

Hello,
You can write variable-length data using the hvl_t type provided by HDF5. It is essentially a struct with 2 members:

  /**
   * Variable Length Datatype struct in memory (This is only used for VL
   * sequences, not VL strings, which are stored in char *'s)
   */
  typedef struct {
      size_t len; /**< Length of VL data (in base type units) */
      void  *p;   /**< Pointer to VL data */
  } hvl_t;

You can then populate a buffer in the form of std::vector<hvl_t> where each member covers the segments in your original buffer std::vector<uint8_t>.

To create the HDF5 datatype you need to use H5Tvlen_create(H5T_NATIVE_UINT8) and pass this along when you create the dataset.
See for instance example 04c, how the hvl_t year member is treated.