[SOLVED] Cannot write a variable length string attribute to HDF5 file

Cannot write a variable length string attribute to HDF5 file

Given the following function, I am trying to write a variable length string to an open HDF5 file. However, the H5Awrite(...) line causes an access violation inside the HDF5 stack, which I can't see. Before I go find or build debug libraries for HDF5, I was hoping there was some obvious issue.

Also, I removed the error handling that would normally be there to make the code more simple while debugging and sharing. I inspected and verified each hid_t is valid, and the HDF5 library has not issued any prior warnings or errors.

#include <hdf5.h>
#include <string>

void writeAttr(hid_t parent, const std::string& name, const std::string& value)
{
    hid_t type = H5Tcopy(H5T_C_S1);
    H5Tset_size(type, H5T_VARIABLE);

    hid_t attr = H5Acreate(parent, name.c_str(), type, H5Screate(H5S_SCALAR), H5P_DEFAULT, H5P_DEFAULT);
    herr_t status = H5Awrite(attr, type, value.c_str());  // <-- access violation

    H5Aclose(attr);
    H5Tclose(type);
}

int main()
{
    const std::string filename = "D:\\test.h5";
    hid_t file = H5Fcreate(filename.c_str(), H5F_ACC_TRUNC, H5P_DEFAULT, H5P_DEFAULT);
    writeAttr(file, "Attribute", "Attribute value");
    H5Fclose(file);
    return 0;
}

I have also tried adding the following with no luck:

H5Tset_cset(type, H5T_CSET_ASCII);
H5Tset_strpad(type, H5T_STR_NULLTERM);

HDF5 library version 1.14.1

Solution

I cheated and asked one of the hdf5 maintainers who was kind enough to resolve the issue:

"Variable-length strings are a bit of a special case in HDF5. It's potentially a bit misleading due to the way the array is declared." ... "The short of it is essentially that the library expects char * for strings and char ** for variable-length strings.".

So:

auto lvptr = value.c_str();
herr_t status = H5Awrite(attr, type, &lvptr);

Then it runs without issues.