IN: Identifier of the attribute datatype (in memory)
const void *buf
IN: Data to be written
H5A_WRITE writes an attribute, specified with attr_id. The attribute's memory datatype is specified with mem_type_id. The entire attribute is written from buf to the file.
If mem_type_id is either a fixed-length or variable-length string, it is important to set the string length when defining the datatype. String datatypes are derived from H5T_C_S1 (or H5T_FORTRAN_S1 for Fortran codes), which defaults to 1 character in size. See H5T_SET_SIZE and Creating variable-length string datatypes.