dataset:write:character:scalar: trim to avoid empty junk

for now we leave empty character arrays as-is, the user would need
to check for junk

test:attr:empty string: seeing if GCC 11.3 needs auxiliary test variable

doc: character write auxiliary variable recommended

Author: scivision

API.md

@@ -2,6 +2,8 @@
 
 This document provides a listing of h5fortran `public` scoped user-facing procedures and methods with a summary of their parameters.
 
+Reading or writing {real64,real32,int32,int64} from scalar to 7d is supported.
+
 All examples assume:
 
 ```fortran
@@ -18,6 +20,40 @@ use h5fortran, only : hdf5version
 print *, hdf5version()
 ```
 
+## Character variables
+
+Character variables are also supported for datasets and attributes.
+On some compilers (e.g. oneAPI), it's more stable to use an auxiliary variable when writing character data to avoid "junk" in the string.
+
+For example:
+
+```fortran
+character(5) :: aux
+
+aux = "hello"
+
+h%write("/mystr", aux)
+```
+
+rather than simply
+
+```fortran
+h%write("/mystr", "hello")
+```
+
+This is ESPECIALLY true if writing empty character to a dataset or attribute:
+
+```fortran
+
+h%write("/mystr", "")  !< DON'T: may have junk characters in file
+
+
+!! instead do like:
+character(123) :: aux  !< arbitrary length
+aux = ""
+h%write("/mystr", aux)
+```
+
 ## Open HDF5 file reference
 
 More than one HDF5 file can be open in a program, by declaring unique file handle (variable) like:
@@ -48,7 +84,7 @@ call h%close(close_hdf5_interface)
 logical, intent(in), optional :: close_hdf5_interface
 ```
 
-To avoid memory leaks or corrupted files, always "close" files before STOPping the Fortran program.
+To avoid memory leaks or corrupted files, always "close" files before Stopping the Fortran program.
 
 ## Flush data to disk while file is open
 

src/write_scalar.f90

@@ -26,7 +26,7 @@
   dtype = H5T_STD_I64LE
 type is (character(*))
   dtype = H5T_NATIVE_CHARACTER
-  charlen = len(A)  !< workaround for GCC 8.3.0 bug
+  charlen = len_trim(A)  !< workaround for GCC 8.3.0 bug
   if(charlen == 0) charlen = 1 !< empty string is OK but charlen is strictly positive.
 class default
   error stop "ERROR:h5fortran:write: unknown variable type for " // dname
@@ -48,7 +48,7 @@
 type is (integer(int64))
   call H5Dwrite_f(dset_id, dtype, A, dset_dims, ier, file_space_id=file_space_id, xfer_prp=xfer_id)
 type is (character(*))
-  call H5Dwrite_f(dset_id, dtype_id, A, dset_dims, ier, file_space_id=file_space_id, xfer_prp=xfer_id)
+  call H5Dwrite_f(dset_id, dtype_id, trim(A), dset_dims, ier, file_space_id=file_space_id, xfer_prp=xfer_id)
 class default
   error stop "ERROR:h5fortran:write: unsupported type for " // dname
 end select

test/test_attributes.f90

@@ -1,7 +1,7 @@
 program test_attributes
 
 use, intrinsic:: iso_fortran_env, only: int32, real32, real64, stderr=>error_unit
-use, intrinsic:: iso_c_binding
+use, intrinsic:: iso_c_binding, only: C_NULL_CHAR
 
 use h5fortran, only: hdf5_file, h5write_attr, h5read_attr, HSIZE_T
 
@@ -34,6 +34,8 @@ subroutine test_write_attributes(path)
 type(hdf5_file) :: h
 character(*), intent(in) :: path
 
+character(123) :: buf
+
 integer :: i2(1,1), i3(1,1,1), i4(1,1,1,1), i5(1,1,1,1,1), i6(1,1,1,1,1,1), i7(1,1,1,1,1,1,1)
 
 call h%open(path, action='w', debug=.true.)
@@ -54,10 +56,16 @@ subroutine test_write_attributes(path)
 call h%writeattr('/x', 'i6', i6)
 call h%writeattr('/x', 'i7', i7)
 
-call h%writeattr("/x", "1d_emptyp", [character(1) :: "", ""])
+buf = ""
+!! This auxiliary buffer fixes GCC 11.3.0 and oneAPI.
+!! It wasn't necessary on GCC 11.4.1 and newer.
+!! with h5dump, what's seen on disk file is "\001" value instead of "\000" or space.
+call h%writeattr("/x", "empty_char", buf)
+
+call h%writeattr("/x", "1d_empty", [character(1) :: "", ""])
 call h%writeattr("/x", "c1d", [character(5) :: 'one', 'two', 'three'])
 call h%writeattr("/x", "c2d", reshape([character(5) :: 'one', 'two', 'three', 'four', 'five', 'six'], [2,3]))
-call h%writeattr("/x", "empty_char", "")
+
 
 call h%close()
 

test/test_string.f90

@@ -1,6 +1,7 @@
 program test_string
 
 use, intrinsic:: iso_fortran_env, only:  stderr=>error_unit
+use, intrinsic:: iso_c_binding, only : C_NULL_CHAR
 
 use hdf5, only: H5T_STR_SPACEPAD_F, HSIZE_T
 use h5fortran, only: hdf5_file
@@ -29,9 +30,14 @@ subroutine test_write(fn)
 
 type(hdf5_file) :: h
 
+character(123) :: aux
+
 call h%open(fn, action='w', debug=.true.)
 
-call h%write("/empty", "")
+aux = ""
+!! compillers like oneAPI need an auxiliary variable not a raw constant
+call h%write("/empty", aux)
+
 call h%write('/little', '42')
 call h%write('/MySentence', 'this is a little sentence.')
 call h%write('/vector_scalar', ['vector scalar'])
@@ -58,7 +64,9 @@ subroutine test_read(fn)
 
 call h%open(fn, action='r')
 
+value = ""
 call h%read('/empty', value)
+print '(L1,1x,i0,1x,a)', trim(value) == C_NULL_CHAR, len_trim(value), trim(value)
 if(len_trim(value) /= 0) error stop 'test_string: empty string failure: len_trim /= 0'
 if (value /= '') error stop 'test_string: empty string failure: value = ' // trim(value)