If out_file.h5 is given, a new file is created with the in_file.h5 without the user block. In this case, infile.h5 is unchanged.
If out_file.h5 is not specified, the user_block is removed and in_file.h5 is rewritten, starting at byte 0.
If user_block is set, the user block will be written to user_block. If user_block is not set, the user block (if any) will be written to stdout. If --delete is selected, the user block will not be not written.
The most efficient way to create a user block is to create the file with a user block (see H5Pset_user_block), and write the user block data into that space from a program.
The user block is completely opaque to the HDF5 library and to the h5jam and h5unjam tools. The user block is simply read or written as a string of bytes, which could be text or any kind of binary data. It is up to the user to know what the contents of the user block means and how to process it.
When the user block is extracted, all the data is written to the output, including any padding or unwritten data.