5.1 Introduction

Attributes may be associated with each netCDF variable to specify such properties as units, special values, maximum and minimum valid values, scaling factors, and offsets. Attributes for a netCDF file are defined when the file is first created, while the netCDF dataset is in define mode. Additional attributes may be added later by reentering define mode. A netCDF attribute has a netCDF variable to which it is assigned, a name, a type, a length, and a sequence of one or more values. An attribute is designated by its variable ID and name. When an attribute name is not known, it may be designated by its variable ID and number in order to determine its name, using the function ncmpi_inq_attname.

The attributes associated with a variable are typically defined immediately after the variable is created, while still in define mode. The data type, length, and value of an attribute may be changed even when in data mode, as long as the changed attribute requires no more space than the attribute as originally defined.

It is also possible to have attributes that are not associated with any variable. These are called global attributes and are identified by using NC_GLOBAL as a variable pseudo-ID. Global attributes are usually related to the netCDF file as a whole and may be used for purposes such as providing a title or processing history for a netCDF file.

Operations supported on attributes are:

• Create an attribute, given its variable ID, name, data type, length, and value.
• Get attribute’s data type and length from its variable ID and name.
• Get attribute’s value from its variable ID and name.
• Copy attribute from one netCDF variable to another.
• Get name of attribute from its number.
• Rename an attribute.
• Delete an attribute.