The domain name is c. See https://www.sphinx-doc.org/en/master/usage/domains/c.html.
The .. c:member:: directive, showing a struct member.
The .. c:var:: directive, showing a variable.
The .. c:function:: directive, showing a function. The signature should be given as in C.
type – description of the first parameter.
nitems – description of the second parameter.
a result.
NULL – under some conditions.
NULL – under some other conditions as well.
The .. c:macro:: directive, showing a macro, i.e., a C-language #define, without the replacement text.
The .. c:macro:: directive, showing a macro, i.e., a C-language #define, without the replacement text.
The .. c:struct:: directive, showing a struct.
The .. c:union:: directive, showing a union.
The .. c:enum:: directive, showing an enum.
The .. c:enumerator:: directive, showing an enumerator.
The .. c:type:: directive, showing a type.