Language.C.Syntax.AST

Complete C tranlsation unit (C99 6.9, K&R A10)

A complete C translation unit, for example representing a C header or source file. It consists of a list of external (i.e. toplevel) declarations.

External C declaration (C99 6.9, K&R A10)

Either a toplevel declaration, function definition or external assembler.

C function definition (C99 6.9.1, K&R A10.1)

A function definition is of the form CFunDef specifiers declarator decllist? stmt.

• specifiers are the type and storage-class specifiers of the function. The only storage-class specifiers allowed are extern and static.

• The declarator must be such that the declared identifier has function type. The return type shall be void or an object type other than array type.

• The optional declaration list decllist is for old-style function declarations.

• The statement stmt is a compound statement.

C declarations (K&R A8, C99 6.7), including structure declarations, parameter declarations and type names.

A declaration is of the form CDecl specifiers init-declarator-list, where the form of the declarator list's elements depends on the kind of declaration:

1) Toplevel declarations (K&R A8, C99 6.7 declaration)

• C99 requires that there is at least one specifier, though this is merely a syntactic restriction

• at most one storage class specifier is allowed per declaration

• the elements of the non-empty init-declarator-list are of the form (Just declr, init?, Nothing). The declarator declr has to be present and non-abstract and the initialization expression is optional.

2) Structure declarations (K&R A8.3, C99 6.7.2.1 struct-declaration)

Those are the declarations of a structure's members.

• do not allow storage specifiers

• in strict C99, the list of declarators has to be non-empty

• the elements of init-declarator-list are either of the form (Just declr, Nothing, size?), representing a member with optional bit-field size, or of the form (Nothing, Nothing, Just size), for unnamed bitfields. declr has to be non-abstract.

• no member of a structure shall have incomplete type

3) Parameter declarations (K&R A8.6.3, C99 6.7.5 parameter-declaration)

• init-declarator-list must contain at most one triple of the form (Just declr, Nothing, Nothing), i.e. consist of a single declarator, which is allowed to be abstract (i.e. unnamed).

4) Type names (A8.8, C99 6.7.6)

• do not allow storage specifiers

• init-declarator-list must contain at most one triple of the form (Just declr, Nothing, Nothing). where declr is an abstract declarator (i.e. doesn't contain a declared identifier)

C structure or union specifiers (K&R A8.3, C99 6.7.2.1)

CStruct tag identifier struct-decls c-attrs represents a struct or union specifier (depending on tag).

• either identifier or the declaration list struct-decls (or both) have to be present.

Example: in struct foo x;, the identifier is present, in struct { int y; } x the declaration list, and in struct foo { int y; } x; both of them.

• c-attrs is a list of attribute annotations associated with the struct or union specifier

C enumeration specifier (K&R A8.4, C99 6.7.2.2)

CEnum identifier enumerator-list attrs represent as enum specifier

• Either the identifier or the enumerator-list (or both) have to be present.

• If enumerator-list is present, it has to be non-empty.

• The enumerator list is of the form (enumeration-constant, enumeration-value?), where the latter is an optional constant integral expression.

• attrs is a list of attribute annotations associated with the enumeration specifier

A tag to determine wheter we refer to a struct or union, see CStructUnion.

C declaration specifiers and qualifiers

Declaration specifiers include at most one storage-class specifier (C99 6.7.1), type specifiers (6.7.2) and type qualifiers (6.7.3).

Separate the declaration specifiers

attribute of a declaration qualify declarations or declarators (but not types), and are therefore separated as well.

C storage class specifier (and typedefs) (K&R A8.1, C99 6.7.1)

C type specifier (K&R A8.2, C99 6.7.2)

Type specifiers are either basic types such as char or int, struct, union or enum specifiers or typedef names.

As a GNU extension, a typeof expression also is a type specifier.

returns True if the given typespec is a struct, union or enum definition

C type qualifiers (K&R A8.2, C99 6.7.3) and attributes.

const, volatile and restrict type qualifiers Additionally, attribute annotations for declarations and declarators, and function specifiers

C function specifiers (C99 6.7.4)

function specifiers inline and _Noreturn

C alignment specifiers (C99 6.7.5)

attribute annotations

Those are of the form CAttr attribute-name attribute-parameters, and serve as generic properties of some syntax tree elements.

C declarator (K&R A8.5, C99 6.7.5) and abstract declarator (K&R A8.8, C99 6.7.6)

A declarator declares a single object, function, or type. It is always associated with a declaration (CDecl), which specifies the declaration's type and the additional storage qualifiers and attributes, which apply to the declared object.

A declarator is of the form CDeclr name? indirections asm-name? attrs _, where name is the name of the declared object (missing for abstract declarators), declquals is a set of additional declaration specifiers, asm-name is the optional assembler name and attributes is a set of attrs is a set of attribute annotations for the declared object.

indirections is a set of pointer, array and function declarators, which modify the type of the declared object as described below. If the declaration specifies the non-derived type T, and we have indirections = [D1, D2, ..., Dn] than the declared object has type (D1 indirect (D2 indirect ... (Dn indirect T))), where

• (CPtrDeclr attrs) indirect T is attributed pointer to T

• (CFunDeclr attrs) indirect T is attributed function returning T

• (CArrayDeclr attrs) indirect T is attributed array of elemements of type T

Examples (simplified attributes):

• x is an int

int x;
CDeclr "x" []

• x is a restrict pointer to a const pointer to int

const int * const * restrict x;
CDeclr "x" [CPtrDeclr [restrict], CPtrDeclr [const]]

• f is an function return a constant pointer to int

int* const f();
CDeclr "f" [CFunDeclr [],CPtrDeclr [const]]

• f is a constant pointer to a function returning int

int (* const f)(); ==>
CDeclr "f" [CPtrDeclr [const], CFunDeclr []]

Derived declarators, see CDeclr

Indirections are qualified using type-qualifiers and generic attributes, and additionally

• The size of an array is either a constant expression, variable length (*) or missing; in the last case, the type of the array is incomplete. The qualifier static is allowed for function arguments only, indicating that the supplied argument is an array of at least the given size.

• New style parameter lists have the form Right (declarations, isVariadic), old style parameter lists have the form Left (parameter-names)

Size of an array

C initialization (K&R A8.7, C99 6.7.8)

Initializers are either assignment expressions or initializer lists (surrounded in curly braces), whose elements are themselves initializers, paired with an optional list of designators.

Initializer List

The members of an initializer list are of the form (designator-list,initializer). The designator-list specifies one member of the compound type which is initialized. It is allowed to be empty - in this case the initializer refers to the 'next' member of the compound type (see C99 6.7.8).

Examples (simplified expressions and identifiers):

-- int x[3][4] = { [0][3] = 4, [2] = 5, 8 };
--   corresponds to the assignments
-- x[0][3] = 4; x[2][0] = 5; x[2][1] = 8;
let init1 = ([CArrDesig 0, CArrDesig 3], CInitExpr 4)
    init2 = ([CArrDesig 2]             , CInitExpr 5)
    init3 = ([]                        , CInitExpr 8)
in  CInitList [init1, init2, init3]

-- struct { struct { int a[2]; int b[2]; int c[2]; } s; } x = { .s = { {2,3} , .c[0] = 1 } };
--   corresponds to the assignments
-- x.s.a[0] = 2; x.s.a[1] = 3; x.s.c[0] = 1;
let init_s_0 = CInitList [ ([], CInitExpr 2), ([], CInitExpr 3)]
    init_s   = CInitList [
                           ([], init_s_0),
                           ([CMemberDesig "c", CArrDesig 0], CInitExpr 1)
                         ]
in  CInitList [(CMemberDesig "s", init_s)]

Designators

A designator specifies a member of an object, either an element or range of an array, or the named member of a struct / union.

C statement (K&R A9, C99 6.8)

C99 Block items

Things that may appear in compound statements: either statements, declarations or nested function definitions.

GNU Assembler statement

CAssemblyStatement type-qual? asm-expr out-ops in-ops clobbers _

is an inline assembler statement. The only type-qualifier (if any) allowed is volatile. asm-expr is the actual assembler epxression (a string), out-ops and in-ops are the input and output operands of the statement. clobbers is a list of registers which are clobbered when executing the assembler statement

Assembler operand

CAsmOperand argName? constraintExpr arg specifies an operand for an assembler statement.

C expression (K&R A7)

• these can be arbitrary expression, as the argument of sizeof can be arbitrary, even if appearing in a constant expression

• GNU C extensions: alignof, __real, __imag, ({ stmt-expr }), && label and built-ins