When anidentifier is encountered in a C program, a lookup is performed to locate thedeclaration that introduced that identifier and that is currentlyin scope. C allows more than one declaration for the same identifier to be in scope simultaneously if these identifiers belong to different categories, calledname spaces:
4) Global attribute name space:attribute tokens defined by the standard or implementation-defined attribute prefixes. 5) Non-standard attribute names: attribute names following attribute prefixes. Each attribute prefix has a separate name space for the implementation-defined attributes it introduces. | (since C23) |
At the point of lookup, the name space of an identifier is determined by the manner in which it is used:
4) identifier that directly appears in an attribute specifier ([[...]]) is looked up in the global attribute name space. 5) identifier that follows the:: token following an attribute prefix is looked in the name space introduced by the attribute prefix. | (since C23) |
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The names ofmacros are not part of any name space because they are replaced by the preprocessor prior to semantic analysis.
It is common practice to inject struct/union/enum names into the name space of the ordinary identifiers using atypedef declaration:
struct A{};// introduces the name A in tag name spacetypedefstruct A A;// first, lookup for A after "struct" finds one in tag name space// then introduces the name A in the ordinary name spacestruct A* p;// OK, this A is looked up in the tag name spaceA* q;// OK, this A is looked up in the ordinary name space
A well-known example of the same identifier being used across two name spaces is the identifierstat from the POSIX headersys/stat.h. Itnames a function when used as an ordinary identifier andindicates a struct when used as a tag.
Unlike in C++, enumeration constants are not struct members, and their name space is the name space of ordinary identifiers, and since there is no struct scope in C, their scope is the scope in which the struct declaration appears:
struct tagged_union{enum{INT, FLOAT, STRING} type;union{int integer;float floating_point;char*string;};} tu; tu.type= INT;// OK in C, error in C++
If a standard attribute, an attribute prefix, or a non-standard attribute name is not supported, the invalid attribute itself is ignored without causing an error. | (since C23) |
void foo(void){return;}// ordinary name space, file scopestruct foo{// tag name space, file scopeint foo;// member name space for this struct foo, file scopeenum bar{// tag name space, file scope RED// ordinary name space, file scope} bar;// member name space for this struct foo, file scopestruct foo* p;// OK: uses tag/file scope name "foo"};enum bar x;// OK: uses tag/file-scope bar// int foo; // Error: ordinary name space foo already in scope//union foo { int a, b; }; // Error: tag name space foo in scope int main(void){goto foo;// OK uses "foo" from label name space/function scope struct foo{// tag name space, block scope (hides file scope)enum bar x;// OK, uses "bar" from tag name space/file scope};typedefstruct foo foo;// OK: uses foo from tag name space/block scope// defines block-scope ordinary foo (hides file scope)(foo){.x=RED};// uses ordinary/block-scope foo and ordinary/file-scope RED foo:;// label name space, function scope}
C++ documentation forName lookup |
