Grammar and syntax of typedef in C language.

Question

Hi folks:

I have a problem with the typedef keywords in C language.

In my program, I use the following codes:

typedef int* a[10];

int main(){
 int a[10];
} 

they work well. But why there are no conflicts between a variable and a type sharing the same name a?

Regards.

Answer 1

The C standard says (Section 6.2.1 - Scopes of identifiers):

An identifier can denote an object; a function; a tag or a member of a structure, union, or enumeration; a typedef name; a label name; a macro name; or a macro parameter. The same identifier can denote different entities at different points in the program.

K&R2 say (A.11.1 - Lexical Scope)

Identifiers fall into several name spaces that do not interfere with one another; the same identifier may be used for different purposes, even in the same scope, if the uses are in different name spaces. These classes are: objects, functions, typedef names, and enum constants; labels; tags of structures or unions, and enumerations; and members of each structure or union individually.

I must admit this confuses me. Reading the second quote it appears that variable names and typedef-ed types should clash.

Answer 2

See msdn C language reference:

Typedef names share the name space with ordinary identifiers (see Name Spaces for more information). Therefore, a program can have a typedef name and a local-scope identifier by the same name.

Answer 3

Variables and typedefs occupy the same namespace, and can not share names with other identifiers in exactly the same scope.

However your second a is inside main, and scope rules apply: the second a overrides the first.

You can do the same thing with just plain variables:

int a;

int main() {
    int a;
}

You'll notice that if you move the variable declaration outside main, the program won't compile.