string_name's current value.

The compiler/linker will have allocated a single byte location and associated the variable name 'string_name' with that location. In C, parameters are passed by value only, so foo()'s str parameter would be set with the value held in the byte location associated with string_name at the point of function invocation.

If you also changed foo()'s str parameter to be a 'char *', this is still very likely not what you want. If you want to call foo() with string_name's value, change the str parameter's type to 'char'. If you want to call foo() with string_name's address, change the str parameter's type to 'char *' and change the statement calling foo() to 'foo(&string_name,1)'.