declare private structure in C - c

Question

Is it possible to declare a structure type that is only visible in the .c file which uses the structure? I know that by putting static in front of a external data object, you change the linkage of the variable to be internal. But is it possible to put static in front of the declaration of a new struct type, like the following?
static struct log{
...;
...;
};
typedef struct log log;
If it is not possible to make the structure type, say log as above, to be "private", does it mean that even though other source files do not know the existence of the name (which is log in my example) of the structure, accidental name collisions can still happen if they name some variables log (assuming I will link all object files) ?
EDIT: I am not familiar with how compiler/linker works. If there is a global variable name log, and the file that contains the global variable is linked to the sole source file in which structure log is defined, wouldn't that cause any confusion when linking, one log is a variable name while another log is a type name?

Answer 1

No. The only way to make a struct private is to only have its definition available in the files that use it -- don't put it in a common header file. If it's only used in one source file, then just define it in that source file, but if it's used in more than one source file, you have a tricky problem: you can define it in each source file, but that's fragile since you have to remember to change each instance of it when you make any changes; or, you can define it in a private header file, and make sure only those source files include the private header.
Name collisions in different source files are ok, as long as they don't try to interface with each other in any way. If you have a struct log defined in one file and a different definition of struct log in a different file, do not ever pass one log to the other. In C, the structure name doesn't become part of any symbol names in the object file -- in particular, there's no name mangling of function names to include the parameter types (like C++ does), since function overloading is illegal in C.

Answer 2

No. static is a storage type; it is not meaningful to apply it to a type outside a variable declaration.
If you don't want to define struct log in your header file, you don't have to. Simply writing the typedef as:
typedef struct log log;
is sufficient, so long as you only deal with log * pointers. However, you will need a full definition of the structure to declare a log (or take sizeof(log)), because the size of the structure depends on what it contains.
With regard to name collisions, keep in mind that structures and types are not managed by the linker. The linker only cares about globally visible symbols, such as functions and variables. That being said, you should probably apply a prefix to your type names (e.g, mylib_log_t) to avoid confusion, particularly because log is a math function in the standard library.

Answer 3

You have a reason to write this:
static int a;
Because it prevents the linker from combining it with a defined somewhere else.
The linker has nothing to do with structs, so there is no worries putting in different c files.
As long as its in different c files, there will be no name confusions.

Answer 4

This isn't possible in general. But I can think of a hack that might work on some compilers.
The reason why this is hard to do is because the C compiler needs to know what the structure looks like in order to generate calls to functions with instances of the structure as argument.
So, suppose that you define a library with the following header:
struct foo {
int32_t a, b;
};
foo make_foo(int arg);
foo do_something(foo p1, foo p2);
Then to compile a program which makes a call to do_something, your compiler usually needs to know what the structure foo is like, so that it can pass it as an argument. The compiler can do all sorts of weird things here, like passing part of the structure via registers and part via the stack, so it really needs to know what the structure looks like.
However, I believe that in some compilers, it is possible to give the indication that the structure should be passed entirely via the stack. For instance, the regparm(0) function attribute should work for GCC if you have i386 as your target architecture (docs).
In that situation, it should be possible to do something like this: create a 'public version' of the header file, and in that file, instead of laying out the full struct, you create an undiferentiated version of it:
struct foo {
uint8_t contents[SIZE_OF_STRUCT_FOO];
}
where SIZE_OF_STRUCT_FOO is whatever sizeof(struct foo) returns when you define the struct in the usual way. You are then basically saying that "foo" is a struct with SIZE_OF_STRUCT_FOO bytes. Then, as long as the calling convention treats these two structs in the same way, it should work.