Using static on typedef struct

Question

I use the following code a lot in C:

typedef struct
  {
  int member;
  } structname;

Now i'm trying to keep that struct definition local to a particular source file, so that no other source file even knows the struct exists. I tried the following:

static typedef struct
  {
  int member;
  } structname;

but GCC whines because of an illegal access specifier. Is it even possible to keep a struct's declaration private to a source file?

Answer 1

If you declare the typedef struct within a .c file, it will be private for that source file.

If you declare this typedef in a .h file, it will be accesible for all the .c files that include this header file.

Your statement:

static typedef struct

Is clearly illegal since you are neither declaring a variable nor defining a new type.

Answer 2

All declarations are always local to a particular translation unit in C. That's why you need to include headers in all source files that intend to use a given declaration.

If you want to restrict the use of your struct , either declare it in the file in which you use it, or create a special header that only your file includes.

Answer 3

A structure definition is private to a source file unless placed in a shared header file. No other source file can access the members of the struct, even if given a pointer to the struct (since the layout is not known in the other compilation unit).

If the struct needs to be used elsewhere, it must be used only as a pointer. Put a forward declaration of the form struct structname; typedef struct structname structname; struct structname; typedef struct structname structname; in the headerfile, and use structname * everywhere else in your codebase. Then, since the structure members appear only in one source file, the structure's contents are effectively 'private' to that file.

Answer 4

Hernan Velasquez's answer is the correct answer: there are several problems with your code snippet. Here's a counter-example:

/* This should go in a .h if you will use this typedef in multiple .c files */
typedef struct {
  int a;
  char b[8];
} mystructdef;

int
main (int argc, char *argv[])
{
  /* "static" is legal when you define the variable ...
    ... but *not* when you declare the typedef */
  static mystructdef ms;