C question: Why would one put 'static' variables in a header?

Static has three distinct uses in C:

Not really. It has two distinct uses: it changes linkage from external to internal, or duration from automatic to static.

(2) A variable declared static within a module, (but outside the body of a function) is accessible by all functions within that module. It is not accessible by functions within any other module. That is, it is a localized global.

(3) Functions declared static within a module may only be called by other functions within that module. That is, the scope of the function is localized to the module within which it is declared.

These two are the same thing, really. Distinguishing between functions and variables serves no purpose in this case. 'static' limits the visibility of external definitions to the containing translation unit, regardless if those definitions are of variables or functions.

Static has three distinct uses in C:

Not really. It has two distinct uses: it changes linkage from external to internal, or duration from automatic to static.

(2) A variable declared static within a module, (but outside the body of a function) is accessible by all functions within that module. It is not accessible by functions within any other module. That is, it is a localized global.

(3) Functions declared static within a module may only be called by other functions within that module. That is, the scope of the function is localized to the module within which it is declared.

These two are the same thing, really. Distinguishing between functions and variables serves no purpose in this case. 'static' limits the visibility of external definitions to the containing translation unit, regardless if those definitions are of variables or functions.

Interestingly, the header mentioned in my original post does also contain static function declarations!

AFAIKS, this really makes no sense at all, since it forces every file which includes the header to provided a definition of the function - otherwise you get a "function declared static but not defined" warning!

Again, this suggests to me that the only reason for all this is, in fact, that the author(s) had no idea what he/she/they were doing!

See reference above:

* Incompetence
* Sloth
* A bad character
* Job security
* Arrogance

this is actually a different question.

There are no externs in my question; just statics in a header file.

PS. Yes, Happry Easter!

"He is risen indeed!"

Andy,

With all due respect, and after much ado, I shall answer that question of "why put 'static' in a header file?"

Simple. ALL data-allocation on a module-wide basis should be declared/allocated within its associated header file. ALL external data-references should also be in it's [external] associated .C file's header.

ONLY routine/function specific data-store should be allocated within the actual .C file.

That's my philosophy, and I'm sticking to it.

A #define MAX_SIZE would also be in that header file, so your 'static UART_Buffer[ MAX_SIZE ]; is clearly identified within that .H file.

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

OK - if that is your philosophy, that's fair enough.

But, presumably, this requires a mechanism to ensure that "file-local" (ie, "internal linkage") variables are not repeated in other files?

Thanks for the 'fair enough' comment.

And yes, the "required" mechanism is my

#ifdef UART_Module
// Data Allocation
#else
// Data reference
#endif

construct.

(Not to mention my "uber" #include "Headers.H" file total control freak method of inclusion).

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

#ifdef UART_Module
// Data Allocation
#else
// Data reference
#endif

But that's where it gets a bit confusing!

The whole point of making something static is so that it can not be externally referenced!

So, in this case, would you have, say:

#ifdef UART_Module
static char private_uart_char;
#else
// No reference for private_uart_char
#endif

yes?

Perhaps you could clarify what you mean by "module" here?

Is it a single compilation unit, or the collection of files which, together, form a "module"?

eg, if your UART "driver" consists of, say, a uart_input.c, a uart_output.c and a uart_isr.c - do you consider that to be three "modules", or one?

Yes, you are correct. There would be no extern static UART_Buffer[ MAX_SIZE ];

Even though I am unsure as to the invalidity of doing so, generally I wouldn't do it. My prior post (with he link to the picture) showed that I did make 'extern static' data references. I don't remember why I was doing so. But out of 700+ only eight were of that type in an unproven code-monkey fragment.

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA
P.S. I've really got to move my house now... later for a while.

Even though I am unsure as to the invalidity of doing so, generally I wouldn't do it. My prior post (with he link to the picture) showed that I did make 'extern static' data references. I don't remember why I was doing so. But out of 700+ only eight were of that type in an unproven code-monkey fragment.

Here's what VC thinks of that:

error C2159: more than one storage class specified

Did you compile that code at all?

If I've managed to unpick your explanation of your header file usage you seem to define all file scope variables in a header file and all function scope variables in the function. That doesn't make a lot of sense - why hide file scope declarations in a separate file, particularly if they are not referenced outwith that file?

More than anything else, though, there's no point in ignoring convention unless you have a compelling reason to do so, and the reasons you have given so far are not compelling. They add up to little more than 'I do it my way because it's my way'.

I guess I forgot about that last question of three files. Each .C module should have an associated .H file. If another .C module needs access to the other .C file functions, or .H data-allocation references, then it would include the associated .H file.

/*
**
** UART_Protocol.C
*/
#include "UART_CONFIG.H"
#include "UART.H"
#include "UART_ISR.H"

But in MY case I would have it this way:

/*
******************************************
** UART_Protocol.C
*/
#define UART_Protocol_Module
#include "Includes.H"
#undef  UART_Protocol_Module // non-MISRA

/*
** Includes.H
*/
#ifdef UART_Protocol_Module
#include "UART_CONFIG.H"
#include "UART.H"
#include "UART_ISR.H"
#else
// Anything or nothing you need contain if
// UART_Protocol.C didn't call this module
#endif
/*
******************************************
** SPI Module
*/
#ifdef SPI_Module
#include "SPI_Config.H"
#include "SPI.H"
#else
// whatever else you might want if SPI.C
// didn't include the file
#endif
/*

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

P.S. I hate moving.

Dear Jack Sprat,

I do it this way because I realized a long time ago that the process of inclusion is typically unstructured, and I wanted to bring a method of structure to the inclusion process.

As most people know, a module that includes "87C51.H" in one file, and another file includes "87C51FB.H in another will run into errors that can be hard to track down.

By having a single and well controlled "Includes.H" file, you can control exactly what all project modules include, and the order of which they get included.

--Cpt. Vince Foster
2nd Cannon Place
Fort Marcy Park, VA

Vince darling,

As most people know, a module that includes "87C51.H" in one file, and another file includes "87C51FB.H in another will run into errors that can be hard to track down.

Indeed, the consequences of compiling the wrong code are well understood.

By having a single and well controlled "Includes.H" file, you can control exactly what all project modules include, and the order of which they get included.

That doesn't require those include files to contain file scope variable definitions, or, for that matter anything with the storage class static. Removing those things from the header files takes away the need for all the #ifdef/#undef/#endif stuff. If you want to stick all your #include statements in one include file that's absolutely fine.

Another concern I have is related to "Includes.H" itself. I don't like "super header files" because they may incur the need to recompile possibly the entire project (which might have a long compile and link time...) in case of only a minor change to one of its subordinates. But this is just my opinion.