Internal static variables in C, would you use them

问题:

In C you can have external static variables that are viewable every where in the file, while internal static variables are only visible in the function but is persistent

For example:

#include <stdio.h>

void foo_bar( void )
{
        static counter = 0;
        printf("counter is %d\n", counter);
        counter++;
}
int main( void )
{
        foo_bar();
        foo_bar();
        foo_bar();
 return 0;
}

the output will be

counter is 0
counter is 1
counter is 2

My question is why would you use an internal static variable? If you don't want your static variable visible in the rest of the file shouldn't the function really be in its own file then?

回答1:

This confusion usually comes about because the static keyword serves two purposes.

When used at file level, it controls the visibility of its object outside the compilation unit, not the duration of the object (visibility and duration are layman's terms I use during educational sessions, the ISO standard uses different terms which you may want to learn eventually, but I've found they confuse most beginning students).

Objects created at file level already have their duration decided by virtue of the fact that they're at file level. The static keyword then just makes them invisible to the linker.

When used inside functions, it controls duration, not visibility. Visibility is already decided since it's inside the function - it can't be seen outside the function. The static keyword in this case, causes the object to be created at the same time as file level objects.

Note that, technically, a function level static may not necessarily come into existence until the function is first called (and that may make sense for C++ with its constructors) but every C implementation I've ever used creates its function level statics at the same time as file level objects.

Also, whilst I'm using the word "object", I don't mean it in the sense of C++ objects (since this is a C question). It's just because static can apply to variables or functions at file level and I need an all-encompassing word to describe that.

Function level statics are still used quite a bit - they can cause trouble in multi-threaded programs if that's not catered for but, provided you know what you're doing (or you're not threading), they're the best way to preserve state across multiple function calls while still providing for encapsulation.

Even with threading, there are tricks you can do in the function (such as allocation of thread specific data within the function) to make it workable without exposing the function internals unnecessarily.

The only other choices I can think of are global variables and passing a "state variable" to the function each time.

In both these cases, you expose the inner workings of the function to its clients and make the function dependent on the good behavior of the client (always a risky assumption).

回答2:

They are used to implement tools like strtok, and they cause problems with reentrancy...

Think carefully before fooling around with this tool, but there are times when they are appropriate.

回答3:

For example, in C++, it is used as one way to get singleton istances

SingletonObject& getInstance()
{
  static SingletonObject o;
  return o;
}

which is used to solve the initialization order problem (although it's not thread-safe).

Ad "shouldn't the function be in its own file"

Certainly not, that's nonsense. Much of the point of programming languages is to facilitate isolation and therefore reuse of code (local variables, procedures, structures etc. all do that) and this is just another way to do that.

BTW, as others pointed out, almost every argument against global variables applies to static variables too, because they are in fact globals. But there are many cases when it's ok to use globals, and people do.

回答4:

I find it handy for one-time, delayed, initialization:

int GetMagic()
{
   static int magicV= -1;

   if(-1 == magicV)
   {
      //do expensive, one-time initialization
      magicV = {something here}
   }
   return magicV;
}

As others have said, this isn't thread-safe during it's very first invocation, but sometimes you can get away with it :)

回答5:

I think that people generally stay away from internal static variables. I know strtok() uses one, or something like it, and because of that is probably the most hated function in the C library.

Other languages like C# don't even support it. I think the idea used to be that it was there to provide some semblance of encapsulation (if you can call it that) before the time of OO languages.

回答6:

Probably not terribly useful in C, but they are used in C++ to guarantee the initialisation of namespace scoped statics. In both C and C++ there are problemns with their use in multi-threaded applications.

回答7:

I wouldn't want the existence of a static variable to force me to put the function into its own file. What if I have a number of similar functions, each with their own static counter, that I wanted to put into one file? There are enough decisions we have to make about where to put things, without needing one more constraint.

回答8:

Some use cases for static variables:

• you can use it for counters and you won't pollute the global namespace.
• you can protect variables using a function that gets the value as a pointer and returns the internal static. This whay you can control how the value is assigned. (use NULL when you just want to get the value)

回答9:

I've never heard this specific construct termed "internal static variable." A fitting label, I suppose.

Like any construct, it has to be used knowledgeably and responsibly. You must know the ramifications of using the construct.

It keeps the variable declared at the most local scope without having to create a separate file for the function. It also prevents global variable declaration.

For example -

char *GetTempFileName()
{
  static int i;
  char *fileName = new char[1024];
  memset(fileName, 0x00, sizeof(char) * 1024);
  sprintf(fileName, "Temp%.05d.tmp\n", ++i);
  return fileName;
}

VB.NET supports the same construct.

Public Function GetTempFileName() As String
  Static i As Integer = 0
  i += 1
  Return String.Format("Temp{0}", i.ToString("00000"))
End Function

One ramification of this is that these functions are not reentrant nor thread safe.

回答10:

Not anymore. I've seen or heard the results of function local static variables in multithreaded land, and it isn't pretty.

回答11:

In writing code for a microcontroller I would use a local static variable to hold the value of a sub-state for a particular function. For instance if I had an I2C handler that was called every time main() ran then it would have its own internal state held in a static local variable. Then every time it was called it would check what state it was in and process I/O accordingly (push bits onto output pins, pull up a line, etc).

回答12:

All statics are persistent and unprotected from simultaneous access, much like globals, and for that reason must be used with caution and prudence. However, there are certainly times when they come in handy, and they don't necessarily merit being in their own file.

I've used one in a fatal error logging function that gets patched to my target's error interrupt vectors, eg. div-by-zero. When this function gets called, interrupts are disabled, so threading is a non-issue. But re-entrancy could still happen if I caused a new error while in the process of logging the first error, like if the error string formatter broke. In that case, I'd have to take more drastic action.

void errorLog(...)
{
    static int reentrant = 0;
    if(reentrant)
    {
        // We somehow caused an error while logging a previous error.
        // Bail out immediately!
        hardwareReset();
    }

    // Leave ourselves a breadcrumb so we know we're already logging.
    reentrant = 1;

    // Format the error and put it in the log.
    ....

    // Error successfully logged, time to reset.
    hardwareReset();
}

This approach is checking against a very unlikely event, and it's only safe because interrupts are disabled. However, on an embedded target, the rule is "never hang." This approach guarantees (within reason) that the hardware eventually gets reset, one way or the other.

回答13:

A simple use for this is that a function can know how many times it has been called.