How do C libraries store state variables?

Question

I've been developing in C for a while now, coming from object-oriented languages like C++ and Java, and whilst using standard library functions like fread() and strtok(), I began to wonder: How do these functions go about storing variables related to their state?

For example, strtok() behaves differently for subsequent calls, and fread() keeps track of your position in the file. This information has to be stored somewhere, and in an object-oriented language there are scope constructs such as private or protected to guard these values from unwanted modification, but as far as I understand C does not have these.

So then, how are they safely stored? Or are they in fact global and modifiable from anywhere? The question spawned when I was writing a header file with utility functions that needed to store similar state information. What is the "right" way to do this sort of thing in C, and how is it handled in existing libraries?

Answer 1

In the case of fread() the state is stored in a FILE structure. The structure is a resource allocated by fopen() which returns a pointer to it, and you pass that pointer to every file operation call. The resource is released when fclose() is called. The FILE resource may be allocated from a static pool or it may be dynamically allocated from the heap - that would be implementation dependent. For example:

RESOURCE* getResource()
{
    return malloc( sizeof(RESOURCE) ) ; 
}

int useResource( RESOURCE* r )
{
   return r.index++ ;
}

void releaseResource( RESOURCE* r )
{
   free( r ) ;
}

strtok() on the other hand contains an internal static pointer that is initialised when a string is passed and used as the starting point when a null pointer is passed.

For example:

int getState()
{
    static int state = 0 ;
    return state++ ;
}

int main()
{
    int s ;

    do
    {
        s = getState() ;
        printf( "state = %d\n", s ;

    } while( s < 10 ) ;
}