Structure Arguments

There are many objects defined throughout the library as structures of varying and possibly large sizes. Due to the inefficiency of passing structures by value (copying the whole structure to a function), structures are generally passed to functions by reference. Because of this, there is always a potential to pass an invalid pointer to a function which expects a pointer to a structure. The following examples illustrate the right and wrong ways to use such a function. Given a structure and a library function which initializes it,

typedef  struct
{
    int   a, b, c, d;
} big_struct;

void    initialize( struct  big_struct   *s )
{
    s->a = 1;   s->b = 2;   s->c = 3;    s->d = 4;
}

int  main()
{
    big_struct   *s;

    initialize( s );   /* WRONG */
}

because the variable s is an uninitialized pointer. The correct method is to define a structure variable, not a pointer to a structure, and pass a pointer to the structure:

int  main()
{
    big_struct   s;

    initialize( &s );
}

Alternately, the incorrect example above could have been corrected by allocating the s pointer before calling initialize.