ANSI X3J11 onexit() implementation

[Doug Gwyn]

/*
	onexit, exit -- register function to be invoked at program termination

	last exit:	17-Jun-1986	D A Gwyn

	Portable version that invokes the previous implementation of exit(),
	now renamed __exit(), just to show how onexit() can be implemented.
*/

#if __STDC__

#include	<stdlib.h>		/* defines onexit_t */

#define	VOID	void			/* empty argument list in prototype */
#define	INT	int			/* int argument in prototype */

#else	/* pre-X3J11 C */

#define	VOID	/* nothing -- prototypes not supported */
#define	INT	/* nothing -- prototypes not supported */

typedef	void	(*onexit_t)(VOID);	/* should be general enough */
/* The above line really needs to be put into a <stdlib.h> header! */

#define	NULL	0

#endif	/* __STDC__ */

extern void	__exit(INT);		/* pre-onexit version of exit();
					   typically _cleanup() then _exit() */

/* functions are registered in a linear list with explicit count: */

#ifndef	MAX_CALLS			/* # functions that can be registered */
#define	MAX_CALLS	32		/* minimum required by X3J11 */
#endif

static onexit_t	(*list[MAX_CALLS])(VOID);	/* function registration list */
static int	n_calls = 0;		/* # functions registered so far */


onexit_t
onexit( func )
	onexit_t	(*func)(VOID);	/* -> function to be registered */
	{
	if ( func == NULL		/* safety net for incorrect usage */
	  || n_calls == MAX_CALLS
	   )
		return (onexit_t) NULL;	/* registration failure */
	else	{
		list[n_calls++] = func;	/* register the function */

		return (onexit_t) func;	/* non-NULL => success */
		}
	}


void
exit( status )
	int	status;			/* termination status code */
	{
	/* execute functions in reverse order of registration */

	while ( n_calls != 0 )
		(void) (*list[--n_calls])();

	__exit( status );		/* former version of exit() */

	/*NOTREACHED*/
	}