fopen.c

最新Minix3源码

/*
 * fopen.c - open a stream
 */
/* $Header: fopen.c,v 1.8 91/02/22 16:29:46 ceriel Exp $ */

#if	defined(_POSIX_SOURCE)
#include	<sys/types.h>
#endif
#include	<stdio.h>
#include	<stdlib.h>
#include	"loc_incl.h"

#define	PMODE		0666

/* The next 3 defines are true in all UNIX systems known to me.
 */
#define	O_RDONLY	0
#define	O_WRONLY	1
#define	O_RDWR		2

/* Since the O_CREAT flag is not available on all systems, we can't get it
 * from the standard library. Furthermore, even if we know that <fcntl.h>
 * contains such a flag, it's not sure whether it can be used, since we
 * might be cross-compiling for another system, which may use an entirely
 * different value for O_CREAT (or not support such a mode). The safest
 * thing is to just use the Version 7 semantics for open, and use creat()
 * whenever necessary.
 *
 * Another problem is O_APPEND, for which the same holds. When "a"
 * open-mode is used, an lseek() to the end is done before every write()
 * system-call.
 *
 * The O_CREAT, O_TRUNC and O_APPEND given here, are only for convenience.
 * They are not passed to open(), so the values don't have to match a value
 * from the real world. It is enough when they are unique.
 */
#define	O_CREAT		0x010
#define	O_TRUNC		0x020
#define	O_APPEND	0x040

int _open(const char *path, int flags);
int _creat(const char *path, Mode_t mode);
int _close(int d);

FILE *
fopen(const char *name, const char *mode)
{
	register int i;
	int rwmode = 0, rwflags = 0;
	FILE *stream;
	int fd, flags = 0;

	for (i = 0; __iotab[i] != 0 ; i++) 
		if ( i >= FOPEN_MAX-1 )
			return (FILE *)NULL;

	switch(*mode++) {
	case 'r':
		flags |= _IOREAD | _IOREADING;	
		rwmode = O_RDONLY;
		break;
	case 'w':
		flags |= _IOWRITE | _IOWRITING;
		rwmode = O_WRONLY;
		rwflags = O_CREAT | O_TRUNC;
		break;
	case 'a': 
		flags |= _IOWRITE | _IOWRITING | _IOAPPEND;
		rwmode = O_WRONLY;
		rwflags |= O_APPEND | O_CREAT;
		break;         
	default:
		return (FILE *)NULL;
	}

	while (*mode) {
		switch(*mode++) {
		case 'b':
			continue;
		case '+':
			rwmode = O_RDWR;
			flags |= _IOREAD | _IOWRITE;
			continue;
		/* The sequence may be followed by additional characters */
		default:
			break;
		}
		break;
	}

	/* Perform a creat() when the file should be truncated or when
	 * the file is opened for writing and the open() failed.
	 */
	if ((rwflags & O_TRUNC)
	    || (((fd = _open(name, rwmode)) < 0)
		    && (rwflags & O_CREAT))) {
		if (((fd = _creat(name, PMODE)) > 0) && flags  | _IOREAD) {
			(void) _close(fd);
			fd = _open(name, rwmode);
		}
			
	}

	if (fd < 0) return (FILE *)NULL;

	if (( stream = (FILE *) malloc(sizeof(FILE))) == NULL ) {
		_close(fd);
		return (FILE *)NULL;
	}

	if ((flags & (_IOREAD | _IOWRITE))  == (_IOREAD | _IOWRITE))
		flags &= ~(_IOREADING | _IOWRITING);

	stream->_count = 0;
	stream->_fd = fd;
	stream->_flags = flags;
	stream->_buf = NULL;
	__iotab[i] = stream;
	return stream;
}