buffer.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

	    corrupt |= ar_read();
	}
	if (chunk > len) {
	    chunk = len;
	}
	bufidx += chunk;
	len -= chunk;
	total += chunk;
    }
    return (corrupt);
}


/* buf_read - read a given number of characters from the input archive
 *
 * DESCRIPTION
 *
 *	Reads len number of characters from the input archive and
 *	stores them in the buffer pointed at by dst.
 *
 * PARAMETERS
 *
 *	char   *dst	- pointer to buffer to store data into
 *	uint	len	- length of data to read
 *
 * RETURNS
 *
 * 	Returns zero with valid data, -1 if unreadable portions were 
 *	replaced by null characters. 
 */

#ifdef __STDC__

int buf_read(char *dst, uint len)

#else

int buf_read(dst, len)
char           *dst;
uint            len;

#endif
{
    int             have;
    int             want;
    int             corrupt = 0;
    char           *endx = dst + len;

    while (want = endx - dst) {
	if (bufend - bufidx < 0) {
	    fatal("Buffer overlow in buf_read\n");
	}
	while ((have = bufend - bufidx) == 0) {
	    have = 0;
	    corrupt |= ar_read();
	}
	if (have > want) {
	    have = want;
	}
	memcpy(dst, bufidx, have);
	bufidx += have;
	dst += have;
	total += have;
    }
    return (corrupt);
}


/* indata - install data from an archive
 *
 * DESCRIPTION
 *
 *	Indata writes size bytes of data from the archive buffer to the output 
 *	file specified by fd.  The filename which is being written, pointed
 *	to by name is provided only for diagnostics.
 *
 * PARAMETERS
 *
 *	int	fd	- output file descriptor
 *	OFFSET	size	- number of bytes to write to output file
 *	char	*name	- name of file which corresponds to fd
 *
 * RETURNS
 *
 * 	Returns given file descriptor. 
 */

#ifdef __STDC__

static int indata(int fd, OFFSET size, char *name)

#else

static int indata(fd, size, name)
int             fd;
OFFSET          size;
char           *name;

#endif
{
    uint            chunk;
    char           *oops;
    int             sparse;
    int             corrupt;
    char           *buf;
    uint            avail;

    corrupt = sparse = 0;
    oops = (char *)NULL;
    while (size) {
	corrupt |= buf_in_avail(&buf, &avail);
	size -= (chunk = size < avail ? (uint) size : avail);
	if (oops == (char *)NULL && (sparse = ar_write(fd, buf, chunk)) < 0) {
	    oops = strerror();
	}
	buf_use(chunk);
    }
    if (corrupt) {
	warn(name, "Corrupt archive data");
    }
    if (oops) {
	warn(name, oops);
    } else if (sparse > 0 && (lseek(fd, (OFFSET) - 1, 1) < 0
			      || write(fd, "", 1) != 1)) {
	warn(name, strerror());
    }
    return (fd);
}


/* outflush - flush the output buffer
 *
 * DESCRIPTION
 *
 *	The output buffer is written, if there is anything in it, to the
 *	archive file.
 */

#ifdef __STDC__

static void outflush(void)

#else

static void outflush()

#endif
{
    char           *buf;
    int             got;
    uint            len;

    /* if (bufidx - buf > 0) */
	for (buf = bufstart; len = bufidx - buf;) {
	    if ((got = write(archivefd, buf, MIN(len, blocksize))) > 0) {
		buf += got;
	    } else if (got < 0) {
		next(AR_WRITE);
	    }
	}
    bufend = (bufidx = bufstart) + blocksize;
}


/* ar_read - fill the archive buffer
 *
 * DESCRIPTION
 *
 * 	Remembers mid-buffer read failures and reports them the next time 
 *	through.  Replaces unreadable data with null characters.   Resets
 *	the buffer pointers as appropriate.
 *
 * RETURNS
 *
 *	Returns zero with valid data, -1 otherwise. 
 */

#ifdef __STDC__

int ar_read(void)

#else

int ar_read()

#endif
{
    int             got;
    static int      failed;

    bufend = bufidx = bufstart;
    if (!failed) {
	if (areof) {
	    if (total == 0) {
		fatal("No input");
	    } else {
		next(AR_READ);
	    }
	}
	while (!failed && !areof && bufstart + blocksize - bufend >= blocksize) {
	    if ((got = read(archivefd, bufend, (unsigned int) blocksize)) > 0) {
		bufend += got;
	    } else if (got < 0) {
		failed = -1;
		warnarch(strerror(), (OFFSET) 0 - (bufend - bufidx));
	    } else {
		++areof;
	    }
	}
    }
    if (failed && bufend == bufstart) {
	failed = 0;
	for (got = 0; got < blocksize; ++got) {
	    *bufend++ = '\0';
	}
	return (-1);
    }
    return (0);
}


/* ar_write - write a filesystem block
 *
 * DESCRIPTION
 *
 * 	Writes len bytes of data data from the specified buffer to the 
 *	specified file.   Seeks past sparse blocks. 
 *
 * PARAMETERS
 *
 *	int     fd	- file to write to
 *	char   *buf	- buffer to get data from
 *	uint	len	- number of bytes to transfer
 *
 * RETURNS
 *
 *	Returns 0 if the block was written, the given length for a sparse 
 *	block or -1 if unsuccessful. 
 */

#ifdef __STDC__

static int ar_write(int fd, char *buf, uint len)

#else

static int ar_write(fd, buf, len)
int             fd;
char           *buf;
uint            len;

#endif
{
    char           *bidx;
    char           *bend;

    bend = (bidx = buf) + len;
    while (bidx < bend) {
	if (*bidx++) {
	    return (write(fd, buf, len) == len ? 0 : -1);
	}
    }
    return (lseek(fd, (OFFSET) len, 1) < 0 ? -1 : len);
}


/* buf_pad - pad the archive buffer
 *
 * DESCRIPTION
 *
 *	Buf_pad writes len zero bytes to the archive buffer in order to 
 *	pad it.
 *
 * PARAMETERS
 *
 *	OFFSET	pad	- number of zero bytes to pad
 *
 */

#ifdef __STDC__

static void buf_pad(OFFSET pad)

#else

static void buf_pad(pad)
OFFSET           pad;

#endif
{
    int             idx;
    int             have;

    while (pad) {
	if ((have = bufend - bufidx) > pad) {
	    have = pad;
	}
	for (idx = 0; idx < have; ++idx) {
	    *bufidx++ = '\0';
	}
	total += have;
	pad -= have;
	if (bufend - bufidx == 0) {
	    outflush();
	}
    }
}


/* buf_use - allocate buffer space
 *
 * DESCRIPTION
 *
 *	Buf_use marks space in the buffer as being used; advancing both the
 *	buffer index (bufidx) and the total byte count (total).
 *
 * PARAMETERS
 *
 *	uint	len	- Amount of space to allocate in the buffer
 */

#ifdef __STDC__

static void buf_use(uint len)

#else

static void buf_use(len)
uint            len;

#endif
{
    bufidx += len;
    total += len;
}


/* buf_in_avail - index available input data within the buffer
 *
 * DESCRIPTION
 *
 *	Buf_in_avail fills the archive buffer, and points the bufp
 *	parameter at the start of the data.  The lenp parameter is
 *	modified to contain the number of bytes which were read.
 *
 * PARAMETERS
 *
 *	char   **bufp	- pointer to the buffer to read data into
 *	uint	*lenp	- pointer to the number of bytes which were read
 *			  (returned to the caller)
 *
 * RETURNS
 *
 * 	Stores a pointer to the data and its length in given locations. 
 *	Returns zero with valid data, -1 if unreadable portions were 
 *	replaced with nulls. 
 *
 * ERRORS
 *
 *	If an error occurs in ar_read, the error code is returned to the
 *	calling function.
 *
 */

#ifdef __STDC__

static int buf_in_avail(char **bufp, uint *lenp)

#else

static int buf_in_avail(bufp, lenp)
char          **bufp;
uint           *lenp;

#endif
{
    uint            have;
    int             corrupt = 0;

    while ((have = bufend - bufidx) == 0) {
	corrupt |= ar_read();
    }
    *bufp = bufidx;
    *lenp = have;
    return (corrupt);
}


/* buf_out_avail  - index buffer space for archive output
 *
 * DESCRIPTION
 *
 * 	Stores a buffer pointer at a given location. Returns the number 
 *	of bytes available. 
 *
 * PARAMETERS
 *
 *	char	**bufp	- pointer to the buffer which is to be stored
 *
 * RETURNS
 *
 * 	The number of bytes which are available in the buffer.
 *
 */

#ifdef __STDC__

static uint buf_out_avail(char **bufp)

#else

static uint buf_out_avail(bufp)
char          **bufp;

#endif
{
    int             have;

    if (bufend - bufidx < 0) {
	fatal("Buffer overlow in buf_out_avail\n");
    }
    if ((have = bufend - bufidx) == 0) {
	outflush();
    }
    *bufp = bufidx;
    return (have);
}