writeidx.c

APUE(第二版)的随书源代码,与第一版的代码相比增加了一些章节的代码(详见第二版的说明)

#include	"db.h"
#include	<sys/uio.h>		/* struct iovec */

/* Write an index record.
 * _db_writedat() is called before this function, to set the fields
 * datoff and datlen in the DB structure, which we need to write
 * the index record. */

void
_db_writeidx(DB *db, const char *key,
					 off_t offset, int whence, off_t ptrval)
{
	struct iovec	iov[2];
	char			asciiptrlen[PTR_SZ + IDXLEN_SZ +1];
	int				len;

	if ( (db->ptrval = ptrval) < 0 || ptrval > PTR_MAX)
		err_quit("invalid ptr: %d", ptrval);

	sprintf(db->idxbuf, "%s%c%d%c%d\n",
				key, SEP, db->datoff, SEP, db->datlen);
	if ( (len = strlen(db->idxbuf)) < IDXLEN_MIN || len > IDXLEN_MAX)
		err_dump("invalid length");
	sprintf(asciiptrlen, "%*d%*d", PTR_SZ, ptrval, IDXLEN_SZ, len);

		/* If we're appending, we have to lock before doing the lseek()
		   and write() to make the two an atomic operation.  If we're
		   overwriting an existing record, we don't have to lock. */
	if (whence == SEEK_END)		/* we're appending */
		if (writew_lock(db->idxfd, ((db->nhash+1)*PTR_SZ)+1,
													SEEK_SET, 0) < 0)
			err_dump("writew_lock error");

		/* Position the index file and record the offset */
	if ( (db->idxoff = lseek(db->idxfd, offset, whence)) == -1)
		err_dump("lseek error");

	iov[0].iov_base = asciiptrlen;
	iov[0].iov_len  = PTR_SZ + IDXLEN_SZ;
	iov[1].iov_base = db->idxbuf;
	iov[1].iov_len  = len;
	if (writev(db->idxfd, &iov[0], 2) != PTR_SZ + IDXLEN_SZ + len)
		err_dump("writev error of index record");

	if (whence == SEEK_END)
		if (un_lock(db->idxfd, ((db->nhash+1)*PTR_SZ)+1, SEEK_SET, 0) < 0)
			err_dump("un_lock error");
}