e8419c90f — BusyBox

Subject

tail: fix "tail -c 20 /dev/huge_disk" (was taking ages) tail: a few variables renamed wc: tiny optimization.

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Architecture & Compiler Flags

Architecture: x86_64-linux-gnu Compiler: gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

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Commit Diff GitHub →

coreutils/tail.c +42 -37 coreutils/wc.c +1 -1

coreutils/tail.c+42 -37

@@ -74,7 +74,7 @@ static unsigned eat_num(const char *p)
	p++;
	else if (*p == '+') {
	p++;
-	G.status = EXIT_FAILURE;
+	G.status = 1; /* mark that we saw "+" */
	}
	return xatou_sfx(p, tail_suffixes);
	}
@@ -92,7 +92,7 @@ int tail_main(int argc, char **argv)
	char *tailbuf;
	size_t tailbufsize;
	int taillen = 0;
-	int newline = 0;
+	int newlines_seen = 0;
	int nfiles, nread, nwrite, seen, i, opt;

	int *fds;
@@ -128,12 +128,12 @@ int tail_main(int argc, char **argv)
	#endif
	argc -= optind;
	argv += optind;
-	from_top = G.status;
+	from_top = G.status; /* 1 if there was "-c +N" or "-n +N" */
+	G.status = EXIT_SUCCESS;

	/* open all the files */
	fds = xmalloc(sizeof(int) * (argc + 1));
	nfiles = i = 0;
-	G.status = EXIT_SUCCESS;
	if (argc == 0) {
	struct stat statbuf;

@@ -155,39 +155,49 @@ int tail_main(int argc, char **argv)
	if (!nfiles)
	bb_error_msg_and_die("no files");

+	/* prepare the buffer */
	tailbufsize = BUFSIZ;
-
-	/* tail the files */
	if (!from_top && COUNT_BYTES) {
-	if (tailbufsize < count) {
+	if (tailbufsize < count + BUFSIZ) {
	tailbufsize = count + BUFSIZ;
	}
	}
+	tailbuf = xmalloc(tailbufsize);

-	buf = tailbuf = xmalloc(tailbufsize);
-
+	/* tail the files */
	fmt = header_fmt + 1; /* Skip header leading newline on first output. */
	i = 0;
	do {
-	/* Be careful. It would be possible to optimize the count-bytes
-	* case if the file is seekable. If you do though, remember that
-	* starting file position may not be the beginning of the file.
-	* Beware of backing up too far. See example in wc.c.
-	*/
-	if (!(count \| from_top) && lseek(fds[i], 0, SEEK_END) >= 0) {
-	continue;
-	}
+	off_t current;

	if (nfiles > header_threshhold) {
	tail_xprint_header(fmt, argv[i]);
	fmt = header_fmt;
	}

+	/* Optimizing count-bytes case if the file is seekable.
+	* Beware of backing up too far.
+	* Also we exclude files with size 0 (because of /proc/xxx) */
+	current = lseek(fds[i], 0, SEEK_END);
+	if (current > 0) {
+	if (!from_top) {
+	if (count == 0)
+	continue; /* showing zero lines is easy :) */
+	if (COUNT_BYTES) {
+	current -= count;
+	if (current < 0)
+	current = 0;
+	xlseek(fds[i], current, SEEK_SET);
+	bb_copyfd_size(fds[i], STDOUT_FILENO, count);
+	continue;
+	}
+	}
+	}
+
	buf = tailbuf;
	taillen = 0;
	seen = 1;
-	newline = 0;
-
+	newlines_seen = 0;
	while ((nread = tail_read(fds[i], buf, tailbufsize-taillen)) > 0) {
	if (from_top) {
	nwrite = nread;
@@ -215,34 +225,32 @@ int tail_main(int argc, char **argv)
	}
	} else {
	int k = nread;
-	int nbuf = 0;
+	int newlines_in_buf = 0;

-	while (k) {
-	--k;
+	do { /* count '\n' in last read */
+	k--;
	if (buf[k] == '\n') {
-	++nbuf;
+	newlines_in_buf++;
	}
-	}
+	} while (k);

-	if (newline + nbuf < count) {
-	newline += nbuf;
+	if (newlines_seen + newlines_in_buf < count) {
+	newlines_seen += newlines_in_buf;
	taillen += nread;
	} else {
-	int extra = 0;
+	int extra = (buf[nread-1] != '\n');

-	if (buf[nread-1] != '\n')
-	extra = 1;
-	k = newline + nbuf + extra - count;
+	k = newlines_seen + newlines_in_buf + extra - count;
	s = tailbuf;
	while (k) {
	if (*s == '\n') {
-	--k;
+	k--;
	}
-	++s;
+	s++;
	}
	taillen += nread - (s - tailbuf);
	memmove(tailbuf, s, taillen);
-	newline = count - extra;
+	newlines_seen = count - extra;
	}
	if (tailbufsize < taillen + BUFSIZ) {
	tailbufsize = taillen + BUFSIZ;
@@ -251,13 +259,10 @@ int tail_main(int argc, char **argv)
	}
	buf = tailbuf + taillen;
	}
-	}
-
+	} /* while (tail_read() > 0) */
	if (!from_top) {
	xwrite(STDOUT_FILENO, tailbuf, taillen);
	}
-
-	taillen = 0;
	} while (++i < nfiles);

	buf = xrealloc(tailbuf, BUFSIZ);

coreutils/wc.c+1 -1

@@ -73,7 +73,7 @@ int wc_main(int argc, char **argv)
	{
	FILE *fp;
	const char s, arg;
-	const char *start_fmt = "%9"COUNT_FMT;
+	const char *start_fmt = " %9"COUNT_FMT + 1;
	const char *fname_fmt = " %s\n";
	COUNT_T *pcounts;
	COUNT_T counts[4];