tailf: count last lines correctly at initial print out

When last lines happen to be greater than string buffer size for fgets()
the number of printed lines resulted to too few.  To avoid miscounts due
insufficient buffer size use mmap() to map the whole file and rewind
until requested number of new lines is found.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

1 files changed, 36 insertions, 41 deletions

diff --git a/text-utils/tailf.c b/text-utils/tailf.c
index 38df99f6f..179dfc3d6 100644
--- a/text-utils/tailf.c
+++ b/text-utils/tailf.c
@@ -36,6 +36,8 @@
 #include <ctype.h>
 #include <errno.h>
 #include <getopt.h>
+#include <sys/mman.h>
+
 #ifdef HAVE_INOTIFY_INIT
 #include <sys/inotify.h>
 #endif
@@ -49,44 +51,38 @@
 #define DEFAULT_LINES  10
 
 static void
-tailf(const char *filename, int lines)
+tailf(const char *filename, unsigned long lines, struct stat *st)
 {
-	char *buf, *p;
-	int  head = 0;
-	int  tail = 0;
-	FILE *str;
-	int  i;
+	int fd;
+	size_t i;
+	char *data;
 
-	if (!(str = fopen(filename, "r")))
+	if (!(fd = open(filename, O_RDONLY)))
 		err(EXIT_FAILURE, _("cannot open %s"), filename);
-
-	buf = xmalloc((lines ? lines : 1) * BUFSIZ);
-	p = buf;
-	while (fgets(p, BUFSIZ, str)) {
-		if (++tail >= lines) {
-			tail = 0;
-			head = 1;
+	data = mmap(0, st->st_size, PROT_READ, MAP_SHARED, fd, 0);
+	i = (size_t)st->st_size - 1;
+	/* humans do not think last new line in a file should be counted,
+	 * in that case do off by one from counter point of view */
+	if (data[i] == '\n')
+		lines++;
+	while (i) {
+		if (data[i] == '\n') {
+			if (--lines == 0) {
+				i++;
+				break;
+			}
 		}
-		p = buf + (tail * BUFSIZ);
+		i--;
 	}
-
-	if (head) {
-		for (i = tail; i < lines; i++)
-			fputs(buf + (i * BUFSIZ), stdout);
-		for (i = 0; i < tail; i++)
-			fputs(buf + (i * BUFSIZ), stdout);
-	} else {
-		for (i = head; i < tail; i++)
-			fputs(buf + (i * BUFSIZ), stdout);
-	}
-
+	while (i < (size_t)st->st_size)
+		putchar(data[i++]);
+	munmap(data, st->st_size);
+	close(fd);
 	fflush(stdout);
-	free(buf);
-	fclose(str);
 }
 
 static void
-roll_file(const char *filename, off_t *size)
+roll_file(const char *filename, struct stat *old)
 {
 	char buf[BUFSIZ];
 	int fd;
@@ -100,12 +96,12 @@ roll_file(const char *filename, off_t *size)
 	if (fstat(fd, &st) == -1)
 		err(EXIT_FAILURE, _("stat of %s failed"), filename);
 
-	if (st.st_size == *size) {
+	if (st.st_size == old->st_size) {
 		close(fd);
 		return;
 	}
 
-	if (lseek(fd, *size, SEEK_SET) != (off_t)-1) {
+	if (lseek(fd, old->st_size, SEEK_SET) != (off_t)-1) {
 		ssize_t rc, wc;
 
 		while ((rc = read(fd, buf, sizeof(buf))) > 0) {
@@ -123,16 +119,16 @@ roll_file(const char *filename, off_t *size)
 	 * avoids data duplication. If we read nothing or hit an error, reset
 	 * to the reported size, this handles truncated files.
 	 */
-	*size = (pos != -1 && pos != *size) ? pos : st.st_size;
+	old->st_size = (pos != -1 && pos != old->st_size) ? pos : st.st_size;
 
 	close(fd);
 }
 
 static void
-watch_file(const char *filename, off_t *size)
+watch_file(const char *filename, struct stat *old)
 {
 	do {
-		roll_file(filename, size);
+		roll_file(filename, old);
 		xusleep(250000);
 	} while(1);
 }
@@ -144,7 +140,7 @@ watch_file(const char *filename, off_t *size)
 #define NEVENTS		4
 
 static int
-watch_file_inotify(const char *filename, off_t *size)
+watch_file_inotify(const char *filename, struct stat *old)
 {
 	char buf[ NEVENTS * sizeof(struct inotify_event) ];
 	int fd, ffd, e;
@@ -176,7 +172,7 @@ watch_file_inotify(const char *filename, off_t *size)
 			struct inotify_event *ev = (struct inotify_event *) &buf[e];
 
 			if (ev->mask & IN_MODIFY)
-				roll_file(filename, size);
+				roll_file(filename, old);
 			else {
 				close(ffd);
 				ffd = -1;
@@ -238,7 +234,6 @@ int main(int argc, char **argv)
 	long lines;
 	int ch;
 	struct stat st;
-	off_t size = 0;
 
 	static const struct option longopts[] = {
 		{ "lines",   required_argument, 0, 'n' },
@@ -280,13 +275,13 @@ int main(int argc, char **argv)
 	if (stat(filename, &st) != 0)
 		err(EXIT_FAILURE, _("stat of %s failed"), filename);
 
-	size = st.st_size;;
-	tailf(filename, lines);
+	if (st.st_size)
+		tailf(filename, lines, &st);
 
 #ifdef HAVE_INOTIFY_INIT
-	if (!watch_file_inotify(filename, &size))
+	if (!watch_file_inotify(filename, &st))
 #endif
-		watch_file(filename, &size);
+		watch_file(filename, &st);
 
 	return EXIT_SUCCESS;
 }