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tail: track file size only in -f mode

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This eliminates extra fstat and lseek calls on every read

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2013-07-02 20:09:15 +02:00
parent cd8eece88e
commit 6651e42609
1 changed files with 14 additions and 10 deletions
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24
coreutils/tail.c
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@ -66,17 +66,21 @@ static void tail_xprint_header(const char *fmt, const char *filename)
bb_perror_nomsg_and_die();
}
static ssize_t tail_read(int fd, char *buf, size_t count)
static ssize_t tail_read(int fd, char *buf, size_t count, int follow)
{
ssize_t r;
off_t current;
struct stat sbuf;
/* /proc files report zero st_size, don't lseek them. */
if (fstat(fd, &sbuf) == 0 && sbuf.st_size > 0) {
current = lseek(fd, 0, SEEK_CUR);
if (sbuf.st_size < current)
xlseek(fd, 0, SEEK_SET);
if (follow) {
/* tail -f keeps following files even if they are truncated */
off_t current;
struct stat sbuf;
/* /proc files report zero st_size, don't lseek them */
if (fstat(fd, &sbuf) == 0 && sbuf.st_size > 0) {
current = lseek(fd, 0, SEEK_CUR);
if (sbuf.st_size < current)
xlseek(fd, 0, SEEK_SET);
}
}
r = full_read(fd, buf, count);
@ -251,7 +255,7 @@ int tail_main(int argc, char **argv)
* Used only by +N code ("start from Nth", 1-based): */
seen = 1;
newlines_seen = 0;
while ((nread = tail_read(fd, buf, tailbufsize - taillen)) > 0) {
while ((nread = tail_read(fd, buf, tailbufsize - taillen, /*follow:*/ 0)) > 0) {
if (G.from_top) {
int nwrite = nread;
if (seen < count) {
@ -363,7 +367,7 @@ int tail_main(int argc, char **argv)
if (nfiles > header_threshhold) {
fmt = header_fmt_str;
}
while ((nread = tail_read(fd, tailbuf, BUFSIZ)) > 0) {
while ((nread = tail_read(fd, tailbuf, BUFSIZ, /*follow:*/ 1)) > 0) {
if (fmt) {
tail_xprint_header(fmt, filename);
fmt = NULL;