From 6651e4260955d159e6a4f794d0a69c1b3f8e670c Mon Sep 17 00:00:00 2001 From: Denys Vlasenko Date: Tue, 2 Jul 2013 20:09:15 +0200 Subject: [PATCH] tail: track file size only in -f mode This eliminates extra fstat and lseek calls on every read Signed-off-by: Denys Vlasenko --- coreutils/tail.c | 26 +++++++++++++++----------- 1 file changed, 15 insertions(+), 11 deletions(-) diff --git a/coreutils/tail.c b/coreutils/tail.c index 19fd8f695..fb07ca27b 100644 --- a/coreutils/tail.c +++ b/coreutils/tail.c @@ -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; -- 2.25.1