tail: optimize "tail -c HUGENUM REGULAR_FILE". Closes 3763.

function                                             old     new   delta
tail_main                                           1541    1547      +6

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 21 insertions, 15 deletions

diff --git a/coreutils/tail.c b/coreutils/tail.c
index eac982735..4b42ebc52 100644
--- a/coreutils/tail.c
+++ b/coreutils/tail.c
@@ -59,7 +59,8 @@ static const struct suffix_mult tail_suffixes[] = {
 };
 
 struct globals {
-        bool status;
+        bool from_top;
+        bool exitcode;
 } FIX_ALIASING;
 #define G (*(struct globals*)&bb_common_bufsiz1)
 
@@ -85,7 +86,7 @@ static ssize_t tail_read(int fd, char *buf, size_t count)
         r = full_read(fd, buf, count);
         if (r < 0) {
                 bb_perror_msg(bb_msg_read_error);
-                G.status = EXIT_FAILURE;
+                G.exitcode = EXIT_FAILURE;
         }
 
         return r;
@@ -99,7 +100,7 @@ static unsigned eat_num(const char *p)
                 p++;
         else if (*p == '+') {
                 p++;
-                G.status = 1; /* mark that we saw "+" */
+                G.from_top = 1;
         }
         return xatou_sfx(p, tail_suffixes);
 }
@@ -109,7 +110,6 @@ int tail_main(int argc, char **argv)
 {
         unsigned count = 10;
         unsigned sleep_period = 1;
-        bool from_top;
         const char *str_c, *str_n;
 
         char *tailbuf;
@@ -152,8 +152,6 @@ int tail_main(int argc, char **argv)
 #endif
         argc -= optind;
         argv += optind;
-        from_top = G.status; /* 1 if there was "-c +N" or "-n +N" */
-        G.status = EXIT_SUCCESS;
 
         /* open all the files */
         fds = xmalloc(sizeof(fds[0]) * (argc + 1));
@@ -171,7 +169,7 @@ int tail_main(int argc, char **argv)
         do {
                 int fd = open_or_warn_stdin(argv[i]);
                 if (fd < 0 && !FOLLOW_RETRY) {
-                        G.status = EXIT_FAILURE;
+                        G.exitcode = EXIT_FAILURE;
                         continue;
                 }
                 fds[nfiles] = fd;
@@ -183,15 +181,19 @@ int tail_main(int argc, char **argv)
 
         /* prepare the buffer */
         tailbufsize = BUFSIZ;
-        if (!from_top && COUNT_BYTES) {
+        if (!G.from_top && COUNT_BYTES) {
                 if (tailbufsize < count + BUFSIZ) {
                         tailbufsize = count + BUFSIZ;
                 }
         }
-        tailbuf = xmalloc(tailbufsize);
+        /* tail -c1024m REGULAR_FILE doesn't really need 1G mem block.
+         * (In fact, it doesn't need ANY memory). So delay allocation.
+         */
+        tailbuf = NULL;
 
         /* tail the files */
-        fmt = header_fmt_str + 1; /* skip header leading newline on first output */
+
+        fmt = header_fmt_str + 1; /* skip leading newline in the header on the first output */
         i = 0;
         do {
                 char *buf;
@@ -209,7 +211,7 @@ int tail_main(int argc, char **argv)
                         fmt = header_fmt_str;
                 }
 
-                if (!from_top) {
+                if (!G.from_top) {
                         off_t current = lseek(fd, 0, SEEK_END);
                         if (current > 0) {
                                 unsigned off;
@@ -242,6 +244,9 @@ int tail_main(int argc, char **argv)
                         }
                 }
 
+                if (!tailbuf)
+                        tailbuf = xmalloc(tailbufsize);
+
                 buf = tailbuf;
                 taillen = 0;
                 /* "We saw 1st line/byte".
@@ -249,7 +254,7 @@ int tail_main(int argc, char **argv)
                 seen = 1;
                 newlines_seen = 0;
                 while ((nread = tail_read(fd, buf, tailbufsize-taillen)) > 0) {
-                        if (from_top) {
+                        if (G.from_top) {
                                 int nwrite = nread;
                                 if (seen < count) {
                                         /* We need to skip a few more bytes/lines */
@@ -313,7 +318,7 @@ int tail_main(int argc, char **argv)
                                 buf = tailbuf + taillen;
                         }
                 } /* while (tail_read() > 0) */
-                if (!from_top) {
+                if (!G.from_top) {
                         xwrite(STDOUT_FILENO, tailbuf, taillen);
                 }
         } while (++i < nfiles);
@@ -368,10 +373,11 @@ int tail_main(int argc, char **argv)
                                 xwrite(STDOUT_FILENO, tailbuf, nread);
                         }
                 } while (++i < nfiles);
-        }
+        } /* while (1) */
+
         if (ENABLE_FEATURE_CLEAN_UP) {
                 free(fds);
                 free(tailbuf);
         }
-        return G.status;
+        return G.exitcode;
 }