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>

diff --git a/coreutils/tail.c b/coreutils/tail.c
index eac9827..4b42ebc 100644 (file)
--- 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;
 }