logread: implement dumpfollow mode of operation

This is basically a combination of the default (dump mode) and -f
(follow mode). Specifying -F makes logread first dump the log buffer and
then immediately start following it.

function                                             old     new   delta
packed_usage                                       30412   30443     +31
logread_main                                         491     497      +6

Signed-off-by: Phil Sutter <phil.sutter@viprinet.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 11 insertions, 5 deletions

diff --git a/sysklogd/logread.c b/sysklogd/logread.c
index bea73d932..da4a4d4df 100644
--- a/sysklogd/logread.c
+++ b/sysklogd/logread.c
@@ -10,10 +10,11 @@
  */
 
 //usage:#define logread_trivial_usage
-//usage:       "[-f]"
+//usage:       "[-fF]"
 //usage:#define logread_full_usage "\n\n"
 //usage:       "Show messages in syslogd's circular buffer\n"
 //usage:     "\n        -f      Output data as log grows"
+//usage:     "\n        -F      Same as -f, but dump buffer first"
 
 #include "libbb.h"
 #include <sys/ipc.h>
@@ -83,7 +84,7 @@ int logread_main(int argc UNUSED_PARAM, char **argv)
         unsigned cur;
         int log_semid; /* ipc semaphore id */
         int log_shmid; /* ipc shared memory id */
-        smallint follow = getopt32(argv, "f");
+        int follow = getopt32(argv, "fF");
 
         INIT_G();
 
@@ -106,7 +107,7 @@ int logread_main(int argc UNUSED_PARAM, char **argv)
         /* Max possible value for tail is shbuf->size - 1 */
         cur = shbuf->tail;
 
-        /* Loop for logread -f, one pass if there was no -f */
+        /* Loop for -f or -F, one pass otherwise */
         do {
                 unsigned shbuf_size;
                 unsigned shbuf_tail;
@@ -129,7 +130,12 @@ int logread_main(int argc UNUSED_PARAM, char **argv)
                         printf("cur:%u tail:%u size:%u\n",
                                         cur, shbuf_tail, shbuf_size);
 
-                if (!follow) {
+                if (!(follow & 1)) { /* not -f */
+                        /* if -F, "convert" it to -f, so that we dont
+                         * dump the entire buffer on each iteration
+                         */
+                        follow >>= 1;
+
                         /* advance to oldest complete message */
                         /* find NUL */
                         cur += strlen(shbuf_data + cur);
@@ -142,7 +148,7 @@ int logread_main(int argc UNUSED_PARAM, char **argv)
                         cur++;
                         if (cur >= shbuf_size) /* last byte in buffer? */
                                 cur = 0;
-                } else { /* logread -f */
+                } else { /* -f */
                         if (cur == shbuf_tail) {
                                 sem_up(log_semid);
                                 fflush_all();