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-rw-r--r--Documentation/ABI/testing/dev-kmsg11
1 files changed, 11 insertions, 0 deletions
diff --git a/Documentation/ABI/testing/dev-kmsg b/Documentation/ABI/testing/dev-kmsg
index f307506eb54c..3c0bb76e3417 100644
--- a/Documentation/ABI/testing/dev-kmsg
+++ b/Documentation/ABI/testing/dev-kmsg
@@ -56,6 +56,17 @@ Description: The /dev/kmsg character device node provides userspace access
seek after the last record available at the time
the last SYSLOG_ACTION_CLEAR was issued.
+ Other seek operations or offsets are not supported because of
+ the special behavior this device has. The device allows to read
+ or write only whole variable length messages (records) that are
+ stored in a ring buffer.
+
+ Because of the non-standard behavior also the error values are
+ non-standard. -ESPIPE is returned for non-zero offset. -EINVAL
+ is returned for other operations, e.g. SEEK_CUR. This behavior
+ and values are historical and could not be modified without the
+ risk of breaking userspace.
+
The output format consists of a prefix carrying the syslog
prefix including priority and facility, the 64 bit message
sequence number and the monotonic timestamp in microseconds,