There are a few ways you might see a Fuchsia program generate log messages:
- the LogSink service
- the kernel's debuglog
- a combination of these, routed through
Connect takes a socket, into which the actual log messages are written by the syslog library.
If the socket's buffer is full, the writing thread will drop logs. Dropped messages on the writer
side are counted and that count is sent in the next successful message, after which the counter is
log_listener prints a warning when it's aware of dropped messages.
LogSink service must drain all of the sockets it receives quickly enough to prevent messages
being dropped on the writer-side.
LogSink is responsible for draining those sockets to fill
internal buffers. This can result in high CPU usage in both the writing component and the
LogSink when logging heavily.
Different languages use different mechanisms to talk to
LogSink. See the relevant pages for more
The kernel allows programs to create debuglog handles from the root resource, each handle allowing
its owner to write messages into the kernel's shared ring buffer. Each message has a limit of
ZX_LOG_RECORD_DATA_MAX bytes, with content in excess being truncated.
In addition to being bindable to file descriptors, debuglog handles can be passed to the
debuglog_read syscalls. The read syscall is used to transfer from the
debuglog to the system log.
Components that want to send their standard streams to the debuglog gain access through the
Most logs written to debuglog handles are written through stdio forwarding.
These concepts are familiar to many from other operating systems but their use in Fuchsia can be complicated to follow because they are routed differently in different parts of the system.
Drivers log to the
LogSink sink service, but do so through the use of
zxlogf. This function
provides a wrapper around the syslog library, so that each driver will have its own log message
stderr to debuglog. This allows
to output critical information to the debuglog, or to fallback to the debuglog for certain product
configurations where the
LogSink service is not available.
The handles are populated in procargs by appmgr when creating processes and are pulled from
fuchsia.sys/LaunchInfo if provided. For example,
run-test-component provides its own
stderr handles for test components so it can prevent that output from reaching the klog.
If no fd's are provided by the caller of
CreateComponent, then the handles are cloned from
stderr. appmgr populates its own stdio with debuglog handles, using the
stdout-to-debuglog library to wire up a handle received from
Components don't have their
stderr streams captured by default.
For ELF components, there are flags used to tell the ELF runner to redirect
the output of these stream to the
LogSink service. For more information, see
the ELF runner section on forwarding stdout and stderr streams.
Forwarding klog to syslog
The Archivist continually reads from the klog and forwards those messages to the main log. Messages from the klog can be dropped by the pipeline if they are rolled out of the klog buffer before the archivist reads them into syslog -- these are not yet tracked.
All kernel log messages are sent to the system log with INFO severity because the debuglog syscalls lack a way to express the severity of a message.