drivers/syslog README File
==========================
SYSLOG Interfaces
=================
Standard SYSLOG Interfaces
--------------------------
The NuttX SYSLOG is an architecture for getting debug and status
information from the system. The syslogging interfaces are defined in the
header file include/syslog.h. The primary interface to SYSLOG sub-system
is the function syslog() and, to a lesser extent, its companion vsyslog():
syslog() and vsyslog()
----------------------
Prototypes:
int syslog(int priority, FAR const IPTR char *format, ...);
void vsyslog(int priority, FAR const IPTR char *src, va_list ap);
Description:
syslog() generates a log message. The priority argument is formed by
ORing the facility and the level values (see include/syslog.h). The
remaining arguments are a format, as in printf and any arguments to the
format.
The NuttX implementation does not support any special formatting
characters beyond those supported by printf.
The function vsyslog() performs the same task as syslog() with the
difference that it takes a set of arguments which have been obtained
using the stdarg variable argument list macros.
setlogmask()
------------
The additional setlogmask() interface can use use to filter SYSLOG output:
Prototypes:
int setlogmask(int mask);
Description:
The setlogmask() function sets the logmask and returns the previous
mask. If the mask argument is 0, the current logmask is not modified.
The SYSLOG priorities are: LOG_EMERG, LOG_ALERT, LOG_CRIT, LOG_ERR,
LOG_WARNING, LOG_NOTICE, LOG_INFO, and LOG_DEBUG. The bit corresponding
to a priority p is LOG_MASK(p); LOG_UPTO(p) provides the mask of all
priorities in the above list up to and including p.
Per OpenGroup.org "If the maskpri argument is 0, the current log mask
is not modified." In this implementation, the value zero is permitted
in order to disable all syslog levels.
REVISIT: Per POSIX the syslog mask should be a per-process value but in
NuttX, the scope of the mask is dependent on the nature of the build:
* Flat Build: There is one, global SYSLOG mask that controls all output.
Protected Build: There are two SYSLOG masks. One within the kernel
that controls only kernel output. And one in user-space that controls
only user SYSLOG output.
* Kernel Build: The kernel build is compliant with the POSIX requirement:
There will be one mask for for each user process, controlling the
SYSLOG output only form that process. There will be a separate mask
accessible only in the kernel code to control kernel SYSLOG output.
*
These are all standard interfaces as defined at http://pubs.opengroup.org/onlinepubs/009695399/functions/closelog.html
Debug Interfaces
----------------
In NuttX, syslog output is really synonymous to debug output and,
therefore, the debugging interface macros defined in the header file
include/debug.h are also syslogging interfaces. Those macros are simply
wrappers around syslog(). The debugging interfaces differ from the syslog
interfaces in that:
* They do not take a priority parameter; the priority is inherent in the
debug macro name.
* They decorate the output stream with information such as the file name
* They can each be disabled via configuration options.
Each debug macro has a base name that represents the priority and a prefix
that represents the sub-system. Each macro is individually initialized by
both priority and sub-system. For example, uerr() is the macro used for
error level messages from the USB subsystem and is enabled with
CONFIG_DEBUG_USB_ERROR.
The base debug macro names, their priority, and configuration variable are
summarized below:
* info(). The info() macro is the lowest priority (LOG_INFO) and is
intended to provide general information about the flow of program
execution so that you can get an overview of the behavior of the
program. info() is often very chatty and voluminous and usually more
information than you may want to see. The info() macro is controlled
via CONFIG_DEBUG_subsystem_INFO
* warn(). The warn() macro has medium priority (LOG_WARN) and is
controlled by CONFIG_DEBUG_subsystem_WARN. The warn() is intended to
note exceptional or unexpected conditions that meigh be potential
errors or, perhaps, minor errors that easily recovered.
* err(). This is a high priority debug macro (LOG_ERROR) and controlled
by CONFIG_DEBUG_subsystem_ERROR. The err() is reserved to indicate
important error conditions.
* alert(). The highest priority debug macro (LOG_EMERG) and is
controlled by CONFIG_DEBUG_ALERT. The alert() macro is reserved for
use solely by assertion and crash handling logic. It also differs
from the other macros in that it cannot be enabled or disabled per
subsystem.
SYSLOG Channels
===============
SYSLOG Channel Interfaces
-------------------------
In the NuttX SYSLOG implementation, the underlying device logic the
supports the SYSLOG output is referred to as a SYSLOG channel. Each
SYSLOG channel is represented by an interface defined in
include/nuttx/syslog/syslog.h:
/* This structure provides the interface to a SYSLOG device */
typedef CODE int (*syslog_putc_t)(int ch);
typedef CODE int (*syslog_flush_t)(void);
struct syslog_channel_s
{
/* I/O redirection methods */
#ifdef CONFIG_SYSLOG_WRITE
syslog_write_t sc_write; /* Write multiple bytes */
#endif
syslog_putc_t sc_putc; /* Normal buffered output */
syslog_putc_t sc_force; /* Low-level output for interrupt handlers */
syslog_flush_t sc_flush; /* Flush buffered output (on crash) */
/* Implementation specific logic may follow */
};
The channel interface is instantiated by calling syslog_channel():
syslog_channel()
----------------
Prototype:
int syslog_channel(FAR const struct syslog_channel_s *channel);
Description:
Configure the SYSLOGging function to use the provided channel to
generate SYSLOG output.
syslog_channel() is a non-standard, internal OS interface and is not
available to applications. It may be called numerous times as
necessary to change channel interfaces.
Input Parameters:
* channel - Provides the interface to the channel to be used.
Returned Value:
Zero (OK) is returned on success. A negated errno value is returned
on any failure.
SYSLOG Channel Initialization
-----------------------------
The initial, default SYSLOG channel is established with statically
initialized global variables so that some level of SYSLOG output may be
available immediately upon reset. This initialized data is in the file
drivers/syslog/syslog_channel.c. The initial SYSLOG capability is
determined by the selected SYSLOG channel:
* In-Memory Buffer (RAMLOG). Full SYSLOG capability as available at
reset.
* Serial Console. If the serial implementation provides the low-level
character output function up_putc(), then that low level serial output
is available as soon as the serial device has been configured.
* For all other SYSLOG channels, all SYSLOG output goes to the bit-
bucket until the SYSLOG channel device has been initialized.
The syslog channel device is initialized when the bring-up logic calls
syslog_initialize():
syslog_initialize()
-------------------
Prototype:
#ifndef CONFIG_ARCH_SYSLOG
int syslog_initialize(void);
#else
# define syslog_initialize()
#endif
Description:
On power up, the SYSLOG facility is non-existent or limited to very
low-level output. This function is called later in the initialization
sequence after full driver support has been initialized. It installs
the configured SYSLOG drivers and enables full SYSLOGing capability.
This function performs these basic operations:
* Initialize the SYSLOG device
* Call syslog_channel() to begin using that device.
* If CONFIG_ARCH_SYSLOG is selected, then the architecture-specific
logic will provide its own SYSLOG device initialize which must include
as a minimum a call to syslog_channel() to use the device.
Returned Value:
Zero (OK) is returned on success; a negated errno value is returned on
any failure.
Different types of SYSLOG devices have different OS initialization
requirements. Some are available immediately at reset, some are available
after some basic OS initialization, and some only after OS is fully
initialized.
There are other types of SYSLOG channel devices that may require even
further initialization. For example, the file SYSLOG channel (described
below) cannot be initialized until the necessary file systems have been
mounted.
Interrupt Level SYSLOG Output
-----------------------------
As a general statement, SYSLOG output only supports //normal// output from
NuttX tasks. However, for debugging purposes, it is also useful to get
SYSLOG output from interrupt level logic. In an embedded system, that is
often where the most critical operations are performed.
There are three conditions under which SYSLOG output generated from
interrupt level processing can a included the SYSLOG output stream:
1. Low-Level Serial Output
--------------------------
If you are using a SYSLOG console channel (CONFIG_SYSLOG_CONSOLE) and if
the underlying architecture supports the low-level up_putc() interface
(CONFIG_ARCH_LOWPUTC), then the SYSLOG logic will direct the output to
up_putc() which is capable of generating the serial output within the
context of an interrupt handler.
There are a few issues in doing this however:
* up_putc() is able to generate debug output in any context because it
disables serial interrupts and polls the hardware directly. These
polls may take many milliseconds and during that time, all interrupts
are disable within the interrupt handler. This, of course, interferes
with the real-time behavior of the RTOS.
* The output generated by up_putc() is immediate and in real-time. The
normal SYSLOG output, on the other hand, is buffered in the serial
driver and may be delayed with respect to the immediate output by many
lines. Therefore, the interrupt level SYSLOG output provided through
up_putc() is grossly out of synchronization with other debug output
2. In-Memory Buffering
----------------------
If the RAMLOG SYSLOG channel is supported, then all SYSLOG output is
buffered in memory. Interrupt level SYSLOG output is no different than
normal SYSLOG output in this case.
3. Serialization Buffer
-----------------------
A final option is the use of an "interrupt buffer" to buffer the
interrupt level SYSLOG output. In this case:
* SYSLOG output generated from interrupt level process in not sent to
the SYSLOG channel immediately. Rather, it is buffered in the
interrupt serialization buffer.
* Later, when the next normal syslog output is generated, it will first
empty the content of the interrupt buffer to the SYSLOG device in the
proper context. It will then be followed by the normal syslog output.
In this case, the interrupt level SYSLOG output will interrupt the
normal output stream and the interrupt level SYSLOG output will be
inserted into the correct position in the SYSLOG output when the next
normal SYSLOG output is generated.
The SYSLOG interrupt buffer is enabled with CONFIG_SYSLOG_INTBUFFER. When
the interrupt buffer is enabled, you must also provide the size of the
interrupt buffer with CONFIG_SYSLOG_INTBUFSIZE.
SYSLOG Channel Options
======================
SYSLOG Console Device
---------------------
The typical SYSLOG device is the system console. If you are using a
serial console, for example, then the SYSLOG output will appear on that
serial port.
This SYSLOG channel is automatically selected by syslog_initialize() in
the LATE initialization phase based on configuration options. The
configuration options that affect this channel selection include:
* CONFIG_DEV_CONSOLE. This setting indicates that the system supports a
console device, i.e., that the character device /dev/console exists.
* CONFIG_SERIAL_CONSOLE. This configuration option is automatically
selected when a UART or USART is configured as the system console.
There is no user selection.
* CONFIG_SYSLOG_CONSOLE. This configuration option is manually selected
from the SYSLOG menu. This is the option that actually enables the
SYSLOG console device. It depends on CONFIG_DEV_CONSOLE.
* CONFIG_ARCH_LOWPUTC. This is an indication from the architecture
configuration that the platform supports the up_putc() interface.
up_putc() is a very low level UART interface that can even be used from
interrupt handling.
Interrupt level SYSLOG output will be lost unless: (1) the interrupt buffer
is enabled to support serialization, or (2) a serial console is used and
up_putc() is supported.
NOTE: The console channel uses the fixed character device at /dev/console.
The console channel is not synonymous with stdout (or file descriptor 1).
stdout is the current output from a task when, say, printf() if used.
Initially, stdout does, indeed, use the /dev/console device. However,
stdout may subsequently be redirected to some other device or file. This
is always the case, for example, when a transient device is used for a
console -- such as a USB console or a Telnet console. The SYSLOG channel
is not redirected as stdout is; the SYSLOG channel will stayed fixed (unless
it is explicitly changed via syslog_channel()).
References: drivers/syslog/syslog_consolechannel.c and
drivers/syslog/syslog_device.c
SYSLOG Character Device
-----------------------
The system console device, /dev/console, is a character driver with some
special properties. However, any character driver may be used as the
SYSLOG output channel. For example, suppose you have a serial console on
/dev/ttyS0 and you want SYSLOG output on /dev/ttyS1. Or suppose you
support only a Telnet console but want to capture debug output
/dev/ttyS0.
This SYSLOG device channel is selected with CONFIG_SYSLOG_CHAR and has no
other dependencies. Differences from the SYSLOG console channel include:
* CONFIG_SYSLOG_DEVPATH. This configuration option string must be set
provide the full path to the character device to be used.
* The forced SYSLOG output always goes to the bit-bucket. This means
that interrupt level SYSLOG output will be lost unless the interrupt
buffer is enabled to support serialization.
* CONFIG_SYSLOG_CHAR_CRLF. If CONFIG_SYSLOG_CHAR_CRLF is selected, then
linefeeds in the SYSLOG output will be expanded to Carriage Return +
Linefeed. Since the character device is not a console device, the
addition of carriage returns to line feeds would not be performed
otherwise. You would probably want this expansion if you use a serial
terminal program with the character device output.
References: drivers/syslog/syslog_devchannel.c and
drivers/syslog/syslog_device.c
SYSLOG File Device
------------------
Files can also be used as the sink for SYSLOG output. There is, however,
a very fundamental difference in using a file as opposed the system
console, a RAM buffer, or character device: You must first mount the
file system that supports the SYSLOG file. That difference means that
the file SYSLOG channel cannot be supported during the boot-up phase but
can be instantiated later when board level logic configures the application
environment, including mounting of the file systems.
The interface syslog_file_channel() is used to configure the SYSLOG file
channel:
syslog_file_channel()
---------------------
Prototype:
#ifdef CONFIG_SYSLOG_FILE
int syslog_file_channel(FAR const char *devpath);
#endif
Description:
Configure to use a file in a mounted file system at 'devpath' as the
SYSLOG channel.
This tiny function is simply a wrapper around syslog_dev_initialize()
and syslog_channel(). It calls syslog_dev_initialize() to configure
the character file at 'devpath' and then calls syslog_channel() to use
that device as the SYSLOG output channel.
File SYSLOG channels differ from other SYSLOG channels in that they
cannot be established until after fully booting and mounting the target
file system. This function would need to be called from board-specific
bring-up logic AFTER mounting the file system containing 'devpath'.
SYSLOG data generated prior to calling syslog_file_channel will, of
course, not be included in the file.
NOTE interrupt level SYSLOG output will be lost in this case unless
the interrupt buffer is used.
Input Parameters:
* devpath - The full path to the file to be used for SYSLOG output.
This may be an existing file or not. If the file exists,
syslog_file_channel() will append new SYSLOG data to the end of the
file. If it does not, then syslog_file_channel() will create the
file.
Returned Value:
Zero (OK) is returned on success; a negated errno value is returned on
any failure.
References: drivers/syslog/syslog_filechannel.c,
drivers/syslog/syslog_device.c, and include/nuttx/syslog/syslog.h.
SYSLOG RAMLOG Device
--------------------
The RAMLOG is a standalone feature that can be used to buffer any
character data in memory. There are, however, special configurations
that can be used to configure the RAMLOG as a SYSLOG channel. The RAMLOG
functionality is described in a more general way in the following
paragraphs.
RAM Logging Device
==================
The RAM logging driver is a driver that was intended to support debugging
output (SYSLOG) when the normal serial output is not available. For
example, if you are using a Telnet or USB serial console, the debug output
will get lost -- or worse. For example, what if you want to debug the
network over Telnet?
The RAM logging driver can also accept debug output data from interrupt
handler with no special serialization buffering. As an added benefit, the
RAM logging driver is much less invasive. Since no actual I/O is performed
with the debug output is generated, the RAM logger tends to be much faster
and will interfere much less when used with time critical drivers.
The RAM logging driver is similar to a pipe in that it saves the debugging
output in a circular buffer in RAM. It differs from a pipe in numerous
details as needed to support logging.
This driver is built when CONFIG_RAMLOG is defined in the NuttX
configuration.
dmesg
-----
When the RAMLOG (with SYSLOG) is enabled, a new NuttShell (NSH) command
will appear: dmesg. The dmsg command will dump the contents of the
circular buffer to the console (and also clear the circular buffer).
RAMLOG Configuration options
----------------------------
* CONFIG_RAMLOG - Enables the RAM logging feature
* CONFIG_RAMLOG_SYSLOG - Use the RAM logging device for the syslogging
interface. If this feature is enabled, then all debug output (only)
will be re-directed to the circular buffer in RAM. This RAM log can
be viewed from NSH using the 'dmesg' command. NOTE: Unlike the
limited, generic character driver SYSLOG device, the RAMLOG *can* be
used to capture debug output from interrupt level handlers.
* CONFIG_RAMLOG_NPOLLWAITERS - The number of threads than can be waiting
for this driver on poll(). Default: 4
If CONFIG_RAMLOG_SYSLOG is selected, then the following must also be
provided:
* CONFIG_RAMLOG_BUFSIZE - The size of the circular buffer to use.
Default: 1024 bytes.
Other miscellaneous settings
* CONFIG_RAMLOG_CRLF - Pre-pend a carriage return before every linefeed
that goes into the RAM log.
* CONFIG_RAMLOG_NONBLOCKING - Reading from the RAMLOG will never block
if the RAMLOG is empty. If the RAMLOG is empty, then zero is returned
(usually interpreted as end-of-file). If you do not define this, the
NSH 'dmsg' command will lock up when called! So you probably do want
this!
* CONFIG_RAMLOG_NPOLLWAITERS - The maximum number of threads that may be
waiting on the poll method.
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