Updated: April 19, 2023

System logger


slogger2 [-C dev_console_buffer_size] [-c out_buffer_size,error_buffer_size]
         [-D slogger2_asinfo_name[:dump_file]] [-d slogger2_asinfo_name[:dump_file]]
         [-g gid1,gid2,...] [-l log_save_path] [-P pps_path] [-S] [-s size]
         [-U string] [-v[v]...]

Runs on:

QNX Neutrino


-C dev_console_buffer_size
(QNX Neutrino 7.0 or later) Specify the buffer size to use for /dev/console, which provides a general-purpose console logging node. The size is in bytes, but you can specify a (case-sensitive) suffix of k to specify a size in kilobytes. By default, this feature is enabled with a buffer size of 16 KB. To disable it, set the size to 0.
-c out_buffer_size,error_buffer_size
(QNX Neutrino 7.0 or later) Create a buffer set containing two logging buffers (out and error) with the given sizes, in bytes. You can write to these buffers by opening the following paths for writing:
Path Buffer
/dev/slog2/critical error
/dev/slog2/error error
/dev/slog2/warning out
/dev/slog2/info out
/dev/slog2/debug out

This option creates one log file that's shared by all processes that use slogger2 APIs. Each log line is prefixed with the process ID of the client, in square brackets.

-D slogger2_asinfo_name[:dump_file]
-d slogger2_asinfo_name[:dump_file]
(QNX Neutrino 7.0 or later) Allow slogger2 buffer sets to be backed against a known range of physical memory, for possible recovery after a warm reset.

The slogger2_asinfo_name argument is the name of the address space memory region in the system page (syspage) that's been set aside for this purpose.

If the pre-reset state of the logs is determined to be intact, it's saved in /dev/shmem/dump_file. The default value of dump_file is slog2phys.

The -d option tries to use physical memory backing only if you pass the SLOG2_ALLOC_TYPE_PHYSICAL flag to slog2_register().

The -D option tries to use physical memory backing for every buffer set. To override this behavior, pass the SLOG2_ALLOC_TYPE_SHMEM flag to slog2_register().

If physical memory is depleted and can't be reclaimed from previous dead processes, then the buffer set is created as a regular shared memory object.

-g gid1,gid2,...
(QNX Neutrino 7.0 or later) Add each group in the list to every buffer set as a read-only ACL entry.
-l log_save_path
(QNX Neutrino 7.0 or later) When a client process stops or terminates, move its associated buffer set(s) under /dev/shmem/slogger2/ to log_save_path.

This is mainly intended as a debugging feature.

The default behavior of slogger2 once a client disconnects is to postpone the immediate cleanup of resources associated with the respective buffer set(s) until a later time. The present implementation relies on a queue of disconnected clients.

-P pps_path
(QNX Neutrino 7.0 or later) Use an optional PPS interface for slogger2 that provides a command-line interface that's equivalent to the slog2_get_verbosity() and slog2_set_verbosity() APIs.

The slogger2 APIs filter which logs actually get appended into any of the logging buffers in the buffer set, based on the verbosity level of the buffer set and the severity level passed to the vslog2*() and slog2*() logging APIs. The requirements for this interface are:

  • The PPS service must be running. It can be started after slogger2; slogger2 waits a few minutes for it.
  • The pps_path must be present in the PPS directory hierarchy.

The slogger2 manager then creates (or opens) a PPS object called pps_path/verbose. You can then change the verbosity level from the command line by typing:

echo buffer_set_name::verbosity_level >> pps_path/verbose

The verbosity_level is the numeric value corresponding to the severity levels defined in <sys/slog2.h>.

(QNX Neutrino 7.0 or later) Force slogger2 to look up an environment variable called SLOG_BUFFER_SIZE in each client to possibly override the buffer size associated with the -s option for that client only. The value of SLOG_BUFFER_SIZE is the desired buffer size in bytes.

This option is intended for debug use only; if you use it, slogger2 must retain all root process abilities.

-s size
(QNX Neutrino 7.0 or later) Allocate each client using the legacy logger APIs (i.e., slogb(), slogf(), slogi(), and vslogf()) a private logging buffer that's managed by slogger2 on its behalf. This option lets you override the default per-client buffer size of 64 KB. The size is in bytes, but you can specify a (case-sensitive) suffix of k to specify a size in kilobytes. The size is rounded up to the nearest multiple of the page size (4 KB). To disable the legacy logging feature, set the size to 0.
-U string
Once running, run as the specified user, so that the program doesn't need to run as root. The string can be in one of these forms:
  • uid[:gid[,sup_gid]*]
  • user_name[,sup_gid]*

In the second form, the primary group is the one specified for user_name in /etc/passwd.

Be verbose; more v characters cause more verbosity.


The slogger2 daemon is the central manager of a system-logging framework. Its primary goals are to:

  • The log currently supports only text messages, not binary data.
  • (QNX Neutrino 7.0 or later) If you want to redirect syslogd output to slogger2, send the log messages to /dev/console (which slogger2 monitors) by doing one of the following:
    • Edit /etc/syslog.conf and specify an action that sends the logs to /dev/console, and then start syslogd.
    • Specify the LOG_CONS flag when you call openlog()

The components of the system logger include:

libslog2, <sys/slog2.h>
The library and header file for the API.
The daemon process that's responsible for:
  • allocating buffers for each process
  • maintaining buffers (cleaning up buffers when processes die, etc.)
  • controlling verbosity
  • gathering logs
  • providing a means for other components to consume the logs
libslog2parse, <slog2_parse.h>
A library that supports log parsing by other tools (e.g., slog2info). For more information, see slog2_parse_all(), slog2_parse_dynamic_buffer(), slog2_parse_static_buffer(), and their related functions in the C Library Reference.
The realtime viewer. It can consume the live stream from slogger2, as well as parse (using libslog2parse) and display saved slog2 files.

Buffer Management

The central slogger2 process manages a region of shared RAM where all slog2 buffers are located. This gives slog2 the performance benefit of writing to RAM (instead of to flash or disk) without losing logs in the case of process crashes. When a process ends, its logs remain in RAM, but slogger2 removes them when it needs to reuse that memory. However, if the power source is disconnected, any log contents in RAM will be lost.

Individual slog2 instances are allocated from this memory region by slogger2 and are provided to the client process, via slog2_register(), in the form of a shared memory file in /dev/shmem/slogger2/. The libslog2 library uses shmem_open() to access each instance.

Within each slog2 instance there may be up to four buffers defined by the client process. These buffers can be any combination of 4 KB pages. The primary intent for these buffers is to enable high-rate logging into one buffer, while preserving a longer history of low-rate logging in another.

Note: The intent isn't for every subcomponent in a client process to have its own buffer. Each buffer introduces overhead, both in memory management and in terms of log-streaming performance. The expectation is that it will be rare for more than two buffers in a single instance to be used. For extreme situations, the client process can register multiple slog2 instances to satisfy a need for additional buffers.

The central slogger2 process is responsible for cleaning up the slog2 instances, including calling the shmem_unlink() function to remove the shared memory file.

Severity, verbosity, and filtering

Each log line is assigned one of the following severity levels (listed here in decreasing order):

Shut down the system now (e.g., for OEM use).
Unexpected unrecoverable error (e.g., hard disk error).
Unexpected recoverable error (e.g., you need to reset a hardware controller).
Expected error (e.g., parity error on a serial port).
Warning (e.g., out of paper).
Information (e.g., printing page 3).
Debug messages (e.g., normal detail).
Debug messages (e.g., fine detail).

The verbosity level controls which log lines are written in the slog2 buffer; if the severity level is greater than the verbosity level, the line is written in the buffer.

Filtering controls which log items are displayed to the user; the log contents aren't affected. You could filter the log by using (for example) grep, slog2info, or a custom log viewer.

Redirecting a program's stdout and stderr to slog2 buffers

You can redirect a program's standard output (stdout) and standard error (stderr) streams to slog2 buffers. The slogger2 service manages two device paths, /dev/slog2/stdout and /dev/slog2/stderr, and when a process writes to them, the service stores the bytes written, along with other information such as the process ID, in slog2 buffers.

The advantage of using this redirection feature over the -c option is that a given program's logging information can be viewed separately from that of other programs. With -c, the same buffer set contains output from all processes using slogger2.

You can append options to these paths. The syntax is:
my_program >/dev/slog2/stdout[/[buffer_set_name],stdout_page_count] 
A basic usage of this feature is:
my_program >/dev/slog2/stdout 2>/dev/slog2/stderr
A more complex usage entails providing a buffer set name and the page counts, like so:
my_program >/dev/slog2/stdout/alt_name,1,32 2>/dev/slog2/stderr/alt_name,1,32

The default behavior (if no options are provided) is equivalent to writing to /dev/slog2/stdout/7 and /dev/slog2/stderr/7,1. This is to make /dev/slog2/stdout/7 become /dev/slog2/stdout/7,1 to align with the need for the same syntax in both. In this example, 7 pages are used for stdout and 1 page for stderr.

A 0 (zero) indicates the default value. So
is the same as
and the same as

Redirecting slogger output to slogger2

If you have programs that use the legacy slogger APIs instead of slogger2, you can use the library to route slogger API calls to slogger2. To use the shim library, assign its name to the LD_PRELOAD environment variable when you start a program: my_program
This sets up a slogger2 buffer set with the following parameters:
buffer_set_name: my_program
num_buffers: 1
verbosity level: SLOG2_DEBUG
num_pages: 8
buffer_name: SLOGGER_LEGACY
Both the slogger and slogger2 APIs take an opcode, which is a user-specified numeric value that becomes part of the log message. The opcode is a 32-bit value that's made up of a 20-bit major value and a 12-bit minor value. You usually declare it with a convenience macro as follows:
_SLOG_SETCODE(major, minor)
The slogger2 process and the library drop the first (or major) value and keep only the second (or minor) value. This second value gets put in the log message.
You can control the behavior of the shim library via the following environment variables:
The number of pages used for the SLOGGER_LEGACY buffer (the default is 8).
If this is nonzero and only one buffer set is used, the buffer is named SLOGGER_DEFAULT, and it allocates the number of pages specified by this environment variable. In the case where 2 buffers are requested (SLOG2_ONE_BUFFER_SET isn't zero), this is the number of pages used by the second buffer (the default is 0).
The default verbosity used by slogger2 (the default is SLOG2_DEBUG2).
If this is zero, slogger2 is set up to use a single buffer (the default). If it's nonzero, slogger2 is configured to use two buffers: one named SLOGGER_LEGACY with the number of pages specified by SLOG2_SHIM_PAGES, and one named SLOGGER_DEFAULT with the number of pages specified by SLOG2_DEFAULT_PAGES.

The two-buffer setting is meaningful only if SLOG2_DEFAULT_PAGES is also nonzero.

If you don't load the shim library, any slogger messages issued by your programs are still handled by slogger2. This is because slogger2 exposes the /dev/slog device path that the legacy slogger APIs write their messages to. The slogger2 process understands the format of messages received this way, but just like when the shim library is used, it also drops the major value from the opcode and keeps only the minor value.

The advantage of using the shim library is that each log written to the logging buffer is handled locally within the process, without any message passing overhead. With the /dev/slog method, the logging buffer is maintained by slogger2 and each log written by a process is passed as a message to slogger2, which in turn appends the log to a buffer that it maintains on behalf of the client process. Almost all advantages related to logging with slogger2 are lost with the /dev/slog approach. The only advantage to this other approach is simplicity; it minimizes the effort required to get a legacy code base working, while retaining the same efficiency as slogger.