asl(3) BSD Library Functions Manual asl(3)
asl_add_log_file, asl_add_outout_file, asl_close, asl_close_auxiliary_file, asl_create_auxiliary_file, asl_free, asl_get, asl_key, asl_log,
asl_log_auxiliary_location, asl_log_descriptor, asl_new, asl_open, asl_open_from_file, asl_remove_log_file, asl_search, asl_send, asl_set,
asl_set_filter, asl_set_output_file_filter, asl_set_query, asl_unset, asl_vlog, aslresponse_free, aslresponse_next -- system log message
sending and searching functions
asl_add_log_file(aslclient asl, int descriptor);
asl_add_output_file(aslclient asl, int descriptor, const char *msg_fmt, const char *time_fmt, int filter, int text_encoding);
asl_create_auxiliary_file(aslmsg msg, const char *title, const char *uti, int *out_descriptor);
const char *
asl_get(aslmsg msg, const char *key);
const char *
asl_key(aslmsg msg, uint32_t n);
asl_log(aslclient asl, aslmsg msg, int level, const char *format, ...);
asl_log_auxiliary_location(aslmsg msg, const char *title, const char *uti, const char *url);
asl_log_descriptor(aslclient asl, aslmsg msg, int level, int descriptor, uint32_t fd_type);
asl_open(const char *ident, const char *facility, uint32_t opts);
asl_open_from_file(int descriptor, const char *ident, const char *facility);
asl_remove_log_file(aslclient asl, int descriptor);
asl_search(aslclient asl, aslmsg msg);
asl_send(aslclient asl, aslmsg msg);
asl_set(aslmsg msg, const char *key, const char *value);
asl_set_filter(aslclient asl, int filter);
asl_set_output_file_filter(aslclient asl, int descriptor, int filter);
asl_set_query(aslmsg msg, const char *key, const char *value, uint32_t op);
asl_unset(aslmsg msg, const char *key);
asl_vlog(aslclient asl, aslmsg msg, int level, const char *format, va_list ap);
These routines provide an interface to the Apple System Log facility. They are intended to be a replacement for the syslog(3) API, which
will continue to be supported for backwards compatibility. The new API allows client applications to create flexible, structured messages
and send them to the syslogd server, where they may undergo additional processing. Messages received by the server are saved in a data store
(subject to input filtering constraints). This API permits clients to create queries and search the message data store for matching mes-
An introduction to the concepts underlying this interface follows the interface summary below.
asl_open(ident, facility, opts) creates and returns a client handle, or NULL if an error occurs in the library. Messages sent using this
handle will default to having the string ident as the value associated with the ASL_KEY_SENDER key, and the value facility associated with
the ASL_KEY_FACILITY key. Several options are available, as described in the CLIENT HANDLES section.
Each client handle holds state information that is used when a message is logged using that handle. This information includes the ident and
facility strings and the options from the opts parameter. Client handles also contain various filter, file descriptor, and control data.
The state information in a client handle is not protected by any locking or thread synchronization mechanism. It is not safe for two or more
threads to use a single client handle simultaneously. Multi-threaded applications should generally create one client handle for each thread
that logs messages. A client handle may only be safely shared amongst multiple threads if the application uses locks or some synchronization
strategy to ensure single-threaded access.
As a special case, the ASL library allows the use of NULL in place of a client handle. In this case, the library uses an internal structure
which contains its own lock. Multiple threads may safely use NULL, although there may be contention for the lock.
Applications that use libdispatch may use NULL in place of a client handle, although this may cause undesirable synchronization behavior and
degraded performance because of lock contention. A better design is often to use one or more serial dispatch queues specifically for log-
ging. Each such serial queue should use a separate client handle.
asl_close(asl) closes the client handle asl and releases its associated resources.
asl_add_output_file(asl, descriptor, msg_fmt, time_fmt, filter, text_encoding) adds the file descriptor descriptor to the a set of file
descriptors associated with the client handle asl. Each log message sent by that client handle is also written to these file descriptors
(depending on the setting of the filter argument). The message format is specified by the msg_fmt argument. The format for timestamps is
specified by the time_fmt argument, although custom format strings may specify more advanced formats for timestamps. Details on custom for-
mat strings are below.
Each output file has an associated filter value. The filter determines which messages are formatted and written to the file based on the
message priority level.
Special handling for certain characters is specified by the text_encoding argument. The supported values and their effect are described
The msg_format argument is a character string that tells the library how to format each message written to the output file. There are sev-
eral pre-defined message formats, described below. Custom formats are also supported, giving complete control over which ASL message keys
should be written and the overall format of each output line. The pre-defined formats are identified by constants in the asl.h header file.
ASL_MSG_FMT_RAW The contents of the ASL message dictionaries are formatted as a list, with each key-value pair formatted as ``[Key Value]''.
ASL_MSG_FMT_STD Messages are formatted using the standard ASL message format of the form
Time Host Sender[PID] <Level>: Message
Time formats are described below.
ASL_MSG_FMT_BSD The legacy format used for plain-text log files. Similar to the ASL_MSG_FMT_STD format, but the message priority level is
ASL_MSG_FMT_MSG The output line contains only the value of the Message key in each ASL message dictionary.
ASL_MSG_FMT_XML Produces multiple lines of output for each ASL message. The message is formatted as an XML dictionary:
<string>Key 1 Value</string>
<string>Key 2 Value</string>
A NULL value for msg_fmt causes the library to use the ``std'' format.
Custom format strings may contain a mix of characters that are directly copied to the output line and variables, which are a dollar sign '$'
followed by specific ASL message dictionary keys, whose values will be interpolated into the output. For example, the format string:
This message from $Sender PID=$PID at $Time *** $Message
would result in lines in the output file like, e.g.:
This message from login PID=982 at Jul 27 08:41:27 *** USER_PROCESS: 330 ttys000
This message from Mail PID=987 at Jul 27 08:42:16 *** Using V2 Layout
Normally, a space character terminates a variable name. However, the name may be wrapped in parentheses if a space character is not desired
in the output. For example:
A third form for specifying variables may be used for the ASL ``Level'' and ``Time'' message keys. Note that a ``Time'' specification using
one of the forms below will override the time_fmt argument to the function.
The following forms are recognized:
$((Level)(str)) Formats a Level value as a string, for example ``Error'', ``Alert'', ``Warning'', and so on. Note that $(Level) or
$Level formats the value as an integer 0 through 7.
$((Time)(sec)) Formats a Time value as the number of seconds since the Epoch.
$((Time)(raw)) Alias for $((Time)(sec)).
$((Time)(local)) Formats a Time value as a string of the form ``Mmm dd hh:mm:ss'', where Mmm is the abbreviation for the month, dd is
the date (1 - 31) and hh:mm:ss is the time. The local timezone is used.
$((Time)(lcl)) Alias for $((Time)(local)).
$((Time)(utc)) Formats a Time value as a string of the form ``yyyy-mm-dd hh:mm:ssZ'', using Coordinated Universal Time, or the
``Zulu'' time zone.
$((Time)(zulu)) Alias for $((Time)(utc)).
$((Time)(X)) Where X may be any letter in the range A - Z or a - z. Formats the Time using the format ``yyyy-mm-dd hh:mm:ssX'',
using the specified nautical timezone. Z is the same as UTC/Zulu time. Timezones A - M (except J) decrease by one
hour to the east of the Zulu time zone. Timezones N - Y increase by one hour to the west of Z. M and Y have the
same clock time, but differ by one day. J is used to indicate the local timezone. When printing using
$((Time)(J)), the output format is ``yyyy-mm-dd hh:mm:ss'', without a trailing timezone letter.
$((Time)(JZ)) Specifies the local timezone. The timezone offset from UTC follows the date and time. The time is formatted as
``yyyy-mm-dd hh:mm:ss[+|-]HH[:MM]''. Minutes in the timezone offset are only printed if they are non-zero.
$((Time)(ISO8601)) Specifies the local timezone, formatted as specified by ISO 8601. The timezone offset from UTC follows the date and
time. The time is formatted as ``yyyy-mm-ddThh:mm:ss[+|-]HH[:MM]''. Minutes in the timezone offset are only
printed if they are non-zero. Note that this differs from ``JZ'' format only in that a ``T'' character separates
the date and time.
$((Time)([+|-]HH[:MM])) Specifies an offset (+ or -) of the indicated number of hours (HH) and optionally minutes (MM) to UTC. The value is
formatted as a string of the form ``yyyy-mm-dd hh:mm:ss[+|-]HH[:MM]''. Minutes in the timezone offset are only
printed if they are non-zero.
Unless a custom message format uses one of the specialized forms for ``Time'' described above, then any timestamps in an output message will
be formatted according the the time_fmt argument. The known formats are identified by constants in the asl.h header file.
ASL_TIME_FMT_SEC Formats timestamps as the number of seconds since the Epoch.
ASL_TIME_FMT_UTC Formats a Time value as a string of the form ``yyyy-mm-dd hh:mm:ssZ'', using Coordinated Universal Time, or the ``Zulu''
ASL_TIME_FMT_LCL Formats a Time value as a string of the form ``Mmm dd hh:mm:ss'', where Mmm is the abbreviation for the month, dd is the
date (1 - 31) and hh:mm:ss is the time. The local timezone is used.
A value of NULL for the time_fmt argument will cause the default format ASL_TIME_FMT_LCL to be used.
The encoding parameter specifies how certain characters are to be treated when preparing a message for output. The known encodings are:
ASL_ENCODE_NONE No special character encode is done.
ASL_ENCODE_ASL Newlines and tabs are also encoded as "
" and " " respectively. In ``ASL_MSG_FMT_RAW'' format, space characters embedded
in log message keys are encoded as "s" and embedded brackets are escaped to print as "[" and "]".
ASL_ENCODE_SAFE Encodes backspace characters as ^H. Carriage returns are mapped to newlines. A tab character is appended after newlines so
that message text is indented.
ASL_ENCODE_XML This encoding should be used when formatting messages using ASL_MSG_FMT_XML. XML format output requires that keys are valid
UTF8 strings. Keys which are not valid UTF8 are ignored, and the associated value is not printed.
Values that contain legal UTF8 are printed as strings. Ampersand, less than, greater than, quotation mark, and apostrophe
characters are encoded according to XML conventions. Embedded control characters are encoded as ``&#xNN;'' where NN is the
character's hexadecimal value.
Values that do not contain legal UTF8 are encoded in base-64 and printed as data objects.
asl_add_output_file Returns 0 on success, non-zero on failure.
asl_add_log_file(asl, descriptor) Is equivalent to
asl_add_output_file(asl, descriptor, ASL_MSG_FMT_STD, ASL_TIME_FMT_LCL, ASL_FILTER_MASK_UPTO(ASL_LEVEL_DEBUG), ASL_ENCODE_SAFE);
Returns 0 on success, non-zero on failure.
asl_set_output_file_filter(asl, descriptor, filter) replaces the current filter value associated with a file descriptor that has been added
to a client handle. Returns the previous filter value.
asl_remove_log_file(asl, descriptor) removes a file descriptor from the set of file descriptors associated with a client handle. Returns 0
on success, non-zero on failure.
asl_new(type) allocates and returns an aslmsg structure, or NULL in the case of a failure in the library. The type argument must be
ASL_TYPE_MSG or ASL_TYPE_QUERY.
asl_free(msg) frees an aslmsg and releases resources associated with the structure.
asl_set(msg, key, value) creates a new key and value in an aslmsg structure, or replaces the value of an existing key. Returns 0 on success,
non-zero on failure.
asl_set_query(msg, key, op, value) is used to construct searches. It is similar to asl_set(), except that it takes an additional op (opera-
tion) argument. Creates a new (key, op, value) triple in an aslmsg structure, or replaces the value and operation for an existing key. See
the SEARCHING section for more information. Returns 0 on success, non-zero on failure.
asl_unset(msg, key) removes a key and its associated value from an aslmsg structure. Returns 0 on success, non-zero on failure.
asl_key(msg, n) returns the nth key in an aslmsg (beginning at zero), allowing an application to iterate through the keys. Returns NULL if n
indexes beyond the number of keys in msg.
asl_get(msg, key) returns the value associated with key in the aslmsg msg. Returns NULL if msg does not contain key.
asl_set_filter(asl, f) sets a filter for messages being sent to the server. The filter is a bitmask representing priority levels. Only mes-
sages having a priority level with a corresponding bit set in the filter mask are sent to the syslogd server. The filter does not control
writes to additional files associated with the client handle using asl_add_log_file(). Returns the previous filter value.
asl_log(asl, msg, level, format, args...) sends a log to the server (subject to filtering, see asl_set_filter() above) and to any file
descriptors associated with the client handle asl. The msg argument may contain any keys and values, which will be formatted as part of the
log message. The value for ASL_KEY_LEVEL is supplied by the level argument. The value for ASL_KEY_MESSAGE is computed from format and the
associated arguments args.... Normal printf() style argument processing is applied to the format and the arguments. The format may also
contain ``%m'' which will be substituted with the string value corresponding to the current errno.
The ASL_PREFILTER_LOG(asl, msg, level, format, ...) macro may be used in place of asl_log(). The macro avoids processing the variable argu-
ment list in those cases where the message would be filtered out due to filter settings, would not be written to a log file associated with
the aslclient, or would not be written to stderr. The macro may provide a performance benefit for some applications. Details on filter set-
ting, additional log files, and aslclient options are described below in this manual.
asl_vlog(asl, msg, level, format, ap) is similar to asl_log() except that it takes a va_list argument.
asl_send(asl, msg) is similar to asl_log(), except the value for ASL_KEY_MESSAGE is taken from msg rather than being constructed using a
printf() style syntax.
asl_log_descriptor(asl, msg, level, descriptor, fd_type) provides functionality to use file descriptors to send logging data to ASL. asl is
retained by ASL and must still be closed by the caller by calling asl_close() if the caller loses reference to it. msg is copied by ASL and
similarly must still be freed by the caller by calling asl_free() if the caller loses reference to it. Any changes made to it after calling
asl_log_descriptor()() are not applicable to the message used. descriptor is treated differently based on the value of fd_type.
If fd_type is ASL_LOG_DESCRIPTOR_READ, the descriptor must be open for read access. ASL uses dispatch(2) to read from the descriptor as data
becomes available. These data are line buffered and passed to asl_log(). When EOF is read, ASL will close(2) descriptor ..
If fd_type is ASL_LOG_DESCRIPTOR_WRITE, the descriptor is closed and a new writable descriptor is created with the same fileno. Any data
written to this new descriptor are line buffered and passed to asl_log(). When EOF is sent, no further data are read. The caller is respon-
sible for closing the new descriptor. One common use for this API is to redirect writes to stdout or stderr to ASL by passing STDOUT_FILENO
or STDERR_FILENO as descriptor.
asl_search(asl, msg) searches for messages that match the keys and values in msg, subject to matching operations associated with those keys
and values. The msg argument should be constructed using asl_set_query(). See the SEARCHING section for details on constructing queries.
Returns an aslresponse structure that contains matching log messages. NULL is returned in case of error or if there are no matching messages
in the ASL database.
aslresponse_next(r) iterates over an aslresponse structure returned by asl_search(). Each call returns the next aslmsg in the response.
Returns NULL when there are no further messages.
aslresponse_free(r) frees the aslresponse structure r and all of its associated resources.
asl_create_auxiliary_file(msg, title, uti, out_descriptor) Creates an auxiliary file that may be used by the client to save arbitrary data.
When the file is closed using asl_close_auxiliary_file(), syslogd will log the specified msg along with the title and the Uniform Type Iden-
tifier provided by uti. If a NULL value is supplied for uti the type ``public.data'' will be used. The Console application will display the
message with a link to the file.
Auxiliary files are saved in the ASL data store. They are automatically deleted at the same time that the log message expires. Messages
expire in 7 days by default. A value set for the ASLExpireTime key will override the default. Read access for the auxiliary file will be
the same as read access for msg. By default, messages (and auxiliary files) are world-readable. Access may be limited by setting values for
the ReadUID and ReadGID keys.
asl_close_auxiliary_file(descriptor) closes the file descriptor descriptor previously returned by a call to asl_create_auxiliary_file().
asl_log_auxiliary_location(msg, title, uti, url) will log the specified msg along with the title, the Uniform Type Identifier provided by
uti, and the Uniform Resource Locator provided by url. The Console application will display the message with a link to the file. This
allows a client to save data in an auxiliary file, but unlike asl_create_auxiliary_file(), the life-cycle of this file must be managed by
some external system. The file will not be removed when the corresponding log message expired from the ASL data store.
asl_open_from_file(descriptor, facility, opts) creates a client handle for an open file descriptor descriptor. This routine may be used in
conjunction with asl_create_auxiliary_file() or asl_log_auxiliary_location() to save ASL format log messages in an auxiliary file. The UTI
type ``com.apple.asl-file'' should be used for ASL format auxiliary files.
Files with this format may be read from the command line using syslog -f file, or from the Console utility.
The file must be open for read and write access. The file will be truncated and its existing contents will be lost. asl_close() must be
called to close the client handle when logging to this file is complete. The file should be closed using asl_close_auxiliary_file() if it
was returned by asl_create_auxiliary_file(), or close() otherwise.
The client handle returned by asl_open_from_file() contains an internal lock, and may be used safely by multiple threads or from independent
dispatch queues. Note that callers will contend for the internal lock when saving log messages to a file.
Note that messages with ReadUID or ReadGID values will simply be saved to the file, and will not effect read access to either the message or
the file itself. Similarly, messages with ASLExpireTime values will be saved, but will not effect the life-cycle of either the individual
messages or the file.
At the core of this API is the aslmsg structure. Although the structure is opaque and may not be directly manipulated, it contains a list of
key/value pairs. All keys and values are NUL-character terminated C language strings. UTF-8 encoding may be used for non-ASCII characters.
Message structures are generally used to send log messages, and are created thusly:
aslmsg m = asl_new(ASL_TYPE_MSG);
Another message type, ASL_TYPE_QUERY, is used to create queries when searching the data store. Query type messages and searching are
described in detail in the SEARCHING section. For the remainder of this section, the messages described will be of the ASL_TYPE_MSG variety.
Each aslmsg contains a default set of keys and values that are associated with them. These keys are listed in the asl.h header file. They
#define ASL_KEY_TIME "Time"
#define ASL_KEY_HOST "Host"
#define ASL_KEY_SENDER "Sender"
#define ASL_KEY_FACILITY "Facility"
#define ASL_KEY_PID "PID"
#define ASL_KEY_UID "UID"
#define ASL_KEY_GID "GID"
#define ASL_KEY_LEVEL "Level"
#define ASL_KEY_MSG "Message"
Many of these correspond to equivalent parts of messages described in the syslog(3) API. Values associated with these message keys are
assigned appropriate defaults. The value for ASL_KEY_HOST is the local host name, the value associated with ASL_KEY_SENDER is the process
name, the ASL_KEY_PID is the client's process ID number, and so on.
Note the addition of the UID and GID keys. The values for UID and GID are set in library code by the message sender. The server will
attempt to confirm the values, but no claim is made that these values cannot be maliciously overridden in an attempt to deceive a log message
reader as to the identity of the sender of a message. The contents of log messages must be regarded as insecure.
The asl(3) API does not require a process to choose a facility name. The syslogd server will use a default value of ``user'' if a facility
is not set. However, a client may set a facility name as an argument in the asl_open call, or by setting a specific value for the
ASL_KEY_FACILITY in a message:
asl_set(m, ASL_KEY_FACILITY, "com.somename.greatservice");
An application may choose any facility name at will. Different facility names may be attached to different messages, perhaps to distinguish
different subsystems in log messages. Developers are encouraged to adopt a ``Reverse ICANN'' naming convention to avoid conflicting facility
Default values are set in the message for each of the keys listed above, except for ASL_KEY_MSG, which may be explicitly set at any time
using the asl_set routine, or implicitly set at the time the message is sent using the asl_log or asl_vlog routines. These two routines also
have an integer-level parameter for specifying the log priority. The ASL_KEY_LEVEL value is set accordingly. Finally, the value associated
with ASL_KEY_TIME is set in the sending routine.
Although it may appear that there is significant overhead required to send a log message using this API, the opposite is actually true. A
simple ``Hello World'' program requires only:
asl_log(NULL, NULL, ASL_LEVEL_INFO, "Hello World!");
Both asl_log and asl_vlog will provide the appropriate default values when passed a NULL aslmsg argument.
In this example, the aslclient argument is NULL. This is sufficient for a single-threaded application, or for an application which only
sends log messages from a single thread. When logging from multiple threads, each thread should open a separate client handle using
asl_open. The client handle may then be closed when it is no longer required using asl_close. Multiple threads may log messages safely
using a NULL aslclient argument, but the library will use an internal lock, so that in fact only one thread will log at a time.
When an application requires additional keys and values to be associated with each log message, a single message structure may be allocated
and set up as ``template'' message of sorts:
aslmsg m = asl_new(ASL_TYPE_MSG);
asl_set(m, ASL_KEY_FACILITY, "com.secrets.r.us");
asl_set(m, "Clearance", "Top Secret");
asl_log(NULL, m, ASL_LEVEL_NOTICE, "Message One");
asl_log(NULL, m, ASL_LEVEL_ERR, "Message Two");
The message structure will carry the values set for the ``Facility'' and ``Clearance'' keys so that they are used in each call to asl_log,
while the log level and the message text are taken from the calling parameters.
The format argument to asl_log and asl_vlog is identical to printf(3), and may include '%m', which is replaced by the current error message
(as denoted by the global variable errno; see strerror(3).)
Key/value pairs may be removed from a message structure with asl_unset. A message may be freed using asl_free.
The asl_send routine is used by asl_log and asl_vlog to transmit a message to the server. This routine sets the value associated with
ASL_KEY_TIME and sends the message. It may be called directly if all of a message's key/value pairs have been created using asl_set.
Messages that are sent to the syslogd server may be saved in a message store. The store may be searched using asl_search, as described
below. By default, all messages are readable by any user. However, some applications may wish to restrict read access for some messages.
To accommodate this, a client may set a value for the "ReadUID" and "ReadGID" keys. These keys may be associated with a value containing an
ASCII representation of a numeric UID or GID. Only the root user (UID 0), the user with the given UID, or a member of the group with the
given GID may fetch access-controlled messages from the database.
Although the ASL system does not require a "Facility" key in a message, many processes specify a "Facility" value similar to the common usage
of the BSD syslog API, although developers are encouraged to adopt facility names that make sense for their application. A ``Reverse ICANN''
naming convention (e.g. "com.apple.system.syslog") should be adopted to avoid conflicting names. The ASL system generally allows any string
to be used as a facility value, with one exception. The value "com.apple.system", or any string that has "com.apple.system" as a prefix, may
only be used by processes running with the UID 0. This allows system processes to log messages that can not be "spoofed" by user processes.
Non-UID 0 client processes that specify "com.apple.system" as a facility, will be assigned the value "user" by the syslogd server.
A client handle contains various parameters and control settings that are used when a message is logged. This includes an identification
string, a facility name, filtering controls, additional file descriptors, and other data. Client handles are not thread-safe. Applications
that log from multiple threads should create a client handle for each thread.
Applications that use libdispatch must also avoid using a single client handle from multiple dispatch queues if those queues may run concur-
rently. A good approach is to create one or more serial dispatch queues specifically for logging. Each such queue should use its own ASL
If a single handle must be accessed by multiple dispatch queues, then the application must use locks, semaphores, or some other mechanism to
prevent concurrent access to a client handle.
A NULL value may be used in any of the routines that require an aslclient argument. In this case, the library will use an internal client
handle. This internal handle contains its own lock, allowing multiple threads to safely use the NULL client handle. Note, however, that
contention for the lock may cause undesirable synchronization behavior or reduced performance.
The asl_open routine may be given an ident argument, which becomes the default value for the ASL_KEY_SENDER key, and a facility argument,
which becomes the value associated with the ASL_KEY_FACILITY key. If NULL is passed as the value for ident, the name of the currently run-
ning program will be used. If NULL is passed as the value for facility, the value ``user'' will be used for non UID 0 processes, and ``''
daemon will be used for UID 0 processes.
Several options are available when creating a client handle. They are:
ASL_OPT_STDERR adds stderr as an output file descriptor
ASL_OPT_NO_DELAY connects to the server immediately
ASL_OPT_NO_REMOTE disables remote-control filter adjustment
ASL_OPT_NO_DELAY makes the client library connect to the syslogd server at the time that asl_open is called, rather than waiting for the
first message to be sent. Opening the connection is quite fast, but some applications may want to avoid any unnecessary delays when calling
asl_log, asl_vlog, or asl_send.
See the FILTERING section below, and the syslog(1) for additional details on filter controls.
A client handle is closed and its resources released using asl_close. Note that if additional file descriptors were added to the handle,
either using the ASL_OPT_STDERR option or afterwards with the asl_add_log_file routine, those file descriptors are not closed by asl_close.
LOGGING TO ADDITIONAL FILES
If a client handle is opened with the ASL_OPT_STDERR option to asl_open, a copy of each log message will be sent to stderr. Additional out-
put streams may be include using asl_add_log_file.
Messages sent to stderr or other files are printed in the "standard" message format also used as a default format by the syslog(1) command
line utility. Non-ASCII characters in a message are encoded using the ``safe'' encoding style used by syslog(1) with the -E safe option.
Backspace characters are printed as ^H. Carriage returns are mapped to newlines. A tab character is appended after newlines so that message
text is indented.
File descriptors may be removed from the list of outputs associated with a client handle with asl_remove_log_file. This routine simply
removes the file descriptor from the output list. The file is not closed as a result.
The ASL_OPT_STDERR option may not be unset after a client handle has been opened.
The syslogd server archives received messages in a data store that may be searched using the asl_search, aslresponse_next, and
aslresponse_free routines. A query message is created using:
aslmsg q = asl_new(ASL_TYPE_QUERY);
Search settings are made in the query using asl_set_query. A search is performed on the data store with asl_search. It returns an
aslresponse structure. The caller may then call aslresponse_next to iterate through matching messages. The aslresponse structure may be
freed with aslresponse_free.
Like other messages, ASL_TYPE_QUERY messages contain keys and values. They also associate an operation with each key and value. The opera-
tion is used to decide if a message matches the query. The simplest operation is ASL_QUERY_OP_EQUAL, which tests for equality. For example,
the following code snippet searches for messages with a Sender value equal to ``MyApp''.
q = asl_new(ASL_TYPE_QUERY);
asl_set_query(q, ASL_KEY_SENDER, "MyApp", ASL_QUERY_OP_EQUAL);
r = asl_search(NULL, q);
More complex searches may be performed using other query operations.
ASL_QUERY_OP_EQUAL value equality
ASL_QUERY_OP_GREATER value greater than
ASL_QUERY_OP_GREATER_EQUAL value greater than or equal to
ASL_QUERY_OP_LESS value less than
ASL_QUERY_OP_LESS_EQUAL value less than or equal to
ASL_QUERY_OP_NOT_EQUAL value not equal
ASL_QUERY_OP_REGEX regular expression search
ASL_QUERY_OP_TRUE always true - use to test for the existence of a key
Regular expression search uses regex(3) library. Patterns are compiled using the REG_EXTENDED and REG_NOSUB options.
Modifiers that change the behavior of these operations may also be specified by ORing the modifier value with the operation. The modifiers
ASL_QUERY_OP_CASEFOLD string comparisons are case-folded
ASL_QUERY_OP_PREFIX match a leading substring
ASL_QUERY_OP_SUFFIX match a trailing substring
ASL_QUERY_OP_SUBSTRING match any substring
ASL_QUERY_OP_NUMERIC values are converted to integer using atoi
The only modifier that is checked for ASL_QUERY_OP_REGEX search is ASL_QUERY_OP_CASEFOLD. This causes the regular expression to be compiled
with the REG_ICASE option.
If a query message contains more than one set of key/value/operation triples, the result will be a logical AND. For example, to find mes-
sages from ``MyApp'' with a priority level less than or equal to ``3'':
q = asl_new(ASL_TYPE_QUERY);
asl_set_query(q, ASL_KEY_SENDER, "MyApp", ASL_QUERY_OP_EQUAL);
asl_set_query(q, ASL_KEY_LEVEL, "3",
ASL_QUERY_OP_LESS_EQUAL | ASL_QUERY_OP_NUMERIC);
r = asl_search(NULL, q);
After calling asl_search to get an aslresponse structure, use aslresponse_next to iterate through all matching messages. To iterate through
the keys and values in a message, use asl_key to iterate through the keys, then call asl_get to get the value associated with each key.
aslmsg q, m;
const char *key, *val;
r = asl_search(NULL, q);
while (NULL != (m = aslresponse_next(r)))
for (i = 0; (NULL != (key = asl_key(m, i))); i++)
val = asl_get(m, key);
FILTERING AND REMOTE CONTROL
Clients may set a filter mask value with asl_set_filter. The mask specifies which messages should be sent to the syslogd daemon by specify-
ing a yes/no setting for each priority level. Clients typically set a filter mask to avoid sending relatively unimportant messages. For
example, Debug or Info priority level messages are generally only useful for debugging operations. By setting a filter mask, a process can
improve performance by avoiding sending messages that are in most cases unnecessary.
asl_set_filter returns the previous value of the filter, i.e. the value of the filter before the routine was called.
As a convenience, the macros ASL_FILTER_MASK(level) and ASL_FILTER_MASK_UPTO(level) may be used to construct a bit mask corresponding to a
given priority level, or corresponding to a bit mask for all priority levels from ASL_LEVEL_EMERG to a given input level.
The default filter mask is ASL_FILTER_MASK_UPTO(ASL_LEVEL_NOTICE). This means that by default, and in the absence of remote-control changes
(described below), ASL_LEVEL_DEBUG and ASL_LEVEL_INFO priority level messages are not sent to the server.
Three different filters exist for each application. The first is the filter mask set using asl_set_filter as described above. The Apple
System Log facility also manages a ``master'' filter mask. The master filter mask usually has a value that indicates to the library that it
is ``off'', and thus it has no effect. However, the mask filter mask may be enabled by giving it a value using the syslog command, using the
-c 0 option. When the master filter mask has been set, it takes precedence over the client's filter mask. The client's mask is unmodified,
and will become active again if remote-control filtering is disabled.
In addition to the master filter mask, The Apple System Log facility also manages a per-client remote-control filter mask. Like the master
filter mask, the per-client mask is usually ``off'', having no effect on a client. If a per-client filter mask is set using the syslog com-
mand, using the -c process option, then it takes precedence over both the client's filter mask and the master filter mask. As is the case
with the master filter mask, a per-client mask ceases having any effect when if is disabled.
The ASL_OPT_NO_REMOTE option to asl_open causes both the master and per-client remote-control masks to be ignored in the library. In that
case, only the client's own filter mask is used to determine which messages are sent to the server. This may be useful for Applications that
produce log messages that should never be filtered, due to security considerations. Note that root (administrator) access is required to set
or change the master filter mask, and that only root may change a per-client remote-control filter mask for a root (UID 0) process.
The per-process remote control filter value is kept as a state value associated with a key managed by notifyd. The key is protected by an
access control mechanism that only permits the filter value to be accessed and modified by the same effective UID as the ASL client at the
time that the first ASL connection was created. Remote filter control using syslog -c will fail for processes that change effective UID
after starting an ASL connection. Those processes should close all ASL client handles and then re-open ASL connections if remote filter con-
trol support is desired.
These functions first appeared in Mac OS X 10.4.
syslog(1), strvis(3), syslogd(8)
Mac OS X October 1, 2011 Mac OS X