asl(3) BSD Library Functions Manual asl(3)
asl_add_log_file, asl_close, asl_free, asl_get, asl_key, asl_log, asl_new, asl_open, asl_remove_log_file, asl_search, asl_send, asl_set,
asl_set_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 fd);
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_open(const char *ident, const char *facility, uint32_t opts);
asl_remove_log_file(aslclient asl, int fd);
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 f);
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 assocated with the ASL_KEY_SENDER key, and the value facility assocated with the
ASL_KEY_FACILITY key. Several options are available, as described in the CLIENT HANDLES section.
Multi-threaded applications should create one client handle for each thread that logs messages. A client may use NULL as a client handle, in
which case a default handle managed by the library will be used. A NULL handle may be used safely by multiple threads, but the threads will
contend for a single internal lock when sending log messages using a NULL handle.
asl_close(asl) closes the client handle asl and releases its associated resources.
asl_add_log_file(asl, fd) adds the file descriptor fd to the a set of file descriptors associated with the client handle asl. Each log mes-
sage sent by that client handle is also written to these file descriptors. Returns 0 on success, non-zero on failure.
asl_remove_log_file(asl, fd) removes a file descriptor from the set of file descriptors associated with a client handle. Returns 0 on suc-
cess, 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.
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(), exceopt the value for ASL_KEY_MESSAGE is taken from msg rather than being constructed using a
printf() style syntax.
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.
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 accomodate 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.
When logging is done from a single thread, a NULL value may be used in any of the routines that require an aslclient argument. In this case,
the library will open an internal client handle on behalf of the application.
If multiple threads must do logging, or if client options are desired, then the application should call asl_open to create a client handle
for each thread. As a convenience, 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.
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 it's 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 January 11, 2007 Mac OS X