Mac OS X Manual Page For asl_add_log

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asl(3) BSD Library Functions Manual asl(3)

NAME
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

SYNOPSIS
#include <asl.h>

int
asl_add_log_file(aslclient asl, int fd);

void
asl_close(aslclient asl);

void
asl_free(aslmsg msg);

const char *
asl_get(aslmsg msg, const char *key);

const char *
asl_key(aslmsg msg, uint32_t n);

int
asl_log(aslclient asl, aslmsg msg, int level, const char *format, ...);

aslmsg
asl_new(uint32_t type);

aslclient
asl_open(const char *ident, const char *facility, uint32_t opts);

int
asl_remove_log_file(aslclient asl, int fd);

aslresponse
asl_search(aslclient asl, aslmsg msg);

int
asl_send(aslclient asl, aslmsg msg);

int
asl_set(aslmsg msg, const char *key, const char *value);

int
asl_set_filter(aslclient asl, int f);

int
asl_set_query(aslmsg msg, const char *key, const char *value,
uint32_t op);

int
asl_unset(aslmsg msg, const char *key);

int
asl_vlog(aslclient asl, aslmsg msg, int level, const char *format,
va_list ap);

void
aslresponse_free(aslresponse a);

aslmsg
aslresponse_next(aslresponse r);

DESCRIPTION
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 messages.

MESSAGES
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 NULL-terminated C language
character strings. UTF-8 encoding may be used for non-ASCII characters.

Message structures are generally used to send log messages, and are cre-ated created
ated 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 below. 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 are:

#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 dif-ferent different
ferent subsystems in log messages. Developers are encouraged to adopt a
``Reverse ICANN'' naming convention to avoid conflicting facility names.

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 inte-ger-level integer-level
ger-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 sim-ple simple
ple ``Hello World'' program requires only:

#include <asl.h>
...
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 must open a separate client handle using asl_open. The client
handle may then be closed when it is no longer required using asl_close.

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 mes-sage message
sage 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.

SECURITY
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 appli-cations applications
cations may wish to restrict read access for some messages. To accomo-date accomodate
date this, a client may set a value for the "ReadUID" and "ReadGID" keys.
These keys may be associated with a value containing an ASCII representa-tion representation
tion 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.

CLIENT HANDLES
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 applica-tion. application.
tion.

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

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 output
streams may be include using asl_add_log_file.

Messages sent to stderr or other files are printed in the "standard" mes-sage message
sage format also used as a default format by the syslog(1) command line
utility. The strvis(3) encoding with the VIS_CSTYLE, VIS_TAB, and VIS_NL
options is used to print the message.

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.

In the present release of Mac OS X, a ``raw'' format is used to format
messages that are sent to file descriptors that have been added to a
client handle. Each message is preceded by a 10-character field contain-ing containing
ing a message length. The message length is padded with leading white
space. The length gives the string length of the remainder of the output
string. Following the length is a space character, and then the message.
The message is encoded as a set of key/value pairs enclosed in square
brackets, which are themselves separated by a space character. The key
is separated from the value by space character. Embedded closing square
brackets are escaped by a backslash. Embedded space characters in keys
are escaped by a backslash; Embedded newlines are summarily turned into
semicolons. The output is terminated by a trailing newline and a NUL
character.

SEARCHING
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 operation
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 fol-lowing following
lowing code snippet searches for messages with a Sender value equal to
``MyApp''.

aslmsg m;
aslresponse r;
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 speci-fied specified
fied by ORing the modifier value with the operation. The modifiers are:

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 messages
from ``MyApp'' with a priority level less than or equal to ``3'':

aslmsg q;
aslresponse r;
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;
int i;
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);
...
}
}
aslresponse_free(r);

FILTERING AND REMOTE CONTROL
Clients may set a filter mask value with asl_set_filter. The mask speci-fies specifies
fies which messages should be sent to the syslogd daemon by specifying 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 unneces-sary. unnecessary.
sary.

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_FIL-TER_MASK_UPTO(level) ASL_FILTER_MASK_UPTO(level)
TER_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 mes-sages messages
sages 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 Sys-tem System
tem Log facility also manages a ``master'' filter mask. The master fil-ter filter
ter 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 command,
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 perclient
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 remotecontrol
control filter mask for a root (UID 0) process.

HISTORY
These functions first appeared in Mac OS X 10.4.

SEE ALSO
syslog(1), strvis(3), syslogd(8)

Mac OS X January 11, 2007 Mac OS X