SYNOPSIS
#include <asl.h>
aslclient
asl_open(const char *ident, const char *facility, uint32_t opts);
void
asl_close(aslclient asl);
aslmsg
asl_new(uint32_t type);
void
asl_free(aslmsg msg);
int
asl_set(aslmsg msg, const char *key, const char *value);
int
asl_set_query(aslmsg msg, const char *key, const char *value, uint32_t op);
const char *
asl_key(aslmsg msg, uint32_t n);
const char *
asl_get(aslmsg msg, const char *key);
int
asl_unset(aslmsg msg, const char *key);
int
asl_log(aslclient asl, aslmsg msg, int level, const char *format, ...);
int
asl_vlog(aslclient asl, aslmsg msg, int level, const char *format, va_list ap);
int
asl_send(aslclient asl, aslmsg msg);
int
asl_add_log_file(aslclient asl, int fd);
int
asl_remove_log_file(aslclient asl, int fd);
int
asl_set_filter(aslclient asl, int f);
aslresponse
asl_search(aslclient asl, aslmsg msg);
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 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 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_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.
Also note the absence of a Facility key. 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 using:
asl_set(m, "Facility", "UsefulService");
An application may choose any facility name at will.
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, "Facility", "Spy vs. Spy");
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.
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 to the server. This routine sets the value associated with
ASL_KEY_TIME and send the message. It may be called directly if all of a
message's key/value pairs have been created using asl_set.
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 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 default facility name for
the application.
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_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 sent to file descriptors added to a client handle. Each message
is preceded by a 10-character field containing 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 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_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 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.
As a convenience, the macros ASL_FILTER_MASK(level) and ASL_FIL-
TER_MASK_UPTO(level) may be used to construct a bit mask corresponding to
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 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.
HISTORY
These functions first appeared in Mac OS X 10.4.
SEE ALSO
syslogd(8), syslog(1)
Mac OS X January 5, 2005 Mac OS X
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