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.Dd January 11, 2007
.Dt asl 3
.Os "Mac OS X"
.Sh NAME
.Nm asl_add_log_file ,
.Nm asl_close ,
.Nm asl_free ,
.Nm asl_get ,
.Nm asl_key ,
.Nm asl_log ,
.Nm asl_new ,
.Nm asl_open ,
.Nm asl_remove_log_file ,
.Nm asl_search ,
.Nm asl_send ,
.Nm asl_set ,
.Nm asl_set_filter ,
.Nm asl_set_query ,
.Nm asl_unset ,
.Nm asl_vlog ,
.Nm aslresponse_free ,
.Nm aslresponse_next
.Nd system log message sending and searching functions
.Sh SYNOPSIS
.Fd #include <asl.h>
.\"
.Ft int
.Fo asl_add_log_file
.Fa "aslclient asl"
.Fa "int fd"
.Fc
.Ft void
.Fo asl_close
.Fa "aslclient asl"
.Fc
.Ft void
.Fo asl_free
.Fa "aslmsg msg"
.Fc
.Ft const char *
.Fo asl_get
.Fa "aslmsg msg"
.Fa "const char *key"
.Fc
.Ft const char *
.Fo asl_key
.Fa "aslmsg msg"
.Fa "uint32_t n"
.Fc
.Ft int
.Fo asl_log
.Fa "aslclient asl"
.Fa "aslmsg msg"
.Fa "int level"
.Fa "const char *format"
.Fa "..."
.Fc
.Ft aslmsg
.Fo asl_new
.Fa "uint32_t type"
.Fc
.Ft aslclient
.Fo asl_open
.Fa "const char *ident"
.Fa "const char *facility"
.Fa "uint32_t opts"
.Fc
.Ft int
.Fo asl_remove_log_file
.Fa "aslclient asl"
.Fa "int fd"
.Fc
.Ft aslresponse
.Fo asl_search
.Fa "aslclient asl"
.Fa "aslmsg msg"
.Fc
.Ft int
.Fo asl_send
.Fa "aslclient asl"
.Fa "aslmsg msg"
.Fc
.Ft int
.Fo asl_set
.Fa "aslmsg msg"
.Fa "const char *key"
.Fa "const char *value"
.Fc
.Ft int
.Fo asl_set_filter
.Fa "aslclient asl"
.Fa "int f"
.Fc
.Ft int
.Fo asl_set_query
.Fa "aslmsg msg"
.Fa "const char *key"
.Fa "const char *value"
.Fa "uint32_t op"
.Fc
.Ft int
.Fo asl_unset
.Fa "aslmsg msg"
.Fa "const char *key"
.Fc
.Ft int
.Fo asl_vlog
.Fa "aslclient asl"
.Fa "aslmsg msg"
.Fa "int level"
.Fa "const char *format"
.Fa "va_list ap"
.Fc
.Ft void
.Fo aslresponse_free
.Fa "aslresponse a"
.Fc
.Ft aslmsg
.Fo aslresponse_next
.Fa "aslresponse r"
.Fc
.Sh DESCRIPTION
These routines provide an interface to the Apple System Log facility.
They are intended to be a replacement for the
.Xr 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
.Nm 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.
.Ss 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.
.Pp
Message structures are generally used to send log messages,
and are created thusly:
.Pp
aslmsg m = asl_new(ASL_TYPE_MSG);
.Pp
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
.Sx SEARCHING
section below.
For the remainder of this section,
the messages described will be of the ASL_TYPE_MSG variety.
.Pp
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:
.Pp
#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"
.Pp
Many of these correspond to equivalent parts of messages described in the
.Xr 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.
.Pp
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.
.Pp
The
.Xr asl 3
API does not require a process to choose a facility name.
The
.Nm syslogd
server will use a default value of
.Dq user
if a facility is not set.
However, a client may set a facility name as an argument in the
.Nm asl_open
call, or by setting a specific value for the ASL_KEY_FACILITY in a message:
.Pp
asl_set(m, ASL_KEY_FACILITY, "com.somename.greatservice");
.Pp
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
.Dq Reverse ICANN
naming convention to avoid conflicting facility names.
.Pp
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
.Nm asl_set
routine, or implicitly set at the time the message is sent using the
.Nm asl_log
or
.Nm 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.
.Pp
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
.Dq Hello World
program requires only:
.Pp
#include <asl.h>
...
asl_log(NULL, NULL, ASL_LEVEL_INFO, "Hello World!");
.Pp
Both
.Nm asl_log
and
.Nm asl_vlog
will provide the appropriate default values
when passed a NULL aslmsg argument.
.Pp
.Pp
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
.Nm asl_open .
The client handle may then be closed when it is no longer required using
.Nm asl_close .
.Pp
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
.Dq template
message of sorts:
.Pp
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");
.Pp
The message structure will carry the values set for the
.Dq Facility
and
.Dq Clearance
keys so that they are used in each call to
.Nm asl_log ,
while the log level and the message text
are taken from the calling parameters.
.Pp
The
.Ar format
argument to
.Nm asl_log
and
.Nm asl_vlog
is identical to
.Xr printf 3 ,
and may include
.Ql %m ,
which is replaced by the current error message
(as denoted by the global variable
.Va errno ;
see
.Xr strerror 3 . )
.Pp
Key/value pairs may be removed from a message structure with
.Nm asl_unset .
A message may be freed using
.Nm asl_free .
.Pp
The
.Nm asl_send
routine is used by
.Nm asl_log
and
.Nm 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
.Nm asl_set .
.Ss SECURITY
Messages that are sent to the
.Nm syslogd
server may be saved in a message store.
The store may be searched using
.Nm 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.
.Pp
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
.Nm syslog
API, although developers are encouraged to adopt facility names that make sense for their application.
A
.Dq 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
.Nm syslogd
server.
.Ss 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.
.Pp
If multiple threads must do logging,
or if client options are desired,
then the application should call
.Nm asl_open
to create a client handle for each thread.
As a convenience,
the
.Nm 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.
.Pp
Several options are available when creating a client handle.
They are:
.Pp
.Bl -tag -width "ASL_OPT_NO_REMOTE" -compact
.It ASL_OPT_STDERR
adds stderr as an output file descriptor
.It ASL_OPT_NO_DELAY
connects to the server immediately
.It ASL_OPT_NO_REMOTE
disables remote-control filter adjustment
.El
.Pp
See the FILTERING section below, and the
.Xr syslog 1
for additional details on filter controls.
.Pp
A client handle is closed and it's resources released using
.Nm asl_close .
Note that if additional file descriptors were added to the handle,
either using the ASL_OPT_STDERR option
or afterwards with the
.Nm asl_add_log_file
routine, those file descriptors are not closed by
.Nm asl_close .
.Ss LOGGING TO ADDITIONAL FILES
If a client handle is opened with the ASL_OPT_STDERR option to
.Nm asl_open ,
a copy of each log message will be sent to stderr.
Additional output streams may be include using
.Nm asl_add_log_file .
.Pp
Messages sent to stderr or other files are printed in the "standard" message format
also used as a default format by the
.Xr syslog 1
command line utility.
The
.Xr strvis 3
encoding with the VIS_CSTYLE, VIS_TAB, and VIS_NL options is used to print the message.
.Pp
File descriptors may be removed from the list of outputs associated
with a client handle with
.Nm asl_remove_log_file .
This routine simply removes the file descriptor from the output list.
The file is not closed as a result.
.Pp
The ASL_OPT_STDERR option may not be unset
after a client handle has been opened.
.Pp
In the present release of Mac OS X, a
.Dq 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 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.
.Ss SEARCHING
The
.Nm syslogd
server archives received messages in a data store
that may be searched using the
.Nm asl_search ,
.Nm aslresponse_next ,
and
.Nm aslresponse_free
routines.
A query message is created using:
.Pp
aslmsg q = asl_new(ASL_TYPE_QUERY);
.Pp
Search settings are made in the query using
.Nm asl_set_query .
A search is performed on the data store with
.Nm asl_search .
It returns an
.Ft aslresponse
structure.
The caller may then call
.Nm aslresponse_next
to iterate through matching messages.
The
.Ft aslresponse
structure may be freed with
.Nm aslresponse_free .
.Pp
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 following code snippet searches for messages
with a Sender value equal to
.Dq MyApp .
.Pp
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);
.Pp
More complex searches may be performed using other query operations.
.Pp
.Bl -tag -width "ASL_QUERY_OP_GREATER_EQUAL" -compact
.It ASL_QUERY_OP_EQUAL
value equality
.It ASL_QUERY_OP_GREATER
value greater than
.It ASL_QUERY_OP_GREATER_EQUAL
value greater than or equal to
.It ASL_QUERY_OP_LESS
value less than
.It ASL_QUERY_OP_LESS_EQUAL
value less than or equal to
.It ASL_QUERY_OP_NOT_EQUAL
value not equal
.It ASL_QUERY_OP_REGEX
regular expression search
.It ASL_QUERY_OP_TRUE
always true - use to test for the existence of a key
.El
.Pp
Regular expression search uses
.Xr regex 3
library.
Patterns are compiled using the REG_EXTENDED and REG_NOSUB options.
.Pp
Modifiers that change the behavior of these operations
may also be specified by ORing the modifier value with the operation.
The modifiers are:
.Pp
.Bl -tag -width "ASL_QUERY_OP_SUBSTRING" -compact
.It ASL_QUERY_OP_CASEFOLD
string comparisons are case-folded
.It ASL_QUERY_OP_PREFIX
match a leading substring
.It ASL_QUERY_OP_SUFFIX
match a trailing substring
.It ASL_QUERY_OP_SUBSTRING
match any substring
.It ASL_QUERY_OP_NUMERIC
values are converted to integer using
.Nm atoi
.El
.Pp
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.
.Pp
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
.Dq MyApp
with a priority level less than or equal to
.Dq 3 :
.Pp
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);
.Pp
After calling
.Nm asl_search
to get an
.Ft aslresponse
structure, use
.Nm aslresponse_next
to iterate through all matching messages.
To iterate through the keys and values in a message, use
.Nm asl_key
to iterate through the keys, then call
.Nm asl_get
to get the value associated with each key.
.Pp
aslmsg q, m;
int i;
const char *key, *val;
.Pp
...
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);
.Pp
.Ss FILTERING AND REMOTE CONTROL
Clients may set a filter mask value with
.Nm asl_set_filter .
The mask specifies which messages should be sent to the
.Nm 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 unnecessary.
.Pp
.Nm asl_set_filter returns the previous value of the filter, i.e. the value of the filter before the routine was called.
.Pp
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.
.Pp
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
.Mn syslogd
server.
.Pp
Three different filters exist for each application.
The first is the filter mask set using
.Nm asl_set_filter
as described above.
The Apple System Log facility also manages a
.Dq master
filter mask.
The master filter mask usually has a value
that indicates to the library that it is
.Dq off ,
and thus it has no effect.
However, the mask filter mask may be enabled
by giving it a value using the
.Nm syslog
command, using the
.Fl 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.
.Pp
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
.Dq off ,
having no effect on a client.
If a per-client filter mask is set using the
.Nm syslog
command, using the
.Fl c Ar 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.
.Pp
The ASL_OPT_NO_REMOTE option to
.Nm 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.
.Sh HISTORY
These functions first appeared in
Mac OS X 10.4.
.Sh SEE ALSO
.Xr syslog 1 ,
.Xr strvis 3 ,
.Xr syslogd 8