asl(3)

NAME
asl_open, asl_close, asl_new, asl_free, asl_set, asl_set_query, asl_get, asl_unset, asl_log, asl_vlog, asl_send, asl_key, asl_add_log_file, asl_remove_log_file, asl_set_cutoff_level, asl_search, aslresponse_next, aslresponse_free - system log message sending and searching functions

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);
aslmsg
aslresponse_next(aslresponse r);
void
aslresponse_free(aslresponse a);

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 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 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.

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:

#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, "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 message 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

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. 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 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 following 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 specified 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 specifies 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 unnecessary.

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 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)