@node Syslog, Mathematics, Low-Level Terminal Interface, Top

@c %MENU% System logging and messaging

@chapter Syslog

This chapter describes facilities for issuing and logging messages of

system administration interest.  This chapter has nothing to do with

programs issuing messages to their own users or keeping private logs

(One would typically do that with the facilities described in

@ref{I/O on Streams}).

Most systems have a facility called ``Syslog'' that allows programs to

submit messages of interest to system administrators and can be

configured to pass these messages on in various ways, such as printing

on the console, mailing to a particular person, or recording in a log

file for future reference.

A program uses the facilities in this chapter to submit such messages.

@menu

* Overview of Syslog::           Overview of a system's Syslog facility

* Submitting Syslog Messages::   Functions to submit messages to Syslog

@end menu

@node Overview of Syslog

@section Overview of Syslog

System administrators have to deal with lots of different kinds of

messages from a plethora of subsystems within each system, and usually

lots of systems as well.  For example, an FTP server might report every

connection it gets.  The kernel might report hardware failures on a disk

drive.  A DNS server might report usage statistics at regular intervals.

Some of these messages need to be brought to a system administrator's

attention immediately.  And it may not be just any system administrator

-- there may be a particular system administrator who deals with a

particular kind of message.  Other messages just need to be recorded for

future reference if there is a problem.  Still others may need to have

information extracted from them by an automated process that generates

monthly reports.

To deal with these messages, most Unix systems have a facility called

"Syslog."  It is generally based on a daemon called ``Syslogd''

Syslogd listens for messages on a Unix domain socket named

@file{/dev/log}.  Based on classification information in the messages

and its configuration file (usually @file{/etc/syslog.conf}), Syslogd

routes them in various ways.  Some of the popular routings are:

@itemize @bullet

@item

Write to the system console

@item

Mail to a specific user

@item

Write to a log file

@item

Pass to another daemon

@item

Discard

@end itemize

Syslogd can also handle messages from other systems.  It listens on the

@code{syslog} UDP port as well as the local socket for messages.

Syslog can handle messages from the kernel itself.  But the kernel

doesn't write to @file{/dev/log}; rather, another daemon (sometimes

called ``Klogd'') extracts messages from the kernel and passes them on to

Syslog as any other process would (and it properly identifies them as

messages from the kernel).

Syslog can even handle messages that the kernel issued before Syslogd or

Klogd was running.  A Linux kernel, for example, stores startup messages

in a kernel message ring and they are normally still there when Klogd

later starts up.  Assuming Syslogd is running by the time Klogd starts,

Klogd then passes everything in the message ring to it.

In order to classify messages for disposition, Syslog requires any process

that submits a message to it to provide two pieces of classification

information with it:

@table @asis

@item facility

This identifies who submitted the message.  There are a small number of

facilities defined.  The kernel, the mail subsystem, and an FTP server

are examples of recognized facilities.  For the complete list,

@xref{syslog; vsyslog}.  Keep in mind that these are

essentially arbitrary classifications.  "Mail subsystem" doesn't have any

more meaning than the system administrator gives to it.

@item priority

This tells how important the content of the message is.  Examples of

defined priority values are: debug, informational, warning and critical.

For the complete list, see @ref{syslog; vsyslog}.  Except for

the fact that the priorities have a defined order, the meaning of each

of these priorities is entirely determined by the system administrator.

@end table

A ``facility/priority'' is a number that indicates both the facility

and the priority.

@strong{Warning:} This terminology is not universal.  Some people use

``level'' to refer to the priority and ``priority'' to refer to the

combination of facility and priority.  A Linux kernel has a concept of a

message ``level,'' which corresponds both to a Syslog priority and to a

Syslog facility/priority (It can be both because the facility code for

the kernel is zero, and that makes priority and facility/priority the

same value).

@Theglibc{} provides functions to submit messages to Syslog.  They

do it by writing to the @file{/dev/log} socket.  @xref{Submitting Syslog

Messages}.

The @glibcadj{} functions only work to submit messages to the Syslog

facility on the same system.  To submit a message to the Syslog facility

on another system, use the socket I/O functions to write a UDP datagram

to the @code{syslog} UDP port on that system.  @xref{Sockets}.

@node Submitting Syslog Messages

@section Submitting Syslog Messages

@Theglibc{} provides functions to submit messages to the Syslog

facility:

@menu

* openlog::                      Open connection to Syslog

* syslog; vsyslog::              Submit message to Syslog

* closelog::                     Close connection to Syslog

* setlogmask::                   Cause certain messages to be ignored

* Syslog Example::               Example of all of the above

@end menu

These functions only work to submit messages to the Syslog facility on

the same system.  To submit a message to the Syslog facility on another

system, use the socket I/O functions to write a UDP datagram to the

@code{syslog} UDP port on that system.  @xref{Sockets}.

@node openlog

@subsection openlog

The symbols referred to in this section are declared in the file

@file{syslog.h}.

@deftypefun void openlog (const char *@var{ident}, int @var{option}, int @var{facility})

@standards{BSD, syslog.h}

@safety{@prelim{}@mtsafe{}@asunsafe{@asulock{}}@acunsafe{@aculock{} @acsfd{}}}

@c openlog @asulock @aculock @acsfd

@c  libc_lock_lock @asulock @aculock

@c  openlog_internal @acsfd [always guarded by syslog_lock, so no race]

@c   strncpy dup ok

@c   socket dup @acsfd

@c   fcntl dup ok

@c   connect dup ok

@c   close dup @acsfd

@c  cancel_handler(NULL) @aculock

@c   libc_lock_unlock @aculock

@code{openlog} opens or reopens a connection to Syslog in preparation

for submitting messages.

@var{ident} is an arbitrary identification string which future

@code{syslog} invocations will prefix to each message.  This is intended

to identify the source of the message, and people conventionally set it

to the name of the program that will submit the messages.

If @var{ident} is NULL, or if @code{openlog} is not called, the default

identification string used in Syslog messages will be the program name,

taken from argv[0].

Please note that the string pointer @var{ident} will be retained

internally by the Syslog routines.  You must not free the memory that

@var{ident} points to.  It is also dangerous to pass a reference to an

automatic variable since leaving the scope would mean ending the

lifetime of the variable.  If you want to change the @var{ident} string,

you must call @code{openlog} again; overwriting the string pointed to by

@var{ident} is not thread-safe.

You can cause the Syslog routines to drop the reference to @var{ident} and

go back to the default string (the program name taken from argv[0]), by

calling @code{closelog}: @xref{closelog}.

In particular, if you are writing code for a shared library that might get

loaded and then unloaded (e.g. a PAM module), and you use @code{openlog},

you must call @code{closelog} before any point where your library might

get unloaded, as in this example:

@smallexample

#include <syslog.h>

void

shared_library_function (void)

@{

  openlog ("mylibrary", option, priority);

  syslog (LOG_INFO, "shared library has been invoked");

  closelog ();

@}

@end smallexample

Without the call to @code{closelog}, future invocations of @code{syslog}

by the program using the shared library may crash, if the library gets

unloaded and the memory containing the string @code{"mylibrary"} becomes

unmapped.  This is a limitation of the BSD syslog interface.

@code{openlog} may or may not open the @file{/dev/log} socket, depending

on @var{option}.  If it does, it tries to open it and connect it as a

stream socket.  If that doesn't work, it tries to open it and connect it

as a datagram socket.  The socket has the ``Close on Exec'' attribute,

so the kernel will close it if the process performs an exec.

You don't have to use @code{openlog}.  If you call @code{syslog} without

having called @code{openlog}, @code{syslog} just opens the connection

implicitly and uses defaults for the information in @var{ident} and

@var{options}.

@var{options} is a bit string, with the bits as defined by the following

single bit masks:

@vtable @code

@item LOG_PERROR

If on, @code{openlog} sets up the connection so that any @code{syslog}

on this connection writes its message to the calling process' Standard

Error stream in addition to submitting it to Syslog.  If off, @code{syslog}

does not write the message to Standard Error.

@item LOG_CONS

If on, @code{openlog} sets up the connection so that a @code{syslog} on

this connection that fails to submit a message to Syslog writes the

message instead to system console.  If off, @code{syslog} does not write

to the system console (but of course Syslog may write messages it

receives to the console).

@item LOG_PID

When on, @code{openlog} sets up the connection so that a @code{syslog}

on this connection inserts the calling process' Process ID (PID) into

the message.  When off, @code{openlog} does not insert the PID.

@item LOG_NDELAY

When on, @code{openlog} opens and connects the @file{/dev/log} socket.

When off, a future @code{syslog} call must open and connect the socket.

@strong{Portability note:}  In early systems, the sense of this bit was

exactly the opposite.

@item LOG_ODELAY

This bit does nothing.  It exists for backward compatibility.

@end vtable

If any other bit in @var{options} is on, the result is undefined.

@var{facility} is the default facility code for this connection.  A

@code{syslog} on this connection that specifies default facility causes

this facility to be associated with the message.  See @code{syslog} for

possible values.  A value of zero means the default, which is

@code{LOG_USER}.

If a Syslog connection is already open when you call @code{openlog},

@code{openlog} ``reopens'' the connection.  Reopening is like opening

except that if you specify zero for the default facility code, the

default facility code simply remains unchanged and if you specify

LOG_NDELAY and the socket is already open and connected, @code{openlog}

just leaves it that way.

@c There is a bug in closelog() (glibc 2.1.3) wherein it does not reset the

@c default log facility to LOG_USER, which means the default default log

@c facility could be whatever the default log facility was for a previous

@c Syslog connection.  I have documented what the function should be rather

@c than what it is because I think if anyone ever gets concerned, the code

@c will change.

@end deftypefun

@node syslog; vsyslog

@subsection syslog, vsyslog

The symbols referred to in this section are declared in the file

@file{syslog.h}.

@c syslog() is implemented as a call to vsyslog().

@deftypefun void syslog (int @var{facility_priority}, const char *@var{format}, @dots{})

@standards{BSD, syslog.h}

@safety{@prelim{}@mtsafe{@mtsenv{} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}

@c syslog @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd

@c  va_start dup ok

@c  vsyslog_chk @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd

@c   syslog(INTERNALLOG) dup @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd

@c   open_memstream @ascuheap @acsmem

@c   stpcpy dup ok

@c   getpid dup ok

@c   mempcpy dup ok

@c   fsetlocking [no @mtasurace:stream @asulock for exclusive stream]

@c   fprintf @mtslocale @ascuheap @acsmem [no @asucorrupt @aculock @acucorrupt on temp memstream]

@c   time dup ok

@c   localtime_r dup @mtsenv @mtslocale @ascuheap @asulock @aculock @acsmem @acsfd

@c   strftime_l(C) dup @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd

@c   ftell dup ok [no @asucorrupt @aculock @acucorrupt on temp memstream]

@c   fputs_unlocked dup ok [no @mtasurace:stream @asucorrupt @acucorrupt on temp memstream]

@c   putc_unlocked dup ok [no @mtasurace:stream @asucorrupt @acucorrupt on temp memstream]

@c   vfprintf/vfprintf_chk dup @mtslocale @ascuheap @acsmem [no @mtasurace:stream @asucorrupt @acucorrupt on temp memstream]

@c   fclose dup @ascuheap @acsmem [no @asulock @aculock @acsfd on caller-locked memstream]

@c   writev dup ok

@c   libc_lock_lock dup @asulock @aculock

@c   memset dup ok

@c   sigemptyset dup ok

@c   sigaction(SIGPIPE) dup @mtasusig:PIPE @acusig:PIPE

@c   openlog_internal dup @acsfd

@c   send dup ok

@c   closelog_internal dup @acsfd

@c   open dup @acsfd

@c   dprintf dup ok

@c   libc_lock_unlock @asulock @aculock

@c   free dup @acsuheap @acsmem

@c  va_end dup ok

@code{syslog} submits a message to the Syslog facility.  It does this by

writing to the Unix domain socket @code{/dev/log}.

@code{syslog} submits the message with the facility and priority indicated

by @var{facility_priority}.  The macro @code{LOG_MAKEPRI} generates a

facility/priority from a facility and a priority, as in the following

example:

@smallexample

LOG_MAKEPRI(LOG_USER, LOG_WARNING)

@end smallexample

The possible values for the facility code are (macros):

@c Internally, there is also LOG_KERN, but LOG_KERN == 0, which means

@c if you try to use it here, just selects default.

@vtable @code

@item LOG_USER

A miscellaneous user process

@item LOG_MAIL

Mail

@item LOG_DAEMON

A miscellaneous system daemon

@item LOG_AUTH

Security (authorization)

@item LOG_SYSLOG

Syslog

@item LOG_LPR

Central printer

@item LOG_NEWS

Network news (e.g. Usenet)

@item LOG_UUCP

UUCP

@item LOG_CRON

Cron and At

@item LOG_AUTHPRIV

Private security (authorization)

@item LOG_FTP

Ftp server

@item LOG_LOCAL0

Locally defined

@item LOG_LOCAL1

Locally defined

@item LOG_LOCAL2

Locally defined

@item LOG_LOCAL3

Locally defined

@item LOG_LOCAL4

Locally defined

@item LOG_LOCAL5

Locally defined

@item LOG_LOCAL6

Locally defined

@item LOG_LOCAL7

Locally defined

@end vtable

Results are undefined if the facility code is anything else.

@strong{NB:} @code{syslog} recognizes one other facility code: that of

the kernel.  But you can't specify that facility code with these

functions.  If you try, it looks the same to @code{syslog} as if you are

requesting the default facility.  But you wouldn't want to anyway,

because any program that uses @theglibc{} is not the kernel.

You can use just a priority code as @var{facility_priority}.  In that

case, @code{syslog} assumes the default facility established when the

Syslog connection was opened.  @xref{Syslog Example}.

The possible values for the priority code are (macros):

@vtable @code

@item LOG_EMERG

The message says the system is unusable.

@item LOG_ALERT

Action on the message must be taken immediately.

@item LOG_CRIT

The message states a critical condition.

@item LOG_ERR

The message describes an error.

@item LOG_WARNING

The message is a warning.

@item LOG_NOTICE

The message describes a normal but important event.

@item LOG_INFO

The message is purely informational.

@item LOG_DEBUG

The message is only for debugging purposes.

@end vtable

Results are undefined if the priority code is anything else.

If the process does not presently have a Syslog connection open (i.e.,

it did not call @code{openlog}), @code{syslog} implicitly opens the

connection the same as @code{openlog} would, with the following defaults

for information that would otherwise be included in an @code{openlog}

call: The default identification string is the program name.  The

default default facility is @code{LOG_USER}.  The default for all the

connection options in @var{options} is as if those bits were off.

@code{syslog} leaves the Syslog connection open.

If the @file{/dev/log} socket is not open and connected, @code{syslog}

opens and connects it, the same as @code{openlog} with the

@code{LOG_NDELAY} option would.

@code{syslog} leaves @file{/dev/log} open and connected unless its attempt

to send the message failed, in which case @code{syslog} closes it (with the

hope that a future implicit open will restore the Syslog connection to a

usable state).

Example:

@smallexample

#include <syslog.h>

syslog (LOG_MAKEPRI(LOG_LOCAL1, LOG_ERROR),

        "Unable to make network connection to %s.  Error=%m", host);

@end smallexample

@end deftypefun

@deftypefun void vsyslog (int @var{facility_priority}, const char *@var{format}, va_list @var{arglist})

@standards{BSD, syslog.h}

@safety{@prelim{}@mtsafe{@mtsenv{} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}

@c vsyslog @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd

@c  vsyslog_chk dup @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd

This is functionally identical to @code{syslog}, with the BSD style variable

length argument.

@end deftypefun

@node closelog

@subsection closelog

The symbols referred to in this section are declared in the file

@file{syslog.h}.

@deftypefun void closelog (void)

@standards{BSD, syslog.h}

@safety{@prelim{}@mtsafe{}@asunsafe{@asulock{}}@acunsafe{@aculock{} @acsfd{}}}

@c closelog @asulock @aculock @acsfd

@c  libc_lock_lock @asulock @aculock

@c  closelog_internal @acsfd [always guarded by syslog_lock, so no race]

@c   close dup@acsfd

@c  cancel_handler(NULL) @aculock

@c   libc_lock_unlock @aculock

@code{closelog} closes the current Syslog connection, if there is one.

This includes closing the @file{/dev/log} socket, if it is open.

@code{closelog} also sets the identification string for Syslog messages

back to the default, if @code{openlog} was called with a non-NULL argument

to @var{ident}.  The default identification string is the program name

taken from argv[0].

If you are writing shared library code that uses @code{openlog} to

generate custom syslog output, you should use @code{closelog} to drop

@theglibc{}'s internal reference to the @var{ident} pointer when you are

done.  Please read the section on @code{openlog} for more information:

@xref{openlog}.

@code{closelog} does not flush any buffers.  You do not have to call

@code{closelog} before re-opening a Syslog connection with @code{openlog}.

Syslog connections are automatically closed on exec or exit.

@end deftypefun

@node setlogmask

@subsection setlogmask

The symbols referred to in this section are declared in the file

@file{syslog.h}.

@deftypefun int setlogmask (int @var{mask})

@standards{BSD, syslog.h}

@safety{@prelim{}@mtunsafe{@mtasurace{:LogMask}}@asunsafe{}@acsafe{}}

@c Read and modify are not guarded by syslog_lock, so concurrent changes

@c or even uses are undefined.  This should use an atomic swap instead,

@c at least for modifications.

@code{setlogmask} sets a mask (the ``logmask'') that determines which

future @code{syslog} calls shall be ignored.  If a program has not

called @code{setlogmask}, @code{syslog} doesn't ignore any calls.  You

can use @code{setlogmask} to specify that messages of particular

priorities shall be ignored in the future.

A @code{setlogmask} call overrides any previous @code{setlogmask} call.

Note that the logmask exists entirely independently of opening and

closing of Syslog connections.

Setting the logmask has a similar effect to, but is not the same as,

configuring Syslog.  The Syslog configuration may cause Syslog to

discard certain messages it receives, but the logmask causes certain

messages never to get submitted to Syslog in the first place.

@var{mask} is a bit string with one bit corresponding to each of the

possible message priorities.  If the bit is on, @code{syslog} handles

messages of that priority normally.  If it is off, @code{syslog}

discards messages of that priority.  Use the message priority macros

described in @ref{syslog; vsyslog} and the @code{LOG_MASK} to construct

an appropriate @var{mask} value, as in this example:

@smallexample

LOG_MASK(LOG_EMERG) | LOG_MASK(LOG_ERROR)

@end smallexample

or

@smallexample

~(LOG_MASK(LOG_INFO))

@end smallexample

There is also a @code{LOG_UPTO} macro, which generates a mask with the bits

on for a certain priority and all priorities above it:

@smallexample

LOG_UPTO(LOG_ERROR)

@end smallexample

The unfortunate naming of the macro is due to the fact that internally,

higher numbers are used for lower message priorities.

@end deftypefun

@node Syslog Example

@subsection Syslog Example

Here is an example of @code{openlog}, @code{syslog}, and @code{closelog}:

This example sets the logmask so that debug and informational messages

get discarded without ever reaching Syslog.  So the second @code{syslog}

in the example does nothing.

@smallexample

#include <syslog.h>

setlogmask (LOG_UPTO (LOG_NOTICE));

openlog ("exampleprog", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);

syslog (LOG_NOTICE, "Program started by User %d", getuid ());

syslog (LOG_INFO, "A tree falls in a forest");

closelog ();

@end smallexample