574 lines
21 KiB
Plaintext
574 lines
21 KiB
Plaintext
@node Syslog, Mathematics, Low-Level Terminal Interface, Top
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@c %MENU% System logging and messaging
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@chapter Syslog
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This chapter describes facilities for issuing and logging messages of
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system administration interest. This chapter has nothing to do with
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programs issuing messages to their own users or keeping private logs
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(One would typically do that with the facilities described in
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@ref{I/O on Streams}).
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Most systems have a facility called ``Syslog'' that allows programs to
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submit messages of interest to system administrators and can be
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configured to pass these messages on in various ways, such as printing
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on the console, mailing to a particular person, or recording in a log
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file for future reference.
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A program uses the facilities in this chapter to submit such messages.
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@menu
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* Overview of Syslog:: Overview of a system's Syslog facility
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* Submitting Syslog Messages:: Functions to submit messages to Syslog
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@end menu
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@node Overview of Syslog
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@section Overview of Syslog
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System administrators have to deal with lots of different kinds of
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messages from a plethora of subsystems within each system, and usually
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lots of systems as well. For example, an FTP server might report every
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connection it gets. The kernel might report hardware failures on a disk
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drive. A DNS server might report usage statistics at regular intervals.
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Some of these messages need to be brought to a system administrator's
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attention immediately. And it may not be just any system administrator
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-- there may be a particular system administrator who deals with a
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particular kind of message. Other messages just need to be recorded for
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future reference if there is a problem. Still others may need to have
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information extracted from them by an automated process that generates
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monthly reports.
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To deal with these messages, most Unix systems have a facility called
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"Syslog." It is generally based on a daemon called ``Syslogd''
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Syslogd listens for messages on a Unix domain socket named
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@file{/dev/log}. Based on classification information in the messages
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and its configuration file (usually @file{/etc/syslog.conf}), Syslogd
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routes them in various ways. Some of the popular routings are:
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@itemize @bullet
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@item
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Write to the system console
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@item
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Mail to a specific user
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@item
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Write to a log file
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@item
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Pass to another daemon
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@item
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Discard
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@end itemize
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Syslogd can also handle messages from other systems. It listens on the
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@code{syslog} UDP port as well as the local socket for messages.
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Syslog can handle messages from the kernel itself. But the kernel
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doesn't write to @file{/dev/log}; rather, another daemon (sometimes
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called ``Klogd'') extracts messages from the kernel and passes them on to
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Syslog as any other process would (and it properly identifies them as
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messages from the kernel).
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Syslog can even handle messages that the kernel issued before Syslogd or
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Klogd was running. A Linux kernel, for example, stores startup messages
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in a kernel message ring and they are normally still there when Klogd
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later starts up. Assuming Syslogd is running by the time Klogd starts,
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Klogd then passes everything in the message ring to it.
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In order to classify messages for disposition, Syslog requires any process
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that submits a message to it to provide two pieces of classification
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information with it:
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@table @asis
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@item facility
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This identifies who submitted the message. There are a small number of
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facilities defined. The kernel, the mail subsystem, and an FTP server
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are examples of recognized facilities. For the complete list,
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@xref{syslog; vsyslog}. Keep in mind that these are
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essentially arbitrary classifications. "Mail subsystem" doesn't have any
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more meaning than the system administrator gives to it.
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@item priority
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This tells how important the content of the message is. Examples of
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defined priority values are: debug, informational, warning and critical.
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For the complete list, see @ref{syslog; vsyslog}. Except for
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the fact that the priorities have a defined order, the meaning of each
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of these priorities is entirely determined by the system administrator.
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@end table
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A ``facility/priority'' is a number that indicates both the facility
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and the priority.
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@strong{Warning:} This terminology is not universal. Some people use
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``level'' to refer to the priority and ``priority'' to refer to the
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combination of facility and priority. A Linux kernel has a concept of a
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message ``level,'' which corresponds both to a Syslog priority and to a
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Syslog facility/priority (It can be both because the facility code for
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the kernel is zero, and that makes priority and facility/priority the
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same value).
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@Theglibc{} provides functions to submit messages to Syslog. They
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do it by writing to the @file{/dev/log} socket. @xref{Submitting Syslog
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Messages}.
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The @glibcadj{} functions only work to submit messages to the Syslog
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facility on the same system. To submit a message to the Syslog facility
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on another system, use the socket I/O functions to write a UDP datagram
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to the @code{syslog} UDP port on that system. @xref{Sockets}.
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@node Submitting Syslog Messages
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@section Submitting Syslog Messages
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@Theglibc{} provides functions to submit messages to the Syslog
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facility:
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@menu
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* openlog:: Open connection to Syslog
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* syslog; vsyslog:: Submit message to Syslog
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* closelog:: Close connection to Syslog
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* setlogmask:: Cause certain messages to be ignored
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* Syslog Example:: Example of all of the above
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@end menu
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These functions only work to submit messages to the Syslog facility on
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the same system. To submit a message to the Syslog facility on another
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system, use the socket I/O functions to write a UDP datagram to the
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@code{syslog} UDP port on that system. @xref{Sockets}.
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@node openlog
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@subsection openlog
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The symbols referred to in this section are declared in the file
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@file{syslog.h}.
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@deftypefun void openlog (const char *@var{ident}, int @var{option}, int @var{facility})
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@standards{BSD, syslog.h}
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@safety{@prelim{}@mtsafe{}@asunsafe{@asulock{}}@acunsafe{@aculock{} @acsfd{}}}
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@c openlog @asulock @aculock @acsfd
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@c libc_lock_lock @asulock @aculock
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@c openlog_internal @acsfd [always guarded by syslog_lock, so no race]
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@c strncpy dup ok
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@c socket dup @acsfd
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@c fcntl dup ok
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@c connect dup ok
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@c close dup @acsfd
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@c cancel_handler(NULL) @aculock
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@c libc_lock_unlock @aculock
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@code{openlog} opens or reopens a connection to Syslog in preparation
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for submitting messages.
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@var{ident} is an arbitrary identification string which future
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@code{syslog} invocations will prefix to each message. This is intended
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to identify the source of the message, and people conventionally set it
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to the name of the program that will submit the messages.
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If @var{ident} is NULL, or if @code{openlog} is not called, the default
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identification string used in Syslog messages will be the program name,
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taken from argv[0].
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Please note that the string pointer @var{ident} will be retained
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internally by the Syslog routines. You must not free the memory that
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@var{ident} points to. It is also dangerous to pass a reference to an
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automatic variable since leaving the scope would mean ending the
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lifetime of the variable. If you want to change the @var{ident} string,
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you must call @code{openlog} again; overwriting the string pointed to by
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@var{ident} is not thread-safe.
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You can cause the Syslog routines to drop the reference to @var{ident} and
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go back to the default string (the program name taken from argv[0]), by
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calling @code{closelog}: @xref{closelog}.
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In particular, if you are writing code for a shared library that might get
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loaded and then unloaded (e.g. a PAM module), and you use @code{openlog},
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you must call @code{closelog} before any point where your library might
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get unloaded, as in this example:
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@smallexample
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#include <syslog.h>
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void
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shared_library_function (void)
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@{
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openlog ("mylibrary", option, priority);
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syslog (LOG_INFO, "shared library has been invoked");
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closelog ();
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@}
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@end smallexample
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Without the call to @code{closelog}, future invocations of @code{syslog}
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by the program using the shared library may crash, if the library gets
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unloaded and the memory containing the string @code{"mylibrary"} becomes
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unmapped. This is a limitation of the BSD syslog interface.
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@code{openlog} may or may not open the @file{/dev/log} socket, depending
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on @var{option}. If it does, it tries to open it and connect it as a
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stream socket. If that doesn't work, it tries to open it and connect it
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as a datagram socket. The socket has the ``Close on Exec'' attribute,
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so the kernel will close it if the process performs an exec.
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You don't have to use @code{openlog}. If you call @code{syslog} without
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having called @code{openlog}, @code{syslog} just opens the connection
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implicitly and uses defaults for the information in @var{ident} and
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@var{options}.
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@var{options} is a bit string, with the bits as defined by the following
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single bit masks:
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@vtable @code
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@item LOG_PERROR
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If on, @code{openlog} sets up the connection so that any @code{syslog}
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on this connection writes its message to the calling process' Standard
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Error stream in addition to submitting it to Syslog. If off, @code{syslog}
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does not write the message to Standard Error.
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@item LOG_CONS
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If on, @code{openlog} sets up the connection so that a @code{syslog} on
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this connection that fails to submit a message to Syslog writes the
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message instead to system console. If off, @code{syslog} does not write
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to the system console (but of course Syslog may write messages it
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receives to the console).
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@item LOG_PID
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When on, @code{openlog} sets up the connection so that a @code{syslog}
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on this connection inserts the calling process' Process ID (PID) into
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the message. When off, @code{openlog} does not insert the PID.
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@item LOG_NDELAY
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When on, @code{openlog} opens and connects the @file{/dev/log} socket.
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When off, a future @code{syslog} call must open and connect the socket.
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@strong{Portability note:} In early systems, the sense of this bit was
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exactly the opposite.
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@item LOG_ODELAY
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This bit does nothing. It exists for backward compatibility.
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@end vtable
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If any other bit in @var{options} is on, the result is undefined.
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@var{facility} is the default facility code for this connection. A
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@code{syslog} on this connection that specifies default facility causes
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this facility to be associated with the message. See @code{syslog} for
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possible values. A value of zero means the default, which is
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@code{LOG_USER}.
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If a Syslog connection is already open when you call @code{openlog},
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@code{openlog} ``reopens'' the connection. Reopening is like opening
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except that if you specify zero for the default facility code, the
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default facility code simply remains unchanged and if you specify
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LOG_NDELAY and the socket is already open and connected, @code{openlog}
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just leaves it that way.
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@c There is a bug in closelog() (glibc 2.1.3) wherein it does not reset the
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@c default log facility to LOG_USER, which means the default default log
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@c facility could be whatever the default log facility was for a previous
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@c Syslog connection. I have documented what the function should be rather
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@c than what it is because I think if anyone ever gets concerned, the code
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@c will change.
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@end deftypefun
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@node syslog; vsyslog
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@subsection syslog, vsyslog
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The symbols referred to in this section are declared in the file
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@file{syslog.h}.
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@c syslog() is implemented as a call to vsyslog().
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@deftypefun void syslog (int @var{facility_priority}, const char *@var{format}, @dots{})
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@standards{BSD, syslog.h}
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@safety{@prelim{}@mtsafe{@mtsenv{} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
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@c syslog @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd
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@c va_start dup ok
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@c vsyslog_chk @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd
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@c syslog(INTERNALLOG) dup @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd
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@c open_memstream @ascuheap @acsmem
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@c stpcpy dup ok
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@c getpid dup ok
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@c mempcpy dup ok
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@c fsetlocking [no @mtasurace:stream @asulock for exclusive stream]
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@c fprintf @mtslocale @ascuheap @acsmem [no @asucorrupt @aculock @acucorrupt on temp memstream]
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@c time dup ok
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@c localtime_r dup @mtsenv @mtslocale @ascuheap @asulock @aculock @acsmem @acsfd
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@c strftime_l(C) dup @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd
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@c ftell dup ok [no @asucorrupt @aculock @acucorrupt on temp memstream]
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@c fputs_unlocked dup ok [no @mtasurace:stream @asucorrupt @acucorrupt on temp memstream]
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@c putc_unlocked dup ok [no @mtasurace:stream @asucorrupt @acucorrupt on temp memstream]
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@c vfprintf/vfprintf_chk dup @mtslocale @ascuheap @acsmem [no @mtasurace:stream @asucorrupt @acucorrupt on temp memstream]
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@c fclose dup @ascuheap @acsmem [no @asulock @aculock @acsfd on caller-locked memstream]
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@c writev dup ok
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@c libc_lock_lock dup @asulock @aculock
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@c memset dup ok
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@c sigemptyset dup ok
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@c sigaction(SIGPIPE) dup @mtasusig:PIPE @acusig:PIPE
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@c openlog_internal dup @acsfd
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@c send dup ok
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@c closelog_internal dup @acsfd
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@c open dup @acsfd
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@c dprintf dup ok
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@c libc_lock_unlock @asulock @aculock
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@c free dup @acsuheap @acsmem
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@c va_end dup ok
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@code{syslog} submits a message to the Syslog facility. It does this by
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writing to the Unix domain socket @code{/dev/log}.
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@code{syslog} submits the message with the facility and priority indicated
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by @var{facility_priority}. The macro @code{LOG_MAKEPRI} generates a
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facility/priority from a facility and a priority, as in the following
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example:
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@smallexample
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LOG_MAKEPRI(LOG_USER, LOG_WARNING)
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@end smallexample
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The possible values for the facility code are (macros):
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@c Internally, there is also LOG_KERN, but LOG_KERN == 0, which means
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@c if you try to use it here, just selects default.
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@vtable @code
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@item LOG_USER
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A miscellaneous user process
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@item LOG_MAIL
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Mail
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@item LOG_DAEMON
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A miscellaneous system daemon
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@item LOG_AUTH
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Security (authorization)
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@item LOG_SYSLOG
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Syslog
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@item LOG_LPR
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Central printer
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@item LOG_NEWS
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Network news (e.g. Usenet)
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@item LOG_UUCP
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UUCP
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@item LOG_CRON
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Cron and At
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@item LOG_AUTHPRIV
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Private security (authorization)
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@item LOG_FTP
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Ftp server
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@item LOG_LOCAL0
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Locally defined
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@item LOG_LOCAL1
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Locally defined
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@item LOG_LOCAL2
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Locally defined
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@item LOG_LOCAL3
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Locally defined
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@item LOG_LOCAL4
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Locally defined
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@item LOG_LOCAL5
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Locally defined
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@item LOG_LOCAL6
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Locally defined
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@item LOG_LOCAL7
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Locally defined
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@end vtable
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Results are undefined if the facility code is anything else.
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@strong{NB:} @code{syslog} recognizes one other facility code: that of
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the kernel. But you can't specify that facility code with these
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functions. If you try, it looks the same to @code{syslog} as if you are
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requesting the default facility. But you wouldn't want to anyway,
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because any program that uses @theglibc{} is not the kernel.
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You can use just a priority code as @var{facility_priority}. In that
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case, @code{syslog} assumes the default facility established when the
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Syslog connection was opened. @xref{Syslog Example}.
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The possible values for the priority code are (macros):
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@vtable @code
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@item LOG_EMERG
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The message says the system is unusable.
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@item LOG_ALERT
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Action on the message must be taken immediately.
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@item LOG_CRIT
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The message states a critical condition.
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@item LOG_ERR
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The message describes an error.
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@item LOG_WARNING
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The message is a warning.
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@item LOG_NOTICE
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The message describes a normal but important event.
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@item LOG_INFO
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The message is purely informational.
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@item LOG_DEBUG
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The message is only for debugging purposes.
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@end vtable
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Results are undefined if the priority code is anything else.
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If the process does not presently have a Syslog connection open (i.e.,
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it did not call @code{openlog}), @code{syslog} implicitly opens the
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connection the same as @code{openlog} would, with the following defaults
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for information that would otherwise be included in an @code{openlog}
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call: The default identification string is the program name. The
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default default facility is @code{LOG_USER}. The default for all the
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connection options in @var{options} is as if those bits were off.
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@code{syslog} leaves the Syslog connection open.
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If the @file{/dev/log} socket is not open and connected, @code{syslog}
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opens and connects it, the same as @code{openlog} with the
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@code{LOG_NDELAY} option would.
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@code{syslog} leaves @file{/dev/log} open and connected unless its attempt
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to send the message failed, in which case @code{syslog} closes it (with the
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hope that a future implicit open will restore the Syslog connection to a
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usable state).
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Example:
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@smallexample
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#include <syslog.h>
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syslog (LOG_MAKEPRI(LOG_LOCAL1, LOG_ERROR),
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"Unable to make network connection to %s. Error=%m", host);
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@end smallexample
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@end deftypefun
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@deftypefun void vsyslog (int @var{facility_priority}, const char *@var{format}, va_list @var{arglist})
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@standards{BSD, syslog.h}
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@safety{@prelim{}@mtsafe{@mtsenv{} @mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
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@c vsyslog @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd
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@c vsyslog_chk dup @mtsenv @mtslocale @asucorrupt @ascuheap @asulock @ascudlopen @acucorrupt @aculock @acsmem @acsfd
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This is functionally identical to @code{syslog}, with the BSD style variable
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length argument.
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@end deftypefun
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@node closelog
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@subsection closelog
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The symbols referred to in this section are declared in the file
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@file{syslog.h}.
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@deftypefun void closelog (void)
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@standards{BSD, syslog.h}
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@safety{@prelim{}@mtsafe{}@asunsafe{@asulock{}}@acunsafe{@aculock{} @acsfd{}}}
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@c closelog @asulock @aculock @acsfd
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@c libc_lock_lock @asulock @aculock
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@c closelog_internal @acsfd [always guarded by syslog_lock, so no race]
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@c close dup@acsfd
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@c cancel_handler(NULL) @aculock
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@c libc_lock_unlock @aculock
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@code{closelog} closes the current Syslog connection, if there is one.
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This includes closing the @file{/dev/log} socket, if it is open.
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@code{closelog} also sets the identification string for Syslog messages
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back to the default, if @code{openlog} was called with a non-NULL argument
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to @var{ident}. The default identification string is the program name
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taken from argv[0].
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If you are writing shared library code that uses @code{openlog} to
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generate custom syslog output, you should use @code{closelog} to drop
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@theglibc{}'s internal reference to the @var{ident} pointer when you are
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done. Please read the section on @code{openlog} for more information:
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@xref{openlog}.
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@code{closelog} does not flush any buffers. You do not have to call
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@code{closelog} before re-opening a Syslog connection with @code{openlog}.
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Syslog connections are automatically closed on exec or exit.
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@end deftypefun
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@node setlogmask
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@subsection setlogmask
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The symbols referred to in this section are declared in the file
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@file{syslog.h}.
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@deftypefun int setlogmask (int @var{mask})
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@standards{BSD, syslog.h}
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@safety{@prelim{}@mtunsafe{@mtasurace{:LogMask}}@asunsafe{}@acsafe{}}
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@c Read and modify are not guarded by syslog_lock, so concurrent changes
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@c or even uses are undefined. This should use an atomic swap instead,
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@c at least for modifications.
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@code{setlogmask} sets a mask (the ``logmask'') that determines which
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future @code{syslog} calls shall be ignored. If a program has not
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called @code{setlogmask}, @code{syslog} doesn't ignore any calls. You
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can use @code{setlogmask} to specify that messages of particular
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priorities shall be ignored in the future.
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A @code{setlogmask} call overrides any previous @code{setlogmask} call.
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Note that the logmask exists entirely independently of opening and
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closing of Syslog connections.
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Setting the logmask has a similar effect to, but is not the same as,
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configuring Syslog. The Syslog configuration may cause Syslog to
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discard certain messages it receives, but the logmask causes certain
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messages never to get submitted to Syslog in the first place.
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@var{mask} is a bit string with one bit corresponding to each of the
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possible message priorities. If the bit is on, @code{syslog} handles
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messages of that priority normally. If it is off, @code{syslog}
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discards messages of that priority. Use the message priority macros
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described in @ref{syslog; vsyslog} and the @code{LOG_MASK} to construct
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an appropriate @var{mask} value, as in this example:
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@smallexample
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LOG_MASK(LOG_EMERG) | LOG_MASK(LOG_ERROR)
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@end smallexample
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or
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@smallexample
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~(LOG_MASK(LOG_INFO))
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@end smallexample
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There is also a @code{LOG_UPTO} macro, which generates a mask with the bits
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on for a certain priority and all priorities above it:
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@smallexample
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LOG_UPTO(LOG_ERROR)
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@end smallexample
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The unfortunate naming of the macro is due to the fact that internally,
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higher numbers are used for lower message priorities.
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@end deftypefun
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@node Syslog Example
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@subsection Syslog Example
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Here is an example of @code{openlog}, @code{syslog}, and @code{closelog}:
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This example sets the logmask so that debug and informational messages
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get discarded without ever reaching Syslog. So the second @code{syslog}
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in the example does nothing.
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@smallexample
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#include <syslog.h>
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setlogmask (LOG_UPTO (LOG_NOTICE));
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openlog ("exampleprog", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);
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syslog (LOG_NOTICE, "Program started by User %d", getuid ());
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syslog (LOG_INFO, "A tree falls in a forest");
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closelog ();
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@end smallexample
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