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File: libc.info, Node: openlog, Next: syslog; vsyslog, Up: Submitting Syslog Messages
18.2.1 openlog
--------------
The symbols referred to in this section are declared in the file
'syslog.h'.
-- Function: void openlog (const char *IDENT, int OPTION, int FACILITY)
Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd | *Note
POSIX Safety Concepts::.
'openlog' opens or reopens a connection to Syslog in preparation
for submitting messages.
IDENT is an arbitrary identification string which future '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 IDENT is NULL, or if '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 IDENT will be retained
internally by the Syslog routines. You must not free the memory
that 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 IDENT string,
you must call 'openlog' again; overwriting the string pointed to by
IDENT is not thread-safe.
You can cause the Syslog routines to drop the reference to IDENT
and go back to the default string (the program name taken from
argv[0]), by calling 'closelog': *Note 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 'openlog', you must call 'closelog' before any point where your
library might get unloaded, as in this example:
#include <syslog.h>
void
shared_library_function (void)
{
openlog ("mylibrary", option, priority);
syslog (LOG_INFO, "shared library has been invoked");
closelog ();
}
Without the call to 'closelog', future invocations of 'syslog' by
the program using the shared library may crash, if the library gets
unloaded and the memory containing the string '"mylibrary"' becomes
unmapped. This is a limitation of the BSD syslog interface.
'openlog' may or may not open the '/dev/log' socket, depending on
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 'openlog'. If you call 'syslog' without
having called 'openlog', 'syslog' just opens the connection
implicitly and uses defaults for the information in IDENT and
OPTIONS.
OPTIONS is a bit string, with the bits as defined by the following
single bit masks:
'LOG_PERROR'
If on, 'openlog' sets up the connection so that any 'syslog'
on this connection writes its message to the calling process'
Standard Error stream in addition to submitting it to Syslog.
If off, 'syslog' does not write the message to Standard Error.
'LOG_CONS'
If on, 'openlog' sets up the connection so that a 'syslog' on
this connection that fails to submit a message to Syslog
writes the message instead to system console. If off,
'syslog' does not write to the system console (but of course
Syslog may write messages it receives to the console).
'LOG_PID'
When on, 'openlog' sets up the connection so that a 'syslog'
on this connection inserts the calling process' Process ID
(PID) into the message. When off, 'openlog' does not insert
the PID.
'LOG_NDELAY'
When on, 'openlog' opens and connects the '/dev/log' socket.
When off, a future 'syslog' call must open and connect the
socket.
*Portability note:* In early systems, the sense of this bit
was exactly the opposite.
'LOG_ODELAY'
This bit does nothing. It exists for backward compatibility.
If any other bit in OPTIONS is on, the result is undefined.
FACILITY is the default facility code for this connection. A
'syslog' on this connection that specifies default facility causes
this facility to be associated with the message. See 'syslog' for
possible values. A value of zero means the default default, which
is 'LOG_USER'.
If a Syslog connection is already open when you call 'openlog',
'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, 'openlog'
just leaves it that way.
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