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File:,  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

 -- 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>

          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

     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 is a bit string, with the bits as defined by the following
     single bit masks:

          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.

          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).

          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.

          When on, 'openlog' opens and connects the '/dev/log' socket.
          When off, a future 'syslog' call must open and connect the

          *Portability note:* In early systems, the sense of this bit
          was exactly the opposite.

          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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