explain_output - output error messages

       Written by Peter Miller <pmiller@opensource.org.au>

                                                                                               explain_output(3)

       libexplain version 1.4
       Copyright (C) 2010 Peter Miller

       These functions may be used to write error messages.

   explain_output_message
       void explain_output_message(const char *text);

       The  explain_output_message  function  is  used  to  print text.  It is printed via the registered output
       class, see explain_output_register(3) for how.

       text    The text of the message to be printed.  It has not been wrapped (yet).

   explain_output_error
       void explain_output_error(const char *fmt, ...);

       The explain_output_error function is used to print a formatted error message.  The printing is  done  via
       the explain_output_message(3) function.

       fmt     The format text of the message to be printed.  See printf(3) for more information.

   explain_output_error_and_die
       void explain_output_error_and_die(const char *fmt, ...);

       The  explain_output_error_and_die  function  is  used to print text, and then terminate immediately.  The
       printing  is  done  via  the  explain_output_message(3)  function,  process  termination   is   via   the
       explain_output_exit_failure(3) function.

       fmt     The format text of the message to be printed.  See printf(3) for more information.

   explain_output_warning
       void explain_output_warning(const char *fmt, ...);

       The  explain_output_warning  function  is  used  to  print  a formatted error message, including the word
       “warning”.  The printing is done via the explain_output_message(3) function.

       fmt     The format text of the message to be printed.  See printf(3) for more information.

   explain_output_exit
       void explain_output_exit(int status);

       The explain_output_exit function is used to terminate execution.   It  is  executed  via  the  registered
       output class, explain_output_register(3) for how.

       status  The exist status requested.

   explain_output_exit_failure
       void explain_output_exit_failure(void);

       The  explain_output_exit_failure  function is used to terminate execution, with exit status EXIT_FAILURE.
       It is executed via the registered output class, see explain_output_register(3) for how.

   explain_option_hanging_indent_set
       void explain_option_hanging_indent_set(int columns);

       The explain_option_hanging_indent_set function is used to  cause  the  output  wrapping  to  use  hanging
       indents.   By  default  no  hanging indent is used, but this can sometimes obfuscate the end of one error
       message and the beginning of another.  A hanging indent results in continuation lines starting with white
       space, similar to RFC822 headers.

       This can be set using the “hanging‐indent=n” string in the  EXPLAIN_OPTIONS  environment  variable.   See
       explain(3) for more information.

       Using this function will override any environment variable setting.

       columns The  number  of  columns of hanging indent to be used.  A value of 0 means no hanging indent (all
               lines flush with left margin).  A common value to use is 4: it doesn't consume too much  of  each
               line, and it is a clear indent.

       explain_output - output error messages

       It  is  possible  to  change how and where libexplain sends its output, and even how it calls the exit(2)
       function.  This functionality is used by the explain_*_or_die and explain_*_on_error functions.

       By default, libexplain will wrap and print error messages on stderr, and call the exit(2) system call  to
       terminate execution.

       Clients  of  the  libexplain  library  may  choose  to  use  some message handling facilities provided by
       libexplain, or they may choose to implement their own.

       syslog
               To cause all output to be sent to syslog, use

                      explain_output_register(explain_output_syslog_new());

               This is useful for servers and daemons.

       stderrandsyslog
               The “tee” output class can be used to duplicate output.  To cause all output to be sent  to  both
               stderr and syslog, use

                      explain_output_register
                      (
                          explain_output_tee_new
                          (
                              explain_output_stderr_new(),
                              explain_output_syslog_new()
                          )
                      );

               If you need more than two, use several instances of “tee”, cascaded.

       stderrandafile
               To cause all output to be sent to both stderr and a regular file, use

                      explain_output_register
                      (
                          explain_output_tee_new
                          (
                              explain_output_stderr_new(),
                              explain_output_file_new(filename, 0)
                          )
                      );

       See the <libexplain/output.h> file for extensive documentation.

   explain_output_new
       explain_output_t *explain_output_new(const explain_output_vtable_t *vtable);

       The  explain_output_new  function  may  be  used  to  create  a  new  dynamically  allocated  instance of
       explain_output_t.

       vtable  The struct containing the pointers to the methods of the derived class.

       returns NULL on error (i.e. malloc failed), or a pointer to a new dynamically allocated instance  of  the
               class.

   explain_output_stderr_new
       explain_output_t *explain_output_stderr_new(void);

       The  explain_output_stderr_new  function may be used to create a new dynamically allocated instance of an
       explain_output_t class that writes to stderr, and exits via exit(2);

       This is the default output handler.

       returns NULL on error (i.e. malloc failed), or a pointer to a new dynamically allocated instance  of  the
               stderr class.

   explain_output_syslog_new
       explain_output_t *explain_output_syslog_new(void);

       The  explain_output_syslog_new  function may be used to create a new dynamically allocated instance of an
       explain_output_t class that writes to syslog, and exits via exit(2);

       The following values are used:

              option = 0
              facility = LOG_USER
              level = LOG_ERR

       See syslog(3) for more information.

       returns NULL on error (i.e. malloc(3) failed), or a pointer to a new dynamically  allocated  instance  of
               the syslog class.

   explain_output_syslog_new1
       explain_output_t *explain_output_syslog_new1(int level);

       The  explain_output_syslog_new1 function may be used to create a new dynamically allocated instance of an
       explain_output_t class that writes to syslog, and exits via exit(2);

       The following values are used:

              option = 0
              facility = LOG_USER

       See syslog(3) for more information.

       level   The syslog level to be used, see syslog(3) for a definition.

       returns NULL on error (i.e. malloc(3) failed), or a pointer to a new dynamically  allocated  instance  of
               the syslog class.

   explain_output_syslog_new3
       explain_output_t *explain_output_syslog_new3(int option, int facility, int level);

       The  explain_output_syslog_new3 function may be used to create a new dynamically allocated instance of an
       explain_output_t class that writes to syslog, and exits via exit(2);

       If you want different facilities or levels, create multiple instances.

       option  The syslog option to be used, see syslog(3) for a definition.

       facility
               The syslog facility to be used, see syslog(3) for a definition.

       level   The syslog level to be used, see syslog(3) for a definition.

       returns NULL on error (i.e. malloc(3) failed), or a pointer to a new dynamically  allocated  instance  of
               the syslog class.

   explain_output_file_new
       explain_output_t *explain_output_file_new(const char *filename, int append);

       The  explain_output_file_new  function  may  be used to create a new dynamically allocated instance of an
       explain_output_t class that writes to a file, and exits via exit(2).

       filename
               The file to be opened and written to.

       append  true (non‐zero) if messages are to be appended to the file, false (zero) if the  file  is  to  be
               replaced with new contents.

       returns NULL  on  error  (i.e.  malloc(3) or open(2) failed), or a pointer to a new dynamically allocated
               instance of the syslog class.

   explain_output_tee_new
       explain_output_t *explain_output_tee_new(explain_output_t *first, explain_output_t *second);

       The explain_output_tee_new function may be used to create a new  dynamically  allocated  instance  of  an
       explain_output_t class that writes to two other output classes.

       first   The first output class to write to.

       second  The second output class to write to.

       returns NULL  on  error  (i.e. malloc(3) failed), or a pointer to a new dynamically allocated instance of
               the syslog class.

       The output subsystem will “own” the first and second objects after this  call.   You  may  not  make  any
       reference  to  these  pointers  ever again.  The output subsystem will destroy these objects and free the
       memory when it feels like it.

   explain_output_register
       void explain_output_register(explain_output_t *op);

       The explain_output_register function is used to change libexplain's default  output  handling  facilities
       with something else.  The NULL pointer restores libexplain's default processing.

       If no output class is registered, the default is to wrap and print to stderr, and to exit via the exit(2)
       system call.

       op      Pointer to the explain_output_t instance to be operated on.

       The  output  subsystem  will  “own”  the pointer after this call.  You may not make any reference to this
       pointer ever again.  The output subsystem will destroy the object and free the memory when it feels  like
       it.

       #include <libexplain/output.h>