FMTMSG(3)                  Linux Programmer's Manual                 FMTMSG(3)

       fmtmsg - print formatted error messages

       #include <fmtmsg.h>

       int fmtmsg(long classification, const char *label,
                  int severity, const char *text,
                  const char *action, const char *tag);

       This  function displays a message described by its arguments on the de-
       vice(s) specified in the classification argument.  For messages written
       to stderr, the format depends on the MSGVERB environment variable.

       The  label  argument  identifies the source of the message.  The string
       must consist of two colon separated parts where the first part has  not
       more than 10 and the second part not more than 14 characters.

       The text argument describes the condition of the error.

       The action argument describes possible steps to recover from the error.
       If it is printed, it is prefixed by "TO FIX: ".

       The tag argument is a reference to the online documentation where  more
       information  can  be  found.   It  should contain the label value and a
       unique identification number.

   Dummy arguments
       Each of the arguments can have a dummy value.  The dummy classification
       value  MM_NULLMC  (0L)  does  not  specify  any  output,  so nothing is
       printed.  The dummy severity value NO_SEV (0) says that no severity  is
       supplied.   The  values  MM_NULLLBL, MM_NULLTXT, MM_NULLACT, MM_NULLTAG
       are synonyms for ((char *) 0), the empty string, and  MM_NULLSEV  is  a
       synonym for NO_SEV.

   The classification argument
       The  classification argument is the sum of values describing 4 types of

       The first value defines the output channel.

       MM_PRINT    Output to stderr.

       MM_CONSOLE  Output to the system console.

                   Output to both.

       The second value is the source of the error:

       MM_HARD     A hardware error occurred.

       MM_FIRM     A firmware error occurred.

       MM_SOFT     A software error occurred.

       The third value encodes the detector of the problem:

       MM_APPL     It is detected by an application.

       MM_UTIL     It is detected by a utility.

       MM_OPSYS    It is detected by the operating system.

       The fourth value shows the severity of the incident:

       MM_RECOVER  It is a recoverable error.

       MM_NRECOV   It is a nonrecoverable error.

   The severity argument
       The severity argument can take one of the following values:

       MM_NOSEV    No severity is printed.

       MM_HALT     This value is printed as HALT.

       MM_ERROR    This value is printed as ERROR.

       MM_WARNING  This value is printed as WARNING.

       MM_INFO     This value is printed as INFO.

       The numeric values are between 0 and 4.  Using  addseverity(3)  or  the
       environment  variable  SEV_LEVEL you can add more levels and strings to

       The function can return 4 values:

       MM_OK       Everything went smooth.

       MM_NOTOK    Complete failure.

       MM_NOMSG    Error writing to stderr.

       MM_NOCON    Error writing to the console.

       The environment variable MSGVERB ("message verbosity") can be  used  to
       suppress  parts of the output to stderr.  (It does not influence output
       to the console.)  When this variable is defined, is non-NULL, and is  a
       colon-separated list of valid keywords, then only the parts of the mes-
       sage corresponding to these keywords is printed.   Valid  keywords  are
       "label", "severity", "text", "action" and "tag".

       The  environment variable SEV_LEVEL can be used to introduce new sever-
       ity levels.  By default, only the five severity levels described  above
       are available.  Any other numeric value would make fmtmsg() print noth-
       ing.  If the user puts SEV_LEVEL with a format like


       in the environment of the process before the first  call  to  fmtmsg(),
       where each description is of the form


       then  fmtmsg()  will also accept the indicated values for the level (in
       addition to the standard levels 0-4), and use the indicated printstring
       when such a level occurs.

       The  severity-keyword  part  is  not  used by fmtmsg() but it has to be
       present.  The level part is a string representation of a  number.   The
       numeric value must be a number greater than 4.  This value must be used
       in the severity argument of fmtmsg() to select this class.  It  is  not
       possible  to  overwrite any of the predefined classes.  The printstring
       is the string printed when a message of  this  class  is  processed  by

       fmtmsg() is provided in glibc since version 2.1.

       For  an  explanation  of  the  terms  used  in  this  section,  see at-

       |Interface | Attribute     | Value                   |
       |fmtmsg()  | Thread safety | glibc >= 2.16: MT-Safe  |
       |          |               | glibc < 2.16: MT-Unsafe |
       Before glibc 2.16, the fmtmsg() function uses a static variable that is
       not protected, so it is not thread-safe.

       Since  glibc  2.16,  the  fmtmsg()  function uses a lock to protect the
       static variable, so it is thread-safe.

       The functions fmtmsg() and addseverity(3),  and  environment  variables
       MSGVERB and SEV_LEVEL come from System V.

       The  function  fmtmsg()  and  the  environment variable MSGVERB are de-
       scribed in POSIX.1-2001 and POSIX.1-2008.

       System V and UnixWare man pages tell us that these functions have  been
       replaced  by  "pfmt() and addsev()" or by "pfmt(), vpfmt(), lfmt(), and
       vlfmt()", and will be removed later.

       #include <stdio.h>
       #include <stdlib.h>
       #include <fmtmsg.h>

           long class = MM_PRINT | MM_SOFT | MM_OPSYS | MM_RECOVER;
           int err;

           err = fmtmsg(class, "util-linux:mount", MM_ERROR,
                       "unknown mount option", "See mount(8).",
           switch (err) {
           case MM_OK:
           case MM_NOTOK:
               printf("Nothing printed\n");
           case MM_NOMSG:
               printf("Nothing printed to stderr\n");
           case MM_NOCON:
               printf("No console output\n");
               printf("Unknown error from fmtmsg()\n");

       The output should be:

           util-linux:mount: ERROR: unknown mount option
           TO FIX: See mount(8).  util-linux:mount:017

       and after

           MSGVERB=text:action; export MSGVERB

       the output becomes:

           unknown mount option
           TO FIX: See mount(8).

       addseverity(3), perror(3)

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                                  2017-09-15                         FMTMSG(3)
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