FMTMSG(3)                 (2020-06-09)                  FMTMSG(3)

     NAME
          fmtmsg - print formatted error messages

     SYNOPSIS
          #include <fmtmsg.h>

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

     DESCRIPTION
          This function displays a message described by its arguments
          on the device(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 syn-
          onym for NO_SEV.

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

          The first value defines the output channel.

          MM_PRINT    Output to stderr.

          MM_CONSOLE  Output to the system console.

          MM_PRINT | MM_CONSOLE

     Page 1                       Plan 9             (printed 5/18/22)

     FMTMSG(3)                 (2020-06-09)                  FMTMSG(3)

                      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 print.

     RETURN VALUE
          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.

     Page 2                       Plan 9             (printed 5/18/22)

     FMTMSG(3)                 (2020-06-09)                  FMTMSG(3)

     ENVIRONMENT
          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 message 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 severity levels.  By default, only the five severity
          levels described above are available.  Any other numeric
          value would make fmtmsg() print nothing.  If the user puts
          SEV_LEVEL with a format like

               SEV_LEVEL=[description[:description[:...]]]

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

               severity-keyword,level,printstring

          then fmtmsg() will also accept the indicated values for the
          level (in addition to the standard levels 0en4), 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 over-
          write any of the predefined classes.  The printstring is the
          string printed when a message of this class is processed by
          fmtmsg().

     VERSIONS
          fmtmsg() is provided in glibc since version 2.1.

     ATTRIBUTES
          For an explanation of the terms used in this section, see
          attributes(7).  allbox; lb lb lbw23 l l l.
          Interface Attribute Value T{ fmtmsg() T}   Thread safety  T{
          glibc >= 2.16: MT-Safe
          glibc < 2.16: MT-Unsafe T}

          Before glibc 2.16, the fmtmsg() function uses a static vari-
          able that is not protected, so it is not thread-safe.

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

     CONFORMING TO

     Page 3                       Plan 9             (printed 5/18/22)

     FMTMSG(3)                 (2020-06-09)                  FMTMSG(3)

          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 described in POSIX.1-2001 and POSIX.1-2008.

     NOTES
          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.

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

          int
          main(void)
          {
              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).",
                          "util-linux:mount:017");
              switch (err) {
              case MM_OK:
                  break;
              case MM_NOTOK:
                  printf("Nothing printed\n");
                  break;
              case MM_NOMSG:
                  printf("Nothing printed to stderr\n");
                  break;
              case MM_NOCON:
                  printf("No console output\n");
                  break;
              default:
                  printf("Unknown error from fmtmsg()\n");
              }
              exit(EXIT_SUCCESS);
          }

          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

     Page 4                       Plan 9             (printed 5/18/22)

     FMTMSG(3)                 (2020-06-09)                  FMTMSG(3)

          the output becomes:

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

     SEE ALSO
          addseverity(3), perror(3)

     COLOPHON
          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

     Page 5                       Plan 9             (printed 5/18/22)