fmtmsg(3)                                             Library Functions Manual                                            fmtmsg(3)

       fmtmsg - print formatted error messages

       Standard C library (libc, -lc)

       #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 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 la‐
       bel 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 information.

       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 print.

       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  key‐
       words,  then  only the parts of the message corresponding to these keywords is printed.  Valid keywords are "label", "sever‐
       ity", "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


       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  in‐
       dicated 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().

       fmtmsg() is provided since glibc 2.1.

       For an explanation of the terms used in this section, see attributes(7).

       │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 described 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 <fmtmsg.h>
       #include <stdio.h>
       #include <stdlib.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)

Linux man-pages 6.03                                         2023-02-05                                                   fmtmsg(3)