fmtmsg(3)

NAME
fmtmsg - print formatted error messages

LIBRARY
Standard C library (libc, -lc)

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 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.

MM_PRINT | MM_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.

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.

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 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().

ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value
 | 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.

STANDARDS

fmtmsg()
MSGVERB
    POSIX.1-2008.

HISTORY

fmtmsg()
    System V. POSIX.1-2001 and POSIX.1-2008. glibc 2.1.

MSGVERB
    System V. POSIX.1-2001 and POSIX.1-2008.

SEV_LEVEL
    System V.

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

    int
    main(void)
    {
        long c = MM_PRINT | MM_SOFT | MM_OPSYS | MM_RECOVER;
        int err;

        err = fmtmsg(c, "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

the output becomes:

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

SEE ALSO
addseverity(3), perror(3)