man3/fmtmsg.3

.\"  Copyright 2002 walter harms (walter.harms@informatik.uni-oldenburg.de)
.\"
.\" %%%LICENSE_START(GPL_NOVERSION_ONELINE)
.\" Distributed under GPL
.\" %%%LICENSE_END
.\"
.\"  adapted glibc info page
.\"
.\"  This should run as 'Guru Meditation' (amiga joke :)
.\"  The function is quite complex and deserves an example
.\"
.\"  Polished, aeb, 2003-11-01
.TH FMTMSG 3 2020-06-09 "" "Linux Programmer's Manual"
.SH NAME
fmtmsg \- print formatted error messages
.SH SYNOPSIS
.nf
.B #include <fmtmsg.h>
.PP
.BI "int fmtmsg(long " classification ", const char *" label ,
.BI "           int " severity ", const char *" text ,
.BI "           const char *" action ", const char *" tag );
.fi
.SH DESCRIPTION
This function displays a message described by its arguments on the device(s)
specified in the
.I classification
argument.
For messages written to
.IR stderr ,
the format depends on the
.B MSGVERB
environment variable.
.PP
The
.I 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.
.PP
The
.I text
argument describes the condition of the error.
.PP
The
.I action
argument describes possible steps to recover from the error.
If it is printed, it is prefixed by "TO FIX: ".
.PP
The
.I tag
argument is a reference to the online documentation where more
information can be found.
It should contain the
.I label
value and a unique identification number.
.SS Dummy arguments
Each of the arguments can have a dummy value.
The dummy classification value
.B MM_NULLMC
(0L) does not specify any output, so nothing is printed.
The dummy severity value
.B NO_SEV
(0) says that no severity is supplied.
The values
.BR MM_NULLLBL ,
.BR MM_NULLTXT ,
.BR MM_NULLACT ,
.B MM_NULLTAG
are synonyms for
.IR "((char\ *)\ 0)" ,
the empty string, and
.B MM_NULLSEV
is a synonym for
.BR NO_SEV .
.SS The classification argument
The
.I classification
argument is the sum of values describing 4 types of information.
.PP
The first value defines the output channel.
.TP 12n
.B MM_PRINT
Output to
.IR stderr .
.TP
.B MM_CONSOLE
Output to the system console.
.TP
.B "MM_PRINT | MM_CONSOLE"
Output to both.
.PP
The second value is the source of the error:
.TP 12n
.B MM_HARD
A hardware error occurred.
.TP
.B MM_FIRM
A firmware error occurred.
.TP
.B MM_SOFT
A software error occurred.
.PP
The third value encodes the detector of the problem:
.TP 12n
.B MM_APPL
It is detected by an application.
.TP
.B MM_UTIL
It is detected by a utility.
.TP
.B MM_OPSYS
It is detected by the operating system.
.PP
The fourth value shows the severity of the incident:
.TP 12n
.B MM_RECOVER
It is a recoverable error.
.TP
.B MM_NRECOV
It is a nonrecoverable error.
.SS The severity argument
The
.I severity
argument can take one of the following values:
.TP 12n
.B MM_NOSEV
No severity is printed.
.TP
.B MM_HALT
This value is printed as HALT.
.TP
.B MM_ERROR
This value is printed as ERROR.
.TP
.B MM_WARNING
This value is printed as WARNING.
.TP
.B MM_INFO
This value is printed as INFO.
.PP
The numeric values are between 0 and 4.
Using
.BR addseverity (3)
or the environment variable
.B SEV_LEVEL
you can add more levels and strings to print.
.SH RETURN VALUE
The function can return 4 values:
.TP 12n
.B MM_OK
Everything went smooth.
.TP
.B MM_NOTOK
Complete failure.
.TP
.B MM_NOMSG
Error writing to
.IR stderr .
.TP
.B MM_NOCON
Error writing to the console.
.SH ENVIRONMENT
The environment variable
.B MSGVERB
("message verbosity") can be used to suppress parts of
the output to
.IR 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".
.PP
The environment variable
.B 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
.BR fmtmsg ()
print nothing.
If the user puts
.B SEV_LEVEL
with a format like
.PP
.RS
SEV_LEVEL=[description[:description[:...]]]
.RE
.PP
in the environment of the process before the first call to
.BR fmtmsg (),
where each description is of the form
.PP
.RS
severity-keyword,level,printstring
.RE
.PP
then
.BR fmtmsg ()
will also accept the indicated values for the level (in addition to
the standard levels 0\(en4), and use the indicated printstring when
such a level occurs.
.PP
The severity-keyword part is not used by
.BR 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
.BR 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
.BR fmtmsg ().
.SH VERSIONS
.BR fmtmsg ()
is provided in glibc since version 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbw23
l l l.
Interface	Attribute	Value
T{
.BR fmtmsg ()
T}	Thread safety	T{
glibc >= 2.16: MT-Safe
.br
glibc < 2.16: MT-Unsafe
T}
.TE
.PP
Before glibc 2.16, the
.BR fmtmsg ()
function uses a static variable that is not protected,
so it is not thread-safe.
.PP
Since glibc 2.16,
.\" Modified in commit 7724defcf8873116fe4efab256596861eef21a94
the
.BR fmtmsg ()
function uses a lock to protect the static variable, so it is thread-safe.
.SH CONFORMING TO
The functions
.BR fmtmsg ()
and
.BR addseverity (3),
and environment variables
.B MSGVERB
and
.B SEV_LEVEL
come from System V.
.PP
The function
.BR fmtmsg ()
and the environment variable
.B MSGVERB
are described in POSIX.1-2001 and POSIX.1-2008.
.SH 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.
.SH EXAMPLES
.EX
#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\en");
        break;
    case MM_NOMSG:
        printf("Nothing printed to stderr\en");
        break;
    case MM_NOCON:
        printf("No console output\en");
        break;
    default:
        printf("Unknown error from fmtmsg()\en");
    }
    exit(EXIT_SUCCESS);
}
.EE
.PP
The output should be:
.PP
.in +4n
.EX
util\-linux:mount: ERROR: unknown mount option
TO FIX: See mount(8).  util\-linux:mount:017
.EE
.in
.PP
and after
.PP
.in +4n
.EX
MSGVERB=text:action; export MSGVERB
.EE
.in
.PP
the output becomes:
.PP
.in +4n
.EX
unknown mount option
TO FIX: See mount(8).
.EE
.in
.SH SEE ALSO
.BR addseverity (3),
.BR perror (3)