fmtmsg(3) Library Functions Manual 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 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
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 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
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 attrib-
utes(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 sta-
tic 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 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
the output becomes:
unknown mount option
TO FIX: See mount(8).
SEE ALSO
addseverity(3), perror(3)
Linux man-pages 6.7 2023-10-31 fmtmsg(3)
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