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FMTMSG(3) Linux Programmer's Manual 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 for‐
mat 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().
VERSIONS
fmtmsg() is provided in glibc since version 2.1.
ATTRIBUTES
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.
CONFORMING TO
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
the output becomes:
unknown mount option
TO FIX: See mount(8).
SEE ALSO
addseverity(3), perror(3)
2021-03-22 FMTMSG(3)