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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ENVIRONMENT | ATTRIBUTES | STANDARDS | HISTORY | EXAMPLES | SEE ALSO | COLOPHON |
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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 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.
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 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().
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.
fmtmsg()
MSGVERB
POSIX.1-2008.
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.
#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).
addseverity(3), perror(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 fmtmsg(3)
Pages that refer to this page: addseverity(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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