FMTMSG(3) Linux Programmer's Manual FMTMSG(3)

fmtmsg - print formatted error messages

#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.

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 is the sum of values describing 4 types of information.
The first value defines the output channel.
Output to stderr.
Output to the system console.
Output to both.
The second value is the source of the error:
A hardware error occurred.
A firmware error occurred.
A software error occurred.
The third value encodes the detector of the problem:
It is detected by an application.
It is detected by a utility.
It is detected by the operating system.
The fourth value shows the severity of the incident:
It is a recoverable error.
It is a nonrecoverable error.

The severity argument can take one of the following values:
No severity is printed.
This value is printed as HALT.
This value is printed as ERROR.
This value is printed as WARNING.
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:
Everything went smooth.
Complete failure.
Error writing to stderr.
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
in the environment of the process before the first call to fmtmsg(), where each description is of the form
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().

fmtmsg() is provided in glibc since version 2.1.

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.

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.

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 <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).

addseverity(3), perror(3)