---
name: "warnc"
section: "3bsd"
platform: "linux"
package: "libbsd"
version: "0.12.2-2"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/errc/3bsd"
---

# warnc(3bsd)

formatted error messages

Part of [errc(3bsd)](/errc/3bsd.md).

## NAME

**errc**, **verrc**, **warnc**, **vwarnc** – formatted error messages

## LIBRARY

library “libbsd”

## SYNOPSIS

**#include \<err.h\>**

(See [libbsd(7)](/libbsd/7.md) for include usage.)

*void*\

**errc**(*int status*, *int code*, *const char \*fmt*, *...*);

*void*\

**verrc**(*int status*, *int code*, *const char \*fmt*, *va\_list args*);

*void*\

**warnc**(*int code*, *const char \*fmt*, *...*);

*void*\

**vwarnc**(*int code*, *const char \*fmt*, *va\_list args*);

## DESCRIPTION

The **err**() and **warn**() family of functions display a formatted error message on the standard error output. In all cases, the last component of the program name, followed by a colon (‘:’) character and a space, are output. The text that follows depends on the function being called. The *fmt* specification (and associated arguments) may be any format allowed by [printf(3)](/printf/3.md) or NULL. If the *fmt* argument is not NULL, the formatted error message is output.

The functions all output an error message string affiliated with an error value (see [strerror(3)](/strerror/3.md)), preceded by a colon character and a space if *fmt* is not NULL. That is, the output is as follows:

```
progname: fmt: error message string
```

if *fmt* is not NULL, or:

```
progname: error message string
```

if it is.

The argument *code* is used as the error value instead of the current value of the global variable *errno*.

In all cases, the output is followed by a newline character.

The **errc**(), and **verrc**() functions do not return, but exit with the value of the argument *status*.

## EXAMPLES

Display the current *errno* information string and exit:

```
if ((p = malloc(size)) == NULL)
	err(1, NULL);
if ((fd = open(file_name, O_RDONLY, 0)) == -1)
	err(1, "%s", file_name);
```

Display an error message and exit:

```
if (tm.tm_hour < START_TIME)
	errx(1, "too early, wait until %s", start_time_string);
```

Warn of an error:

```
if ((fd = open(raw_device, O_RDONLY, 0)) == -1)
	warnx("%s: %s: trying the block device",
	    raw_device, strerror(errno));
if ((fd = open(block_device, O_RDONLY, 0)) == -1)
	err(1, "%s", block_device);
```

## SEE ALSO

[err(3)](/err/3.md) [exit(3)](/exit/3.md), [perror(3)](/perror/3.md), [printf(3)](/printf/3.md), [strerror(3)](/strerror/3.md)

## HISTORY

The functions **errc**(), **verrc**(), **warnc**(), and **vwarnc**() first appeared in FreeBSD 3.0, NetBSD 7.0 and OpenBSD 5.6.

## CAVEATS

It is important never to pass a string with user-supplied data as a format without using ‘%s’. An attacker can put format specifiers in the string to mangle the stack, leading to a possible security hole. This holds true even if the string has been built “by hand” using a function like **snprintf**(), as the resulting string may still contain user-supplied conversion specifiers for later interpolation by the **err**() and **warn**() family of functions.

Always be sure to use the proper secure idiom:

```
errc(1, 0, "%s", string);
```