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

# fmtcheck(3bsd)

sanitizes user-supplied printf(3)-style format string

## NAME

**fmtcheck** – sanitizes user-supplied [printf(3)](/printf/3.md)-style format string

## LIBRARY

library “libbsd”

## SYNOPSIS

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

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

*const* *char* *\**\

**fmtcheck**(*const char \*fmt\_suspect*, *const char \*fmt\_default*);

## DESCRIPTION

The **fmtcheck** function scans *fmt\_suspect* and *fmt\_default* to determine if *fmt\_suspect* will consume the same argument types as *fmt\_default* and to ensure that *fmt\_suspect* is a valid format string.

The [printf(3)](/printf/3.md) family of functions can not verify the types of arguments that they are passed at run-time. In some cases, like [catgets(3)](/catgets/3.md), it is useful or necessary to use a user-supplied format string with no guarantee that the format string matches the specified parameters.

The **fmtcheck** function was designed to be used in these cases, as in:

```
printf(fmtcheck(user_format, standard_format), arg1, arg2);
```

In the check, field widths, fillers, precisions, etc. are ignored (unless the field width or precision is an asterisk ‘\*’ instead of a digit string). Also, any text other than the format specifiers is completely ignored.

Note that the formats may be quite different as long as they accept the same parameters. For example, "%ld %o %30s %#llx %-10.\*e %n" is compatible with "This number %lu %d%% and string %s has %qd numbers and %.\*g floats (%n)." However, "%o" is not equivalent to "%lx" because the first requires an integer and the second requires a long, and "%p" is not equivalent to "%lu" because the first requires a pointer and the second requires a long.

## RETURN VALUES

If *fmt\_suspect* is a valid format and consumes the same argument types as *fmt\_default*, then the **fmtcheck** function will return *fmt\_suspect*. Otherwise, it will return *fmt\_default*.

## SEE ALSO

[printf(3)](/printf/3.md)