---
name: "gnu::format"
section: "3attr"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/gnu::format"
---

# gnu::format(3attr)

specify the style of format string

## NAME

gnu::format - specify the style of format string

## SYNOPSIS

```
[[gnu::format(style, fmt-idx, first-idx)]]
```

## DESCRIPTION

This attribute can be applied to a function. It specifies that a function parameter is a format string, and specifies the style of the format string. This allows checking the syntax of the format string, as well as the types of the variadic arguments. The *style* can be one of the following.

- **printf** **scanf** **strftime** **strfmon**

  *fmt-idx* is a 1-based index that specifies the position of the format string within the parameter list.

*first-idx* is a 1-based index that specifies the position of the first argument that corresponds to the format string. If the first argument is part of a *va\_list* argument, it should be specified as **0**.

## VERSIONS

### GNU syntax

- **\_\_attribute\_\_((format(***style***,** *fmt-idx***,** *first-idx***)))**

  ### Styles

On some targets, other styles are additionally supported.

- MinGW Microsoft Windows **ms\_printf** **ms\_scanf** **ms\_strftime**

  These correspond to the formats supported by the *msvcrt.dll* library.

  GCC-only.

- Solaris **cmn\_err**

  - Darwin **CFString**

  - OpenBSD **kprintf** **syslog** **syslog**

  Clang-only.

- FreeBSD freebsd\_kprintf

  Clang-only.

In some languages, other styles are additionally supported.

- Objective-C **NSString**

  ### Non-variadic functions

Clang accepts the attribute on non-variadic functions as an extension.

## STANDARDS

GNU.

## HISTORY

gcc, g++, clang 2.8, clang++ 2.8.

## EXAMPLES

```
[[gnu::format(printf, 3, 0)]]
int
vstprintf(int size;
    char buf[restrict size], int size,
    const char *restrict fmt, va_list args)
{
	int len;
	if (size == 0) {
		errno = EOVERFLOW;
		return -1;
	}
	len = vsnprintf(buf, size, fmt, args);
	if (len >= size) {
		errno = E2BIG;
		return -1;
	}
	return len;
}
[[gnu::format(printf, 3, 4)]]
int
stprintf(int size;
    char buf[restrict size], int size,
    const char *restrict fmt, ...)
{
	int len;
	va_list args;
	va_start(args, fmt);
	len = vstprintf(buf, size, fmt, args);
	va_end(args);
	return len;
}
```