gnu::format - specify the style of format string

NAME  SYNOPSIS  DESCRIPTION  VERSIONS  GNU syntax  Styles  Non-variadic functions  STANDARDS  HISTORY  EXAMPLES 

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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;

}

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