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;
           }

Linux man-pages 6.16                               2025-09-25                                 gnu::format(3attr)