The __format_arg attribute tells gcc that it can use a specific argument to the annotated function as the format string for the purpose of type-checking a surrounding __printf function call. For example, assuming one has a fmtcheck function declared as
const char *fmtcheck(const char *, const char *, unsigned) __format_arg(2); and this is used in sprintf(buf, fmtcheck(what->ever, "%d %lx", 0), i, m) gcc checks that the varargs (i and m) matches the second argument to the fmtcheck function, i.e. that they are (int, long). With sprintf(buf, what->ever, i, m) the compiler cannot do any type checking. Even a static inline fmtcheck() that just returns its first argument would provide documentation for which specifiers what->ever is supposed to contain, but we'll implement an actual run-time check later. Signed-off-by: Rasmus Villemoes <li...@rasmusvillemoes.dk> --- include/linux/compiler_attributes.h | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h index 6b28c1b7310c..08264df52322 100644 --- a/include/linux/compiler_attributes.h +++ b/include/linux/compiler_attributes.h @@ -32,6 +32,7 @@ # define __GCC4_has_attribute___assume_aligned__ (__GNUC_MINOR__ >= 9) # define __GCC4_has_attribute___designated_init__ 0 # define __GCC4_has_attribute___externally_visible__ 1 +# define __GCC4_has_attribute___format_arg__ 1 # define __GCC4_has_attribute___noclone__ 1 # define __GCC4_has_attribute___optimize__ 1 # define __GCC4_has_attribute___nonstring__ 0 @@ -140,6 +141,18 @@ #define __printf(a, b) __attribute__((__format__(printf, a, b))) #define __scanf(a, b) __attribute__((__format__(scanf, a, b))) +/* + * Optional + * + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-format_005farg-function-attribute + * clang: apparently supported, but undocumented + */ +#if __has_attribute(__format_arg__) +# define __format_arg(n) __attribute__((__format_arg__(n))) +#else +# define __format_arg(n) +#endif + /* * gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-gnu_005finline-function-attribute * clang: https://clang.llvm.org/docs/AttributeReference.html#gnu-inline -- 2.19.1.6.gbde171bbf5