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Clang only allows you to use `__attribute__((format))` on variadic functions.
There are legit use cases for `__attribute__((format))` on non-variadic
functions, such as:
(1) variadic templates
template<typename… Args>
void print(const char *fmt, Args… &&args) __attribute__((format(1, 2))); //
error: format attribute requires variadic function
(2) functions which take fixed arguments and a custom format:
void print_number_string(const char *fmt, unsigned number, const char
*string) __attribute__((format(1, 2)));
// ^error: format attribute requires variadic function
void foo(void) {
print_number_string(“%08x %s\n”, 0xdeadbeef, “hello”);
print_number_string(“%d %s”, 0xcafebabe, “bar”);
}
This change allows Clang users to attach `__attribute__((format))` to
non-variadic functions, including functions with C++ variadic templates. It
replaces the error with a GCC compatibility warning and improves the type
checker to ensure that received arrays are treated like pointers (this is a
possibility in C++ since references to template types can bind to arrays).
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D112579
Files:
clang/include/clang/Basic/DiagnosticSemaKinds.td
clang/lib/AST/FormatString.cpp
clang/lib/Sema/SemaDeclAttr.cpp
clang/test/Sema/attr-format.c
clang/test/Sema/attr-format.cpp
Index: clang/test/Sema/attr-format.cpp
===================================================================
--- /dev/null
+++ clang/test/Sema/attr-format.cpp
@@ -0,0 +1,16 @@
+//RUN: %clang_cc1 -fsyntax-only -verify %s
+
+#include <stdarg.h>
+
+template<typename... Args>
+void format(const char *fmt, Args &&... args) // expected-warning{{GCC requires functions with 'format' attribute to be variadic}}
+ __attribute__((format(printf, 1, 2)));
+
+void do_format() {
+ int x = 123;
+ int &y = x;
+ const char *s = "world";
+ format("bare string");
+ format("%s %s %u %d %i %p\n", "hello", s, 10u, x, y, &do_format);
+ format("bad format %s"); // expected-warning{{more '%' conversions than data arguments}}
+}
Index: clang/test/Sema/attr-format.c
===================================================================
--- clang/test/Sema/attr-format.c
+++ clang/test/Sema/attr-format.c
@@ -5,7 +5,7 @@
void a(const char *a, ...) __attribute__((format(printf, 1,2))); // no-error
void b(const char *a, ...) __attribute__((format(printf, 1,1))); // expected-error {{'format' attribute parameter 3 is out of bounds}}
void c(const char *a, ...) __attribute__((format(printf, 0,2))); // expected-error {{'format' attribute parameter 2 is out of bounds}}
-void d(const char *a, int c) __attribute__((format(printf, 1,2))); // expected-error {{format attribute requires variadic function}}
+void d(const char *a, int c) __attribute__((format(printf, 1, 2))); // expected-warning {{GCC requires functions with 'format' attribute to be variadic}}
void e(char *str, int c, ...) __attribute__((format(printf, 2,3))); // expected-error {{format argument not a string type}}
typedef const char* xpto;
@@ -39,7 +39,7 @@
void a2(const char *a, ...) __attribute__((format(printf0, 1,2))); // no-error
void b2(const char *a, ...) __attribute__((format(printf0, 1,1))); // expected-error {{'format' attribute parameter 3 is out of bounds}}
void c2(const char *a, ...) __attribute__((format(printf0, 0,2))); // expected-error {{'format' attribute parameter 2 is out of bounds}}
-void d2(const char *a, int c) __attribute__((format(printf0, 1,2))); // expected-error {{format attribute requires variadic function}}
+void d2(const char *a, int c) __attribute__((format(printf0, 1,2))); // expected-warning {{GCC requires functions with 'format' attribute to be variadic}}
void e2(char *str, int c, ...) __attribute__((format(printf0, 2,3))); // expected-error {{format argument not a string type}}
// FreeBSD usage
@@ -61,7 +61,7 @@
void a3(const char *a, ...) __attribute__((format(freebsd_kprintf, 1,2))); // no-error
void b3(const char *a, ...) __attribute__((format(freebsd_kprintf, 1,1))); // expected-error {{'format' attribute parameter 3 is out of bounds}}
void c3(const char *a, ...) __attribute__((format(freebsd_kprintf, 0,2))); // expected-error {{'format' attribute parameter 2 is out of bounds}}
-void d3(const char *a, int c) __attribute__((format(freebsd_kprintf, 1,2))); // expected-error {{format attribute requires variadic function}}
+void d3(const char *a, int c) __attribute__((format(freebsd_kprintf, 1,2))); // expected-warning {{GCC requires functions with 'format' attribute to be variadic}}
void e3(char *str, int c, ...) __attribute__((format(freebsd_kprintf, 2,3))); // expected-error {{format argument not a string type}}
@@ -87,3 +87,9 @@
__attribute__ ((__format__(__gcc_tdiag__, 1, 2)));
const char *foo3(const char *format) __attribute__((format_arg("foo"))); // expected-error{{'format_arg' attribute requires parameter 1 to be an integer constant}}
+
+void call_nonvariadic(void) {
+ d3("%i", 123);
+ d3("%d", 123);
+ d3("%s", 123); // expected-warning{{format specifies type 'char *' but the argument has type 'int'}}
+}
Index: clang/lib/Sema/SemaDeclAttr.cpp
===================================================================
--- clang/lib/Sema/SemaDeclAttr.cpp
+++ clang/lib/Sema/SemaDeclAttr.cpp
@@ -3612,8 +3612,7 @@
if (isFunctionOrMethodVariadic(D)) {
++NumArgs; // +1 for ...
} else {
- S.Diag(D->getLocation(), diag::err_format_attribute_requires_variadic);
- return;
+ S.Diag(D->getLocation(), diag::warn_gcc_requires_variadic_function) << AL;
}
}
Index: clang/lib/AST/FormatString.cpp
===================================================================
--- clang/lib/AST/FormatString.cpp
+++ clang/lib/AST/FormatString.cpp
@@ -322,6 +322,12 @@
clang::analyze_format_string::ArgType::MatchKind
ArgType::matchesType(ASTContext &C, QualType argTy) const {
+ // When using the format attribute with variadic templates in C++, you can
+ // receive an array that will necessarily decay to a pointer when passed to
+ // the final format consumer. Apply decay before type comparison.
+ if (C.getAsArrayType(argTy))
+ argTy = C.getArrayDecayedType(argTy);
+
if (Ptr) {
// It has to be a pointer.
const PointerType *PT = argTy->getAs<PointerType>();
Index: clang/include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3045,8 +3045,6 @@
"declared with index %0 here">;
def err_format_strftime_third_parameter : Error<
"strftime format attribute requires 3rd parameter to be 0">;
-def err_format_attribute_requires_variadic : Error<
- "format attribute requires variadic function">;
def err_format_attribute_not : Error<"format argument not %0">;
def err_format_attribute_result_not : Error<"function does not return %0">;
def err_format_attribute_implicit_this_format_string : Error<
@@ -4049,6 +4047,9 @@
def warn_gcc_ignores_type_attr : Warning<
"GCC does not allow the %0 attribute to be written on a type">,
InGroup<GccCompat>;
+def warn_gcc_requires_variadic_function : Warning<
+ "GCC requires functions with %0 attribute to be variadic">,
+ InGroup<GccCompat>;
// Clang-Specific Attributes
def warn_attribute_iboutlet : Warning<
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