[clang] [clang] Catch missing format attributes (PR #70024)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,277 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wmissing-format-attribute %s
+
+typedef unsigned short char16_t;
+typedef unsigned int char32_t;
+typedef __WCHAR_TYPE__ wchar_t;
+typedef __SIZE_TYPE__ size_t;
+typedef __builtin_va_list va_list;
+
+__attribute__((__format__(__printf__, 1, 2)))
+int printf(const char *, ...); // #printf
+
+__attribute__((__format__(__scanf__, 1, 2)))
+int scanf(const char *, ...); // #scanf
+
+__attribute__((__format__(__printf__, 1, 0)))
+int vprintf(const char *, va_list); // #vprintf
+
+__attribute__((__format__(__scanf__, 1, 0)))
+int vscanf(const char *, va_list); // #vscanf
+
+__attribute__((__format__(__printf__, 2, 0)))
+int vsprintf(char *, const char *, va_list); // #vsprintf
+
+__attribute__((__format__(__printf__, 3, 0)))
+int vsnprintf(char *ch, size_t, const char *, va_list); // #vsnprintf
+
+__attribute__((__format__(__scanf__, 1, 4)))
+void f1(char *out, const size_t len, const char *format, ... /* args */) // #f1
+{
+    va_list args;
+    vsnprintf(out, len, format, args); // expected-warning@#f1 {{diagnostic behavior may be improved by adding the 'printf' format attribute to the declaration of 'f1'}}
+                                       // CHECK-FIXES: __attribute__((format(printf, 3, 4)))
+}
+
+__attribute__((__format__(__printf__, 1, 4)))
+void f2(char *out, const size_t len, const char *format, ... /* args */) // #f2
+{
+    va_list args;
+    vsnprintf(out, len, format, args); // expected-warning@#f2 {{diagnostic behavior may be improved by adding the 'printf' format attribute to the declaration of 'f2'}}
+                                       // CHECK-FIXES: __attribute__((format(printf, 3, 4)))
+}
+
+void f3(char *out, va_list args) // #f3
+{
+    vprintf(out, args); // expected-warning@#f3 {{diagnostic behavior may be improved by adding the 'printf' format attribute to the declaration of 'f3'}}
+                        // CHECK-FIXES: __attribute__((format(printf, 1, 0)))
+    vscanf(out, args); // expected-warning@#f3 {{diagnostic behavior may be improved by adding the 'scanf' format attribute to the declaration of 'f3'}}
+                       // CHECK-FIXES: __attribute__((format(scanf, 1, 0)))
----------------
AaronBallman wrote:

Oops, sorry that I missed your question!

> My view of this comment is next: it refers to function which calls both vprintf and vscanf format functions (let's call this function fn).

Correct -- that's what I meant; a function that uses both printing and scanning functions. Concretely:
```
#include <stdio.h>

void foo(const char *out, ...) {
    va_list args;
    va_start(args);
    vscanf(out, args);
    vprintf(out, args);
    va_end(args)
}
```
there's no way for someone to call `foo("%d", ???)` that would work because the `???` would be of the wrong type for one of the formatted io calls, so I don't think such a function should have a suggestion to add both attributes (the way GCC does).

https://github.com/llvm/llvm-project/pull/70024