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Fix format attribute for printf

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Since a long time (GCC 4.4?) GCC does support annotating functions
with either the format attribute "gnu_printf" or "ms_printf" to
distinguish between different format string interpretations.

However, it seems like the attribute is ignored for the "printf"
symbol; regardless what the function declaration says, GCC treats
it as "ms_printf". This has become an issue now that mingw-w64
supports using the UCRT instead of msvcrt.dll, and in this case
the stdio functions are declared with the gnu_printf attribute,
and inttypes.h uses the same format specifiers as in GNU mode.

A reproducible example of the problem:

$ cat format.c
__attribute__((__format__ (gnu_printf, 1, 2))) int printf (const char *__format, ...);
__attribute__((__format__ (gnu_printf, 1, 2))) int othername (const char *__format, ...);

void function(void) {
    long long unsigned x = 42;
    othername("%llu\n", x);
    printf("%llu\n", x);
}
$ x86_64-w64-mingw32-gcc -c -Wformat format.c
format.c: In function 'function':
format.c:7:15: warning: unknown conversion type character 'l' in format [-Wformat=]
    7 |     printf("%llu\n", x);
      |               ^
format.c:7:12: warning: too many arguments for format [-Wformat-extra-args]
    7 |     printf("%llu\n", x);
      |            ^~~~~~~~

Note how both functions, printf and othername, are declare with
identical gnu_printf format attributes - GCC does take this into
account for "othername" and doesn't produce a warning, but GCC
seems to disregard the attribute in the printf declaration and
behave as if it was declared as ms_printf.

If the printf function declaration is changed into a static inline
function, the actual attribute used is honored though.

gcc/c-family/ChangeLog:

	PR c/95130
	* c-format.cc: skip default format for printf symbol if
	explicitly declared by prototype.

Signed-off-by: Tomas Kalibera <tomas.kalibera@gmail.com>
Signed-off-by: Jonathan Yong <10walls@gmail.com>
This commit is contained in:
Tomas Kalibera 2023-08-20 02:16:16 +00:00 • committed by Jonathan Yong
parent 1ba3363668
commit 966f3c134b
1 changed files with 36 additions and 0 deletions
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36
gcc/c-family/c-format.cc
View file

@ -1175,6 +1175,7 @@ check_function_format (const_tree fn, tree attrs, int nargs,
tree a; tree a;
tree atname = get_identifier ("format"); tree atname = get_identifier ("format");
bool skipped_default_format = false;
/* See if this function has any format attributes. */ /* See if this function has any format attributes. */
for (a = attrs; a; a = TREE_CHAIN (a)) for (a = attrs; a; a = TREE_CHAIN (a))
@ -1185,6 +1186,38 @@ check_function_format (const_tree fn, tree attrs, int nargs,
function_format_info info; function_format_info info;
decode_format_attr (fn, atname, TREE_VALUE (a), &info, decode_format_attr (fn, atname, TREE_VALUE (a), &info,
/*validated=*/true); /*validated=*/true);
/* Mingw32 targets have traditionally used ms_printf format for the
printf function, and this format is built in GCC. But nowadays,
if mingw-w64 is configured to target UCRT, the printf function
uses the gnu_printf format (specified in the stdio.h header). This
causes GCC to check both formats, which means that GCC would
warn twice about the same issue when both formats are violated,
e.g. for %lu used to print long long unsigned.
Hence, if there is a built-in attribute specifier and at least
one another, we skip the built-in one. See PR 95130 (but note that
GCC ms_printf already supports %llu) and PR 92292. */
if (!skipped_default_format
&& fn
&& TREE_CODE (fn) == FUNCTION_DECL
&& fndecl_built_in_p (fn, BUILT_IN_NORMAL)
&& (tree_to_uhwi (TREE_PURPOSE (TREE_VALUE (a)))
& (int) ATTR_FLAG_BUILT_IN))
{
tree aa;
for (aa = attrs; aa; aa = TREE_CHAIN (aa))
if (a != aa
&& is_attribute_p ("format", get_attribute_name (aa)))
{
skipped_default_format = true;
break;
}
if (skipped_default_format)
continue;
}
if (warn_format) if (warn_format)
{ {
/* FIXME: Rewrite all the internal functions in this file /* FIXME: Rewrite all the internal functions in this file
@ -5190,6 +5223,9 @@ handle_format_attribute (tree node[3], tree atname, tree args,
if (TREE_CODE (TREE_VALUE (args)) == IDENTIFIER_NODE) if (TREE_CODE (TREE_VALUE (args)) == IDENTIFIER_NODE)
TREE_VALUE (args) = canonicalize_attr_name (TREE_VALUE (args)); TREE_VALUE (args) = canonicalize_attr_name (TREE_VALUE (args));
/* record the flags for check_function_format */
TREE_PURPOSE (args) = build_int_cst (unsigned_type_node, flags);
if (!decode_format_attr (fndecl ? fndecl : type, atname, args, &info, if (!decode_format_attr (fndecl ? fndecl : type, atname, args, &info,
/* validated_p = */false)) /* validated_p = */false))
{ {