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From:   Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
To:     Ammar Faizi <ammarfaizi2@gnuweeb.org>
Cc:     GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Subject: [PATCH 1/1] tools/nolibc/stdio: Add format attribute to enable printf warnings
Date:   Mon,  9 May 2022 06:54:45 +0000
Message-Id: <20220509065445.3912334-2-alviro.iskandar@gnuweeb.org>
In-Reply-To: <20220509065445.3912334-1-alviro.iskandar@gnuweeb.org>
References: <20220509065445.3912334-1-alviro.iskandar@gnuweeb.org>
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When we use printf and fprintf functions from nolibc, we don't get any
warning from the compiler if we have wrong arguments, for example the
following calls will compile silently:
```
  printf("%s %s\n", "aaa");
  fprintf(stdout, "%s %s\n", "xxx", 1);
```
Those calls are undefined behavior. We can catch it at compile time by
adding format attribute to those function declarations. After this
patch, we get the following warnings:
```
  warning: format `%s` expects a matching `char *` argument [-Wformat=]
  warning: format `%s` expects argument of type `char *`, but argument 4 has type `int` [-Wformat=]
```

Signed-off-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
---
 tools/include/nolibc/stdio.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index 15dedf8d0902..009dd6ae68f2 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -273,7 +273,7 @@ int vfprintf(FILE *stream, const char *fmt, va_list args)
 	return written;
 }
 
-static __attribute__((unused))
+static __attribute__((unused)) __attribute__((format(printf, 2, 3)))
 int fprintf(FILE *stream, const char *fmt, ...)
 {
 	va_list args;
@@ -285,7 +285,7 @@ int fprintf(FILE *stream, const char *fmt, ...)
 	return ret;
 }
 
-static __attribute__((unused))
+static __attribute__((unused)) __attribute__((format(printf, 1, 2)))
 int printf(const char *fmt, ...)
 {
 	va_list args;
-- 
Alviro Iskandar Setiawan