Why does GCC show duplicate warnings for bad printf format specifier?
I'm curious why GCC shows me two identical warnin开发者_如何学Pythongs when compiling this file:
$ cat test.c
#include <stdio.h>
int main (int argc, char const *argv[])
{
long foo = 0l;
printf("%i\n", foo);
return 0;
}
$ gcc-4.2 -Wall test.c
test.c: In function ‘main’:
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
Interestingly, Clang also gives two warnings:
$ clang test.c
test.c:6:14: warning: conversion specifies type 'int' but the argument has type 'long' [-Wformat]
printf("%i\n", foo);
~^ ~~~
%ld
test.c:6:14: warning: conversion specifies type 'int' but the argument has type 'long' [-Wformat]
printf("%i\n", foo);
~^ ~~~
%ld
2 warnings generated.
Any ideas?
For info:
$ gcc-4.2 -v
Using built-in specs.
Target: i686-apple-darwin11
Configured with: /private/var/tmp/gcc/gcc-5666.3~278/src/configure
--disable-checking --enable-werror --prefix=/usr --mandir=/share/man
--enable-languages=c,objc,c++,obj-c++
--program-transform-name=/^[cg][^.-]*$/s/$/-4.2/ --with-slibdir=/usr/lib
--build=i686-apple-darwin11 --program-prefix=i686-apple-darwin11-
--host=x86_64-apple-darwin11 --target=i686-apple-darwin11
--with-gxx-include-dir=/include/c++/4.2.1
Thread model: posix
gcc version 4.2.1 (Apple Inc. build 5666) (dot 3)
$ clang -v
Apple clang version 2.1 (tags/Apple/clang-163.7.1) (based on LLVM 3.0svn)
Target: x86_64-apple-darwin11.1.0
Thread model: posix
EDIT: the 'multi-architecture' hypothesis a few have suggested sounded good, but I'm not sure it's right. If I force a single architecture with -arch
, I get two warnings. If I specify -arch x86_64 -arch i386
, I get two sets of duplicate warnings!
$ gcc-4.2 -Wall -arch x86_64 test.c
test.c: In function ‘main’:
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
$ gcc-4.2 -Wall -arch x86_64 -arch i386 test.c
test.c: In function ‘main’:
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
test.c: In function ‘main’:
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
test.c:6: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
EDIT: I don't get dupes for all warning types. -Wformat
is the only one I've come across so far. For example, if I throw in an unused variable I only get one warning for that:
$ cat test.c
#include <stdio.h>
int main (int argc, char const *argv[])
{
long foo = 0l;
long bar;
printf("%i\n", foo);
return 0;
}
$ gcc-4.2 -Wall test.c
test.c: In function ‘main’:
test.c:7: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
test.c:7: warning: format ‘%i’ expects type ‘int’, but argument 2 has type ‘long int’
test.c:6: warning: unused variable ‘bar’
This is because Apple's stdio.h
header attaches a GCC format
attribute to its declaration of printf()
...
(e.g. see the declaration of printf()
here and the declaration of the __printflike()
macro here)
...but GCC (and Clang, due to it trying to be very GCC-compatible!) already has built-in knowledge that printf()
is a function which takes printf
-style arguments. You're getting one warning due to the built-in knowledge, and a second warning due to the explicit attribute.
You can demonstrate the same behaviour on other platforms (with at least several versions of GCC) by doing the same thing yourself:
extern int printf(const char *, ...) __attribute__((__format__ (__printf__, 1, 2)));
int main (int argc, char const *argv[])
{
long foo = 0l;
printf("%i\n", foo);
return 0;
}
If you're targetting two CPU architectures (ARMv6/ARMv7 on iOS, for instance, or i386/x86_64 on Mac), you'll see two copies of each warning because the compiler runs twice for each file (once for each architecture.)
On a Mac, you can get it up to 4 warnings per line if you enable PPC/PPC64 support. ;)
Edit: Matthew's got it spot-on in the accepted answer.
It looks like you're compiling for iOS. The code is being compiled multiple times for multiple architectures. The warning is being generating for each architecture.
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