using ‘-Wcast-qual’ option to check the compilation - c

Question

I am reading an introduction to gcc in which it says:
‘-Wcast-qual’ this option warns about pointers that are cast to remove
a type qualifier, such as const. For example, the following function
discards the const qualifier from its input argument, allowing it to
be overwritten:
void
f (const char * str)
{
char * s = (char *)str;
s[0] = ’\0’; }
The modification of the original contents of str is a violation of its
const property. This option will warn about the improper cast of the
variable str which allows the string to be modified.
I tried to repeat this, expecting a warning, but there's no warning when it is compiled. My code is as follows:
#include <stdio.h>
void f(const char * str);
int main() {
char Str = 'a';
char * myStr;
myStr = & Str;
printf ("result: %c \n", * myStr);
f(myStr);
printf ("result: %c \n", * myStr);
return 0;
}
void
f (const char * str)
{
char * s = (char *)str;
s[0] = '\0';
}
the command is : gcc -Wcast-qual castqual.c
Can anyone explain this inconsistency?

Answer 1

Per the comments, it turns out that the OP is using a mac, where gcc as a command is a wrapper around clang - the llvm based compiler.
The version of clang that's being used accepts the option but does nothing with it. Support for the warning is being added and should appear in the XCode 7/clang 7 timeframe; however the question was about gcc and not about clang.
In this case, you need to install a proper copy of gcc using, either manually or using a package manager such as homebrew or macports. Once you've installed gcc, it becomes available as, for example, gcc-4.8, gcc-5, etc (depends on the package manager). When you want to compile code with actual gcc, you use: gcc-4.8 -Wcast-qual testcode.c. If you're using autoconf tools you can use export CC=gcc-4.8; export CXX=g++-4.8, etc. and then that compiler will be picked up by the package.
Verifying that you're calling gcc, rather than clang requires checking the output from gcc --version. On a clang wrapper you see things like:
$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.4.0
Thread model: posix
whereas on an actual gcc instance you see:
$ gcc-5 --version
gcc-5 (Homebrew gcc 5.1.0) 5.1.0
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
imho aside: you can make a symlink to the specific gcc-4.8 called gcc, and get it to invoke directly like that but it can break other things.