Re: [Tinycc-devel] Warning message about string assignment

[Stefanos]

On Sun, 25 Apr 2021 03:03:40 +0200
Vincent Lefevre <vincent@vinc17.net> wrote:

> On 2021-04-25 03:08:44 +0300, Stefanos wrote:
> > I use `tcc version 0.9.27 - 1432574 (x86_64 Linux)` and the code I'm 
> > testing is:
> > 
> >     #include <stdio.h>
> >     #include <string.h>
> > 
> >     int main(void)
> >     {
> >         char *s = "Hello, world!";
> >         printf("String length is %zu characters long.\n", strlen(s));
> >     }
> > 
> > I execute it with the following command:
> > 
> >     tcc -Wall -std=c11 -o test_strlen test_strlen.c
> > 
> > The warning is 
> > 
> >     tmp.c:6: error: assignment discards qualifiers from pointer target type
> > 
> > With GCC and Clang does not throw any warning message.
> > 
> > The command I used for both is:
> > 
> >     gcc -Wall -Wextra -Wpedantic -std=c11 -o test_strlen test_strlen.c
> >     clang -Wall -Wextra -Wpedantic -std=c11 -o test_strlen test_strlen.c
> 
> zira:~> gcc -Wwrite-strings -o test_strlen test_strlen.c
> test_strlen.c: In function ‘main’:
> test_strlen.c:6:13: warning: initialization discards ‘const’ qualifier from 
> pointer target type [-Wdiscarded-qualifiers]
>     6 |   char *s = "Hello, world!";
>       |             ^~~~~~~~~~~~~~~
> 
> Here's what the GCC man page says:
> 
>   -Wwrite-strings
>     When compiling C, give string constants the type "const char[length]"
>     so that copying the address of one into a non-"const" "char *" pointer
>     produces a warning.  These warnings help you find at compile time code
>     that can try to write into a string constant, but only if you have
>     been very careful about using "const" in declarations and prototypes.
>     Otherwise, it is just a nuisance.  This is why we did not make -Wall
>     request these warnings.
> 
> This allows one to detect buggy code like
> 
> #include <string.h>
> 
> int main(void)
> {
>   char *s = "Hello, world!";
>   memcpy (s, "foo", 4);
> }
> 
> zira:~> gcc -o test_strlen test_strlen.c
> zira:~> ./test_strlen
> zsh: segmentation fault (core dumped)  ./test_strlen
> 

I had the impression that `-Wall -Wextra -Wpedantic` would include this specific
flag by default, but seems like it didn't.

Both GCC and Clang fail to detect the issue without the `-Wwrite-strings` flag.

Thank you Vincent for the valuable information; I learned something important 
today!

Cheers mate.