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

[Vincent Lefevre]

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

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Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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