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

[Elijah Stone]

On Sun, 25 Apr 2021, Stefanos wrote:

>        char *s = "Hello, world!";
>    tmp.c:6: error: assignment discards qualifiers from pointer target type
>
> With GCC and Clang does not throw any warning message.

(Aside: for me this is just a warning; are you compiling with -Werror?)

Without formal verification it's not possible to have static analysis 
that's 100% correct in all cases, so I'm not going to say this warning is 
'correct', or that gcc and clang are 'incorrect' to emit it. However, I do 
think the warning is justified and will defend it.

String literals in c are not mutable.  In this snippet:

char *s = "hello world";
s[0] = 'H';

the behaviour of the assignment on the second line is undefined.  On most 
systems the string literal will be placed in a read-only section of memory 
and the write will cause a fault.  It is not _incorrect_ to store the 
address of the string literal in a variable of type char*, nor is it 
incorrect to read from that variable, but it is _misleading_ because it 
implies that it would be correct to write to the values pointed thereby.

Due to a historical oversight (which happens to have been corrected in 
c++), string literals have type char[], not const char[].  Passing 
-Wwrite-strings makes them have const type; this is the same in gcc and 
tcc.  The only difference is that -Wall enables -Wwrite-strings in tcc and 
not in gcc.

 -E