C中的char * vs const char *

Question

char* str ="Hello";

In the above code the literal "Hello" is stored ... in the DATA segment and it is read-only. So isn't it better to always declare it:

const char* str = "Hello";

to avoid improper code such as:

*(str+1) = 't';

Answer 1

"Hello" is stored ... in the DATA segment

"DATA" or .data would refer to the segment where initialized read/write variables with static storage duration go. String literals are not stored there, but more likely in something called .rodata , or possibly in .text along with the code. See String literals: Where do they go?

So isn't it better to always declare it: const char* str = "Hello";

Yes, you should always const qualify pointers to string literals. This is universally considered best practice in C (and mandatory in C++).

Answer 2

Yes, it is better to declare pointers to string literals as const char * . The name "literal" very strongly suggests that this is something that is supposed to be immutable. Unfortunately, C treats string literals as arrays of char , not const char , and unlike numeric literals storage for the string literal must be allocated from somewhere. It is not guaranteed that this storage will be taken from a read-only segment like .rodata - I've worked with implementations in the past where string literals were writable.

The behavior on attempting to modify the contents of a string literal is undefined - the language places no requirement on the compiler or the runtime environment to handle the situation in any particular way. You may get a runtime error, or the string may be left unchanged, or the string may be modified, any something entirely different could happen.

So by declaring the pointer as const char * , you're at least making it harder to accidentally modify a literal through that pointer. Just remember that const doesn't mean "put this thing in read-only memory" - it only means "issue a diagnostic if someone tries to modify this thing".