What should happen, when we try to modify a string constant?

问题

#include<stdio.h>
#include<string.h>

int main()
{
    int i, n;
    char *x="Alice"; // ....... 1
    n = strlen(x);   // ....... 2
    *x = x[n];       // ....... 3
    for(i=0; i<=n; i++)
    {
        printf("%s ", x);
        x++;
    }
    printf("\n");
    return 0;
}

String constant cannot be modified. In the above code *x means 'A'. In line 3 we are trying to modify a string constant. Is it correct to write that statement? When I run this code on Linux, I got segmentation fault. But on www.indiabix.com, they have given answer:

If you compile and execute this program in windows platform with Turbo C, it will give lice ice ce e It may give different output in other platforms (depends upon compiler and machine). The online C compiler given in this site will give Alice lice ice ce e as output (it runs on Linux platform).

回答1:

Your analysis is correct. The line

*x = x[n];

is trying to modify a string literal, so it's undefined behavior.

BTW, I checked the website that you linked. Just browsing it for two minutes, I've already found multiple incorrect code samples (to name a few, using gets, using char(not int) to assign return value of getchar, etc), so my suggestion is don't use it.

回答2:

Your analysis is correct, but doesn't contradict what you quoted.

The code is broken. The answer already acknowledges that it may behave differently on different implementations, and has given two different outputs by two different implementations. You happen to have found an implementation that behaves in a third way. That's perfectly fine.

回答3:

Modification of a string literal is Undefined Behaviour. So the behaviour you observe, and the two described, are consistent with the requirements of the C standard (as is emailing your boss and your spouse, or making demons fly out of your nose). Those three are all actually quite reasonable actions (modify the 'constant', ignore the write, or signal an error).

With GCC, you can ask to be warned when you assign the address of a string literal to a pointer to (writable) char:

cc -g -Wall -Wextra -Wwrite-strings   -c -o 27211884.o 27211884.c
27211884.c: In function ‘main’:
27211884.c:7:13: warning: initialization discards ‘const’ qualifier from pointer target type [enabled by default]
     char *x="Alice"; // ....... 1
             ^

This warning is on by default when compiling C++, but not for C, because char* is often used for string literals in old codebases. I recommend using it when writing new code.

There are two correct ways to write the code of the example, depending on whether you want your string to actually be constant or not:

const char *x = "Alice";

char x[] = "Alice";

回答4:

In this code, Memory for "Alice" will be in read-only data section of executable file and x is a pointer pointing to that read-only location. When we try to modify read-only data section, it should not allow. But char *x="Alice"; is telling the compiler that x is declared as a pointer to character.i.e., x is pointing to a character which can be modified(not read-only). So compiler will think that it can be modified. Thus line *x = x[n]; will behave differently on different compilers. So it will be undefined behavior.
Correct way of declaration to assign string literal is, as below

 const char *x ="Alice";

Then only compiler behavior can be predicted.

来源：https://stackoverflow.com/questions/27211884/what-should-happen-when-we-try-to-modify-a-string-constant

标签

Linux

pointers

string-constant