Can we cast const char * to char * and change the contents: is this safe operation?

Question

char *p, *y;

const char *x ;

p = (char *) malloc(5);
snprintf(p, 4, "test");

x = p;

y = (char *) x;

snprintf(y, 4, "abcd");

Can we cast const char * to char * and change the contents?

Thanks Kumar

@self. Ok, strictly speaking not unsafe, but it is just really not a good idea as outlined in the linked answer. — Kninnug, Dec 05 '13 at 00:16
@Kninnug It is safe as far as C language is concerned, please don't be misleading. — this, Dec 05 '13 at 00:18
@self. Fair enough: removing comment and changing to: The [cast of `malloc`'s result](http://stackoverflow.com/a/605858/249237) is not a good idea. — Kninnug, Dec 05 '13 at 00:19

Mahmoud Fayez · Answer 1 · 2013-12-05T00:42:12.187

Depending on the compiler warning level it may cause build break. If it is not causing a build break because your warning level is low then I guess you better be sure that you are trying to manipulate a var that other developers would think it is a constant!!

Also keep in mind you will end with non null terminated strings here, Always remember I am writing this code for a company and other developers would touch it sooner or later. So stick to the normal coding standards and avoid any mysterious code that may waste your college's time or your time in the future.

score 1 · Answer 2 · answered Dec 05 '13 at 00:40

While in this situation it is safe, as we know that the value pointed to by x is not constant. It is generally not safe to cast a const char * to char * and then change its contents. That's what the const qualifier is for: it indicates to compiler that the value that the pointer points to will not – and the compiler will make sure that it cannot – be changed. In some cases the compiler can then store this in read-only memory and so changing it then would likely cause segmentation faults.

If we remove the cast from the assignment we get the following warning:

warning: assignment discards 'const' qualifier from pointer target type

But with the cast you're basically telling the compiler that you know better and it will obey.

A (trivial) case where this would go disastrously wrong is as such:

const char * x = "Hello world!";
// If I now try to change x the compiler will throw an error
char * y = (char *) x; // <-- cast curbs compiler complaints
// I can now change y (and through it x) which will cause undefined behaviour
// as the string in x is most likely placed in read-only memory
y[0] = 'h'; // <-- Problems!

In the end, if you ever need to do this you should really consider what is really going on in your program.

score 1 · Answer 3 · answered Dec 05 '13 at 00:52

The C standard does permit you to cast away the const qualifier.

You may use the resulting pointer to modify the pointed-to object if it was not originally defined as const and is not a string literal. The behavior is not defined if the object was defined as const or is a string literal.

Can we cast const char * to char * and change the contents: is this safe operation?

3 Answers3