Modifying string literals or char arrays with pointer arithmetic

Question

I understand char* pString is read only, therefor pString[0]='A' will give a Segmentation fault, but why does pString2[0]='A' work in below code:

    char* pString  = "abcdefg";
    char tmpStr[80];
    strcpy(tmpStr, pString);
    char* pString2  = tmpStr;

    //pString[0] = 'A'; // gives segmentation error since string literal is read-only.
    pString2[0] = 'A'; // why this one works?

`pString2` is pointing to the first element of the (non-constant) array `tmpStr`. You can modify the contents of `tmpStr` as much as you'd like, using the array directly or through a pointer like `pString2`. — Some programmer dude, Dec 02 '21 at 14:42

score 2 · Answer 1 · answered Dec 02 '21 at 14:42

2

pString2 is not pointing to a string literal. It's pointing to the first element of a char array, and that array is not const, so writing to it is allowed.

answered Dec 02 '21 at 14:42

dbush

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Vlad from Moscow · Answer 2 · 2021-12-02T14:58:08.537

In this declaration

char* pString  = "abcdefg";

the string literal is the record "abcdefg" not the declared pointer.

The string literal in C has the type char[8]. Though in the type there is absent the qualifier const nevertheless you may not change a string literal. Any attempt to change a string literal results in undefined behavior.

In C++ opposite to C string literals have types of constant character arrays.

As for the declared pointer then it points to the first element of the string literal.

In this code snippet

char tmpStr[80];
strcpy(tmpStr, pString);

there is declared character array tmpStr. You may change the array.

In this statement

strcpy(tmpStr, pString);

elements of the string literal are copied in the character array.

And in the declaration below the declared pointer point to the first element of the array.

char* pString2  = tmpStr;

pString2[0] = 'A';

So you may change the array. You could use even the pointer pString instead of pString2 like

pString  = tmpStr;

pString[0] = 'A';

Pay attention to that you may initialize a character array with a string literal. So you could write for example

char tmpStr[] = "abcdefg";
*tmpStr = 'A';

or

tmpStr[0] = 'A';

Modifying string literals or char arrays with pointer arithmetic

2 Answers2