char * to string literal ?

Question

var1 must hold a string whether it is empty or not or my program will segfault. But gcc complains that the empty string literal is constant while var1 is not. The following is an example of what I am talking about.

How can I fix this ?

warning: assignment discards qualifiers from pointer target type

char *var1 = NULL;
if(var1 == NULL)
{
    var1 = malloc(strlen(var2) + 1);
    strcpy(var1, var2);
}else{
    var1 = ""; // warning points here
}

EDIT:

String literals are not modifiable so I use this flag to warn me:

-Wwrite-strings

Answer 1

Declare var1 as constant:

const char *var1 = NULL;

The const there means you won't change the contents of the thing pointed to (so, eg, you won't try to change the contents of the string literal). You're still free to change what the variable points to.

Answer 2

To assign a empty string, do something like this:

var1 = malloc(1); /* 1 byte */
var1[0] = '\0';

Answer 3

A string literal is of type char[]. When used in most contexts, a string literal (of type char[]) decays to a pointer to its first element (it becomes type char*).

If your compiler complains about assigning a value of type char* (your "") to an object of type char* (your var1), your compiler is broken.

Or, most likely, you are not invoking it as a 'C' compiler, but as a compiler of a language similar to C but not quite C.

Try: gcc -Wno-write-strings ... to disable that specific gcc extension :-)

---

Edit

A string literal is an array of characters (of type char[]), but it is not modifiable. For historic (or some other peculiar) reasons they are not of type const char[] as a "unmodifiable character array" looks like it should be.

With the "-Wwrite-strings" compiler option, gcc tries to be helpful and changes its definition of string literals.

If you want to be safe and have your string literals be of type const char[], just remember you're no longer compiling C (C89, C90, C99, whatever) and be happy. As other answers said: declare your pointers as const or copy the unmodifiable characters to modifiable objects.

Answer 4

String literals have type const char* (or maybe const char[]) in recent versions of C. It is unsafe to assign a string literal to a char* because modifying that char* results in undefined behaviour, reflected in their const type.