C Static variables on function inputs

Question

I'd like to save an input to a function and manipulate it across multiple calls. However, if I do the following...

int testFunc(char *toString) {
        static char *toChange = toString;
        static int counter = 0;

        toChange[counter] = 'A';
        printf("String is being corrupted... %s\n", toChange);
        counter++;

        return 0;
}

I get an error saying that the input toChange cannot be set to a non-static variable. I have been trying to figure out how to get around this but I cannot find any answers.

You should initiate your pointer to NULL, so, in the first call, you copy the string to your local variable, then, after that, you do not change its content anymore. — Eduardo Fernandes
– Eduardo Fernandes, Commented Oct 16, 2019 at 21:00

Vlad from Moscow · Accepted Answer · 2019-10-16 21:07:47Z

2

Static variables shall be initialized by constant expressions.

Write instead something like

 int testFunc(char *toString) {
    static char *toChange;
    static int counter;

    if ( toChange == NULL || toString == NULL ) 
    {
        toChange = toString;
        counter = 0;
    } 

    toChange[counter] = 'A';
    printf("String is being corrupted... %s\n", toChange);
    counter++;

    return 0;

}

edited Oct 16, 2019 at 21:07

answered Oct 16, 2019 at 20:58

Vlad from Moscow

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3 Comments

Adrian Mole Over a year ago

It would be even better to explicitly initialize toChange to NULL! But I think this will be the default, anyway (static data default to zero, I think?)

Eduardo Fernandes Over a year ago

I am not sure if every compiler initializes local variables to zero. I recommend you never count on that.

Vlad from Moscow Over a year ago

@EduardoFernandes It is not just a local variable it is also a static variable and static variables are initialized by the compiler.

Eduardo Fernandes · Accepted Answer · 2019-10-16 21:07:41Z

0

Do this:

int testFunc(char *toString) {
    static char *toChange = NULL;
    static int counter = 0;
    if (toChange == NULL) {
        toChange = (char*)malloc(strlen(toString) + 1);
        memset(toChange, 0, strlen(toString) + 1);
        strcpy(toChange, toString);
    }
    toChange[counter] = 'A';
    printf("String is being corrupted... %s\n", toChange);
    counter++;
    return 0;
}

answered Oct 16, 2019 at 21:07

Eduardo Fernandes

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