Question

Possible Duplicate:
Is it possible to modify a string of char in C?

I have a function to convert a char* to lower case. This is the function:

void toLower(char* word, int length)
{
    int i;

    for(i = 0; i < length; i++) {
        // this I can do
        // printf("%c", tolower(word[i]));

        // this throws segfault
        word[i] = tolower(word[i]);
    }
}

When I call it like this from main, it throws a segfault error:

char* needle = "foobar";
toLower(needle, strlen(needle));

I’m certain the issue is in the assignment here:

word[i] = tolower(word[i]);

But I can’t seem to figure out the corect way to do it. I tried passing it as char**, or doing *(word+i), but all cause the same issue.

Answer 1

You are attempting to change the constant string "foobar". Try:

char needle[] = "foobar";

This will create an array containing the string "foobar" (the compiler arranges to copy the data from the constant string "foobar" to your array needle where you can modify it).