Question

Sorry, I’m not even sure how to ask, since I’m a complete newbie at C, pointers and stuff like that. There’s a function that accepts an argument: char **arg. If I write that argument like so:

char *cargs[] = {"blah", NULL};

and pass it to the function:

function(cargs);

it works. but … I have an NSArray of NSStrings and I need to make this array out of values from NSArray. I figured it should be a matter of creating a C array of the same element count as NSArray and copy the strings, converting them with cStringUsingEncoding. But I honestly have no idea how to do this, since I get confused with all those pointers and such. Any help would be appreciated.

Answer 1

Well, the rough steps can be:

1. use count method of NSArray to know how many NSStrings are there in the NSArray.

2. use malloc to allocate memory for cargs, something like this

   char **cargs = (char **) malloc(sizeof(char *) * count);
   

   by your example, you may need to one more room for NULL which will be at the end of cargs.

3. use a loop and objectAtIndex: of NSArray to get out the NSStrings, like
   NSString *nsstring = [array objectAtIndex:index];

4. use method cStringUsingEncoding: to get the c-string out, better make a copy

5. put these c-string pointers in cargs

6. pass cargs to your function, clean and free things needed to.

It’s a lot of work. ‘Cause the mix of c and obj-c stuff. And a lot of manual malloc and free , messy stuff. Can’t you avoid it?

–add sample code–

I’m not quite sure what your real intent is. Hope this will help.

void func(char **arg)
{
    int i;
    for(i = 0; arg[i] != NULL; i++) {
        printf("%d=%s\n", i, arg[i]);
}
}
int main(int argc, const char *argv[])
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    NSString *s1 = [NSString stringWithString:@"first"];
    NSString *s2 = [NSString stringWithString:@"second"];
    NSString *s3 = [NSString stringWithString:@"third"];

    NSArray *array = [NSArray arrayWithObjects: s1, s2, s3, nil];
    //by now, we have an NSArray of three NSStrings

    int count = [array count];
    char **cargs = (char **) malloc(sizeof(char *) * (count + 1));
    //cargs is a pointer to 4 pointers to char

    int i;
    for(i = 0; i < count; i++) {
        NSString *s = [array objectAtIndex:i];//get a NSString
        const char *cstr = [s cStringUsingEncoding:NSUTF8StringEncoding];//get cstring
        int len = strlen(cstr);//get its length
        char *cstr_copy = (char *) malloc(sizeof(char) * (len + 1));//allocate memory, + 1 for ending '\0'
        strcpy(cstr_copy, cstr);//make a copy
        cargs[i] = cstr_copy;//put the point in cargs
    }
    cargs[i] = NULL;

    func(cargs);//call the function to do something

    for(i = 0; i < count; i++) {
        free(cargs[i]);
    }
    free(cargs);

    [pool drain];
    return 0;
}