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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.

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2 Answers 2

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6

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;
}
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11 Comments

Why send lengthOfBytesUsingEncoding:? cStringUsingEncoding: will automatically create a string of the right length; indeed, you can't tell it to use a different length.
Well, i'd be glad to avoid it, but i'm using AuthorizationExecuteWithPrivileges to run a shell script with elevated privileges and i don't know any other way to do it.
char* arg; arg = (char *)malloc([a count] + 1); for (int i = 0; i < [a count]; i++) { arg[i] = [[a get:i] cStringUsingEncoding:NSUTF8StringEncoding][0]; } arg[[a count]] = 0; This is what i've come up with with the help of my friend, but it breaks the AuthorizationExecuteWithPrivileges ... How do i format the code in the comment? :(
Well, the problem actually is ... how do i assign a cString to that array's element? I've tried: arg[1] = "test" - works. *arg[1] = [@"test" cStringUsingEncoding:NSUTF8StringEncoding][0]; - doesn't work
Marius: cStringUsingEncoding: returns a C string. A C string is an array of characters (char), ending with a zero byte ('\0'); cStringUsingEncoding: returns the address of that array of characters. Saying […cStringUsingEncoding:…][0] takes the first char in the array—the first character of the C string. Don't try to access a specific character of the string; set arg[i] to the pointer to the whole string, as returned by cStringUsingEncoding:.
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@yehnan your answer is GREAT - especially because I thought that const ** cStyle[arrays] were NOT able to be created dynamically... (I really wish there was somewhere with just basic information about C pointers and variables and crap... I find the most basic "C primitives" to be far more bewildering than anything else...) Here's your nice little example "bundled up" into a function... "for the children..."

char ** cArrayFromNSArray ( NSArray* array ){
   int i, count = array.count;
   char **cargs = (char**) malloc(sizeof(char*) * (count + 1));
   for(i = 0; i < count; i++) {        //cargs is a pointer to 4 pointers to char
      NSString *s      = array[i];     //get a NSString
      const char *cstr = s.UTF8String; //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;
  return cargs;
}
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