How to convert an array of NSString to an array of c-strings?

Question

I have an NSArray<NSString*>* object, and I need to invoke a C API which takes in an array of strings as char**.

What's the best way to do this? The important note is the c-strings must not have a const modifier, so something like the following isn't good enough since UTF8String returns const char*:

NSArray<NSString*>* names = ...;
int len = args.count;
char* cNames[len];
for( int i = 0; i < len; i++ ) {
    cNames[i] = names[i].UTF8String;
};

If it does, nothing will make that modify the original NSArray. The above is basically the right approach. You just want to add the tedious malloc and strncpy to create a full copy. The answer is: the above, plus C. — Rob Napier
– Rob Napier, Commented Sep 23, 2016 at 2:09
Don't know where your strings are coming from, but be aware of multi-byte "characters" in these emoji-filled times. If your C API is not Unicode-aware and your strings are not ASCII-except-in-name you'll get strange results. — jscs
– jscs, Commented Sep 23, 2016 at 2:37
Good point, I may need to use the platform specific encoding. — zeroimpl
– zeroimpl, Commented Sep 23, 2016 at 3:19

Mobile Ben · Accepted Answer · 2016-09-25 03:34:10Z

1

You will want to do some dynamic memory as you cannot rely on the backing memory for UTF8String being released.

NSArray *strings = @[ @"String 1", @"Other string", @"Random String"];
char **cStrings = NULL;
NSInteger numCStrings = strings.count;

if (numCStrings) {
    cStrings = (char **)calloc(numCStrings, sizeof(char*))  ;

    if (cStrings) {
        // Safer to allocate memory for each string

        for (NSInteger i=0;i<numCStrings;i++) {
            NSString *nsString = strings[i];
            char *cString = (char *)malloc([nsString lengthOfBytesUsingEncoding:NSUTF8StringEncoding] + 1); // + 1 for \0

            if (cString) {
                strcpy(cString, nsString.UTF8String);
                cStrings[i] = cString;
            } else {
                // You should handle your error
            }
        }
    } else {
        // You should handle your error
    }
}

for (NSInteger i=0;i<numCStrings;i++) {
    NSLog(@"C-String (%ld): %s", i, cStrings[i]);
}

// Note you need to free your memory later!
// Do any additional setup after loading the view, typically from a nib.
for (NSInteger i=0;i<numCStrings;i++) {
    if (cStrings[i]) {
        // Free each string
        free(cStrings[i]);
    }
}

// Free the array
free(cStrings);

cStrings = NULL;
numCStrings = 0;

edited Sep 25, 2016 at 3:34

answered Sep 23, 2016 at 2:08

Mobile Ben

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2 Comments

zeroimpl Over a year ago

Thanks. Ugly but I guess it will have to do!

zeroimpl Over a year ago

One minor correction - nsString.length returns the length in the UTF-16 encoding. Need to use [nsString lengthOfBytesUsingEncoding:NSUTF8StringEncoding]

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