How can I pass a string to a C function from Objective-C and get a value back?

Question

I have an Objective-C view controller class, from which I am trying to call a straight-C (not Objective-C) function. I want to pass in a string variable by reference, set its value inside the C function, and then back in my view controller I want to convert this to a normal NSString object.

Since I can't pass in an NSString object directly, I need to create and pass in either a char pointer or a char array, and then convert it to an NSString object after the function returns.

Can anyone point me to a simple code example that shows how to do this? I'm not strong in either Objective-C or regular C, so manipulating strings is extremely difficult for me.

You can pass Obj-C objects and manipulate them using pure C if you really want to. What does the function you are trying to use do? Is it something you wrote yourself or are you using someone else's code? — kubi
– kubi, Commented Oct 15, 2010 at 13:30

filipe · Accepted Answer · 2010-10-15 13:33:06Z

7

Maybe something like this

bool doSomethingToMyString(const char* originalString, char *buffer, unsigned int size)
{
    bool result = 0;
    if (size >= size_needed)
    {
        sprintf(buffer, "The new content for the string, maybe dependent on the originalString.");
        result = 1;
    }
    return result;
}

...
- (void) objectiveCFunctionOrSomething:(NSString *)originalString
{
    char myString[SIZE];
    if (doSomethingToMyString([originalString UTF8String], myString, SIZE))
    {
        NSString *myNSString = [NSString stringWithCString:myString encoding:NSUTF8StringEncoding];
        // alright!
    }
}

or, you know, something to that effect.

answered Oct 15, 2010 at 13:33

filipe

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2 Comments

MusiGenesis Over a year ago

I think stringWithCString is deprecated (not that there's anything wrong with that). :)

filipe Over a year ago

I'm not sure, but I think stringWithCString: is deprecated but stringWithCString:encoding: isn't. I may be mistaken though.

Nimrod · Accepted Answer · 2010-10-15 13:16:10Z

1

Check out the following in the NSString docs:

– cStringUsingEncoding:
– getCString:maxLength:encoding:
– UTF8String
+ stringWithCString:encoding:

answered Oct 15, 2010 at 13:16

Nimrod

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2 Comments

MusiGenesis Over a year ago

My main problem is passing a char array into a C function and getting a value back out, so it's more of a C problem than an Objective-C problem.

Nimrod Over a year ago

Try char *foo = [myString UTF8String]; char *bar = cfunction(foo); NSString *newString = [NSString stringWithCString:bar encoding:NSUTF8StringEncoding];

MusiGenesis · Accepted Answer · 2010-10-15 13:34:39Z

1

Well, I got it to work. Here is my C function:

int testPassingChar(char buffer[]) {    
    strcpy(buffer, "ABCDEFGHIJ");
    return 0;
}

And then from Objective-C:

char test[10];
int i;
i = testPassingChar(test);
NSString* str = [[NSString alloc] initWithBytes:test length:sizeof(test) 
    encoding:NSASCIIStringEncoding];

answered Oct 15, 2010 at 13:34

MusiGenesis

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5 Comments

walkytalky Over a year ago

You should generally avoid having functions do stuff with passed-in buffers without knowing what size they are. (And by "generally" I really mean "always".)

filipe Over a year ago

what walkytalky said. And in your example if you strcpy "ABCDEFGHIJ" into a buffer that is 10 in size, you'll get a buffer overflow from the "\0" terminating character.

MusiGenesis Over a year ago

@walkytalky: I would indeed prefer to pass in a pointer instead of a pre-allocated buffer, but the above code is the only thing I'm able to get working right now. Also, if I allocate the size of my buffer in the C function, I then have to clean up the memory in Objective-C, which seems odd to me.

rpetrich Over a year ago

@MusiGenesis: You most certainly do have a buffer overflow. Whether or not it clobbers something important depends on how the compiler decides to arrange the array on the stack (often, the optimization level will affect this)

walkytalky Over a year ago

@MusiGenesis: The problem of not knowing the length is the same whether you pass in a pointer or a stack-allocated array (although the consequences of overrunning might differ). It's perfectly acceptable to do C-style dynamic allocations with malloc/free in Objective-C -- there are objecty alternatives if you really want them, but probably superfluous in a case like this. Have your function tell you how much space it needs so you can avoid using a fixed length.

diatrevolo · Accepted Answer · 2010-10-17 03:37:16Z

1

Why don't you wrap your C function in Objective-C?

-(NSString*)testPassingCharWithStringLength:(int)whateverLength {
     char *test = malloc(sizeof(char) * whateverLength);  
     //do whatever you need to *test here in C
     NSString *returnString = [NSString stringWithUTF8String:test];
     free(test);
     return returnString;
}

...for example...

answered Oct 17, 2010 at 3:37

diatrevolo

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