Objective-C - Converting NSString to a C string [duplicate]

Question

Possible Duplicate:
objc warning: “discard qualifiers from pointer target type”

I'm having a bit of trouble converting an NSString to a C string.

const char *input_image = [[[NSBundle mainBundle] pathForResource:@"iphone" ofType:@"png"] UTF8String];
const char *output_image = [[[NSBundle mainBundle] pathForResource:@"iphone_resized" ofType:@"png"] UTF8String];

const char *argv[] = { "convert", input_image, "-resize", "100x100", output_image, NULL };

// ConvertImageCommand(ImageInfo *, int, char **, char **, MagickExceptionInfo *);
// I get a warning: Passing argument 3 'ConvertImageCommand' from incompatible pointer type.
ConvertImageCommand(AcquireImageInfo(), 2, argv, NULL, AcquireExceptionInfo());

Also when I debug argv it doesn't seem right. I see values like:

argv[0] contains 99 'c' // Shouldn't this be "convert"?
argv[1] contains 0 '\100' // and shouldn't this be the "input_image" string?

Joe · Accepted Answer · 2011-09-21 13:22:41Z

3

Richard is correct, here is an example using strdup to suppress the warnings.

char *input_image = strdup([@"input" UTF8String]);
char *output_image = strdup([@"output" UTF8String]);

char *argv[] = { "convert", input_image, "-resize", "100x100", output_image, NULL };

ConvertImageCommand(AcquireImageInfo(), 2, argv, NULL, AcquireExceptionInfo());

free(input_image);
free(output_image);

answered Sep 21, 2011 at 13:22

Joe

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Richard · Accepted Answer · 2011-09-21 12:56:29Z

2

The warning is because you are passing a char const ** (pointer-to-pointer-to-const-char) where the API expects a char ** (pointer-to-pointer-to-char, in particular a NULL-terminated list of non-const C strings).
My point being the const makes the pointer types incompatible. The safe way to get around that is to copy the UTF8 strings into non-const C-string buffers; the unsafe way is to cast.

answered Sep 21, 2011 at 12:56

Richard

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3 Comments

gotnull Over a year ago

I thought as much thanks to @LucasTizma's comments. How would I go about casting the API char * to a const char *?

Joe Over a year ago

You will need to use something like strdup to copy the strings, then you will be responsible for freeing all duplicated strings. As I said in the other comment this is the fault of ConvertImageCommand. If this is your code the best way is to change the parameter to be const char **.

Richard Over a year ago

don't forget to free() the string strdup returns.

Manlio · Accepted Answer · 2011-09-21 12:21:15Z

1

Use cStringUsingEncoding or getCString:maxLength:encoding:

answered Sep 21, 2011 at 12:21

Manlio

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7 Comments

CIFilter Over a year ago

UTF8String also returns a C string.

gotnull Over a year ago

@LucasTizma: See my updated code. I'm still receiving the warning.

CIFilter Over a year ago

See stackoverflow.com/questions/329555/…

CIFilter Over a year ago

But you're declaring a C array of char *s, not const char *s, right?

Joe Over a year ago

That is the fault of ConvertImageCommand(), if ConvertImageCommand() does not modify the strings in the array in any way it should have declared it as a const

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