OSX Mavericks get tags of a file in c program

Question

I need to get the usertags for a file in c program. I am aware of "mdls -name kMDItemUserTags FILENAME" command to get this details. But i need to do it in c code. Is there any way from which i can get the values directly instead of running this command and parsing.

Answer 1

You can do it via the NSURL Resource Key NSURLLabelColorKey which uses an NSColor to specify the colour.

Therefore it cannot be done in C per se, however you can write an Objective-C implementation file with a C function entry point so it can be called from C (as noted by @Alex MDC in the comments you can use CoreFoundation and do it in C directly, but I would always favour Foundation where possible as it's easier to use when using ARC).

Given NSColor is used to specify the colour, you'll need to create a struct to hold the RGB values and translate between that struct and NSColor yourself.

Something like (untested):

OSXFileLabelColour.h:

#ifdef __cplusplus
extern "C" {
#endif

typedef struct {
    int isSet;          // if 0 then no colour set
    float r;
    float g;
    float b;
} LabelColour;

/* Return 1 for success or 0 for failure */
extern int getFileLabelColour(const char *filename, LabelColour *colour);

#ifdef __cplusplus
}    // extern "C"
#endif

OSXFileLabelColour.m:

#import <Foundation/Foundation.h>
#import "OSXFileLabelColour"

int getFileLabelColour(const char *filename, LabelColour *colour)
{
    int retval = 0;
    NSURL *url = [NSURL fileURLWithPath:@(filename)];
    if (url) {
        NSColor *nscolor = nil;
        NSError *error = nil;
        if ([url getResourceValue:&nscolor
                           forKey:NSURLLabelColorKey
                            error:&error]) {
            if (nscolor) {
                CGFloat r, g, b, a;
                [nscolor getRed:&r green:&g blue:&b alpha:&a];
                colour->isSet = 1;
                colour->r = r;
                colour->g = g;
                colour->b = b;
            } else {
                colour->isSet = 0;
            }
            retval = 1;                
        } else {
            NSLog(@"Failed to get colour label for file '%s': %@", filename, [error localizedDescription]);
        }
    } else {
        NSLog(@"Failed to create URL for file '%s'", filename);
    }
    return retval;
}