初始化结构时EXC_BAD_ACCESS

Question

I'm developing an app in Objective-C/OSX (first try). I had a struct (NSDevice custom type) which need to be accessed as a public/extern variable to allow the different interface of the class to access it.

Here is the declaration in the header.

struct NSDevice{
    LIBMTP_raw_device_t * usbrawdevice;
    int numusbrawdevice;
    uint32_t rawdeviceID;

    LIBMTP_mtpdevice_t *device;
};

extern struct NSDevice *Device;

@interface DeviceManager : NSObject

- (void) openDevice;
- (void) closeDevice;

@end

and how I implement it in the source file

#import "DeviceManager.h"

struct NSDevice *Device = NULL;


@implementation DeviceManager



- (id)init{
    self = [super init];
    if(self){
        NSLog(@"Init");
        LIBMTP_Init();

        Device->device = NULL;
        Device->numusbrawdevice = 0;
        Device->rawdeviceID = 0;
        Device->usbrawdevice = NULL;
    }
    return self;
}

- (NSMtp_error) openDevice {
    LIBMTP_error_number_t error = LIBMTP_ERROR_GENERAL;

    NSLog(@"Opening Device");

    error = LIBMTP_Detect_Raw_Devices(&Device->usbrawdevice, &Device->numusbrawdevice);

When trying to init the struct in the init interface, I got an EXC_BAD_ACCESS. Any idea ?

Can I use the "struct" or is there a better way in Objective-C ?

Thx

Answer 1

Seb, you need to allocate the space for Device .

Try doing this:

- (id)init{
    self = [super init];
    if(self){
        NSLog(@"Init");
        LIBMTP_Init();

        Device = malloc(sizeof(NSDevice));
        Device->device = NULL;
        Device->numusbrawdevice = 0;
        Device->rawdeviceID = 0;
        Device->usbrawdevice = NULL;
    }
    return self;
}

and declare your structure like this:

typedef struct {
    LIBMTP_raw_device_t * usbrawdevice;
    int numusbrawdevice;
    uint32_t rawdeviceID;

    LIBMTP_mtpdevice_t *device;
} NSDevice;

Also, I highly recommend changing the name of NSDevice to SebDevice or something that doesn't start with NS as those prefix characters usually signify something built into the MacOS SDK and it's going to confuse anyone else who has to look at your code after you depart the project.

And one last thing, global variables like this shouldn't start with capital letters. In Objective-C, best practice is for variables to start with lower case letters or maybe a g (for global) or an underscore. Change Device to gDevice .