CODEWITHSUNDEEP
iosobjective-ciphoneipadendianness
Zach
Zach

Reputation: 1964

How to detect endianess to keep a consistent byte order with iOS

I am having an issue with iOS byte orders. We are sending messages over BLE, some of the values are stored as NSInteger objects

I have an older (but still supported) iPad appears to store integers with big-endian byte order which is required for our peripheral to correctly interpret the value, however when I run the same application on a newer iPhone it doesn't work and the endianness is reversed.

NSInteger number = 230; /* this is actually declared elsewhere, this is just for the example */

short value = (short)number;

NSData* data = [NSData dataWithBytes:(void*)&value length:sizeof(value)];

int intData = [self intFromNSData:data];

Result values coming through are

On iPad: 230 (00E6)
On iPhone: 58880 (E600)

As you can see the byte order is different on the two devices, I just want to know how I can detect and, if necessary, swap the bytes before I send them to the peripheral device. Or is there a different way to store these numbers that will be consistent across all devices?

Upvotes: 3

Views: 1769

Answers (1)

rmaddy
rmaddy

Reputation: 318824

You don't need to detect the endianness of the device running your app. To ensure that you always send the bytes as big-endian, use the appropriate NSSwapHostXXXToBig function.

short value = 230;
short beValue = NSSwapHostShortToBig(value);
NSData *data = ...

Or you can use the CFSwapXXXHostToBig functions. These work well with the various fixed size data types such as uint16_t.

uint16_t value = 230;
uint16_t beValue = CGSwapInt16HostToBig(value);
NSData *data = ...

Each of these have corresponding functions to convert the big-endian value back to a proper host value if needed.

Upvotes: 4

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