how to encode a UInt32 scalar type into a NSData object

Question

I am currently creating this NSData object. I would like to put in sever different objects that are of type NSString and UInt32. I know how to put a NSString into my NSData object, but I don't know how to do this with a UInt32 scalar type.

this is how I do it with a NSString

- (void) constructRequest
{
    NSString *mystring = [[NSString alloc] initWithString:[self addMethodName]];
    UInt32 protocolInt = [self addProtocolVersion];

    NSData* data=[mystring dataUsingEncoding:NSUTF8StringEncoding];
    [data writeToFile:@"/Users/imac/Desktop/_dataDump.dat" atomically:YES];

}

score 4 · Answer 1 · answered Feb 24 '12 at 01:14

4

So I have figured it out, and instead of just updating my question I will put in the answer so others can see that this question has been answered if they are looking to do something similar.

code is as follows

- (void) constructRequest
{
    //NSString *mystring = [[NSString alloc] initWithString:[self addMethodName]];
    UInt32 protocolInt = [self addProt];

    NSData * data = [[NSData alloc] initWithBytes:&protocolInt length:sizeof(protocolInt)];

    //NSData* data=[mystring dataUsingEncoding:NSUTF8StringEncoding];
    [data writeToFile:@"/Users/imac/Desktop/_dataDump.dat" atomically:YES];

}

answered Feb 24 '12 at 01:14

C.Johns

10,185
20
102
156

2

Don't do that. It's not endian-safe (PowerPC Macs and Intel Macs will read it differently.) – Jonathan Grynspan Feb 24 '12 at 01:28
hrmm. So how to I make it endian-safe? currently I am trying to make it big-endian, I have outputted the result to a .dat folder on my desktop and opened that with HexEdit to check the results, and its currently reading it as big-endian.. any suggestions on how I can make this safe. – C.Johns Feb 24 '12 at 01:48
2

Byte-swap it to a known order first using `CFSwapInt32HostToBig()` or `CFSwapInt32HostToLittle()`. When reading it in, swap the resulting integer back to native order with `CFSwapInt32BigToHost()` or `CFSwapInt32LittleToHost()` as appropriate. – Jonathan Grynspan Feb 24 '12 at 15:02
okay cool, thanks for the heads up. I'm going to have to give this more investigation.. Although I know what your talking about I have only ever heard of this while doing some c++ – C.Johns Feb 26 '12 at 19:16

score 1 · Answer 2 · answered Feb 24 '12 at 01:24

1

Does it need to be NSData? You could use NSString or NSNumber (both can be saved in a property list).

Your scheme doesn't really distinguish between a 4-byte string and a UInt32, if that matters.

answered Feb 24 '12 at 01:24

David Dunham

8,139
3
28
41

I am planning on adding a leading Uint16 number to the start of each property so when it makes it to the server the server can deal with each section in the reciver class. Is that what you were referring to? The only issue being is that I am not sure on how to create one NSData structure out of several... im investigating that currently. – C.Johns Feb 24 '12 at 01:47
NSData is just data. But what you said you wanted to do was save different objects. Instead of focusing on NSData, just save the objects (NSString and NSNumber), as keyed values in a property list (NSDictionary). – David Dunham Feb 29 '12 at 17:50

rotoava · Answer 3 · 2015-10-29T07:52:13.097

You can use htonl(),htons(), ntohl() and ntohs() to make it endian-safe.

 htonl()--"Host to Network Long int"     32Bytes
 ntohl()--"Network to Host Long int"     32Bytes
 htons()--"Host to Network Short int"   16Bytes
 ntohs()--"Network to Host Short int"   16Bytes

Example:

- (void)testExample {

UInt32 length = 0x1a2b3c4d;
NSLog(@"%x", length);
length = htonl(length);
NSLog(@"%x", length);
NSMutableData *data = [[NSMutableData alloc] init];
[data appendBytes:&length length:4];
NSLog(@"%@", data);

}

print:

2015-10-29 15:46:49.224 UPHTTP-iOS[3896:101301] 1a2b3c4d
2015-10-29 15:46:49.224 UPHTTP-iOS[3896:101301] 4d3c2b1a
2015-10-29 15:46:49.224 UPHTTP-iOS[3896:101301] <1a2b3c4d>

how to encode a UInt32 scalar type into a NSData object

3 Answers3