I am running Xcode and I would like to dump out a NSData*. The variable in question is buffer. Is there a way to do this through the UI or the GDB debugger?
Edit
I've moved my notes into an answer.
No one has ever correctly answered the question. After 2 years I think it's time for one :)
Assuming you have in your code
NSData* myData;
Then in lldb you type
me read `[myData bytes]` -c`[myData length]`
If the format of the dump is not to your liking you can add '-t ' for example
me read `[myData bytes]` -c`[myData length]` -t int
For more help type
help me read
in lldb
From Xcode 5 (lldb), you can use the following:
po (NSString *)[[NSString alloc] initWithData:buffer encoding:4]
Note that this assumes your NSData instance is encoded with NSUTF8StringEncoding, but you can look up the other values in the headers or the documentation.
So if you're debugging something like a JSON request that's wrapped up in an NSURLSessionDataTask, the request data is in task.originalRequest.httpBody, and you can view that in the debugger with
po (NSString *)[[NSString alloc] initWithData:task.originalRequest.HTTPBody encoding:4]
In lldb, the following works to let you examine the contents of NSData objects:
You can get the address of the bytes for use with various debugger commands like this:
p (void *)[buffer bytes]
You see something like this:
(void *) $32 = 0x0b5e11f0
If you know the underlying data is a string, you can do this:
p (char *)[buffer bytes]
and the debugger will output:
(char *) $33 = 0x0b5e11f0 "This is the string in your NSData, for example."
In Swift this should do the trick:
po String(data:buffer!, encoding: NSUTF8StringEncoding)
Right click buffer and click on Print description of "buffer".
The console should say
Printing description of buffer:
<your data here ...>
Unfortunately, none of the suggestions so far solved the problem of actually being able to quickly display the data inside NSData.
I wrote a simple method that works the way I need it to. From the GDB window, I can type in print [Util dumpData:theData] and I will get nice, formatted output.
+(void) dumpData:(NSData *)data
{
unsigned char* bytes = (unsigned char*) [data bytes];
for(int i=0;i< [data length];i++)
{
NSString* op = [NSString stringWithFormat:#"%d:%X",i,bytes[i],nil];
NSLog(#"%#", op);
}
}
NsLog Output
0:24
1:0
2:4
3:0
4:0
5:0
Your data instance is empty.
It wouldn't only display the address otherwise. -[NSData description] includes a printout of the contents of the data. The bytes are grouped in fours and printed in hex with a leading 0 placeholder:
char arr[] = {0x1, 0x2, 0xA, 0x4, 0xF};
NSData * dat = [NSData dataWithBytes:arr length:5];
NSLog(#"%#", dat);
2012-07-17 22:24:48.973 PrintDat[61264:403] <01020a04 0f>
Using po dat at the debugger's command line will give you the same results, including the address:
(NSData *) $1 = 0x00007f98da500180 <01020a04 0f>
The contextual menu route that Anshu suggested also uses the description method.
I think I have it now.
Right click on NSData in the list displayed there, and click 'Show Memory Of "x"'.
I posted this as an answer to this relevant question:
Once you place a breakpoint, run, and the program stops at the breakpoint, hover your cursor over the variable/value you want to see like this:
You could also place an NSLog(#"%#", yourLabel.text); to view the contents of that label/other object type.
One other option is to run GDB in the console like this:
gdb
attach <your process name>
And then use the po (print-object) command to view the value of a variable like this:
po variableName
To view the value of primitive types (int, float, long, double, char, etc.), you can just use the print command while running GDB in the console like this:
print yourPrimitiveVariable
Hope this helps!
EDIT:
With the po command, you can print out the value of an object using both the property name (self.myProperty) or the ivar name (possibly _myProperty). I demonstrate this here:
Xcode 11.4 (and probably earlier) supports examining the raw bytes of (NS)Data in the visual debugger. Set a breakpoint after the Data has been assigned, hover over the symbol until the popup appears, and click on the eye or info icon. Roll for dexterity to prevent the popup from closing on you by moving outside of the tiny target.
Tested with Swift but presumably works with C family languages, too.
(Note: the eye icon produces the output below which is nicely formatted but, uh... seems to be missing the final column of bytes?)
The easiest way for me (during local development only!) is to convert to unused NSString instances. Then the values show right up in the debugger. Once I'm finished, I nuke those lines of code.
From this old thread
NSString* newStr = [[NSString alloc] initWithData:theData encoding:NSUTF8StringEncoding];
For Swift code I'm doing the following:
When my program is stopped in a breakpoint I use the "Add expression" option and enter [UInt8](data) where data is my Data instance:
After that I can see the data contents:
Related
I'm facing a problem that I don't understand. Before beginning to explain it, even if I have worked on a Swift project this year that was using some Objective-C, I am new to this language and its concepts.
So, there is my problem : I want to access the bytes of an NSData object. I know there several ways to do so :
[data bytes];
data.bytes;
[data getBytes: dest, length: [data length]];
But each method doesn't return the same value as the console, when I'm using po [data bytes].
Can you explain me why this happens ? I don't really understand what I'm missing.
Thanks.
data and data.bytes are of two totally different types. data is an instance of NSData, while data.bytes is a raw pointer (const void *). When you call po in the debugger (short for "print object"), it will call -description on things which inherit from NSObject, or just print the value if they do not.
In this case, since data is an NSData (which has -description), if you po data, it calls [data description] and prints the result of that out; since NSData knows how to nicely format its contents, it will print nicely.
However, since data.bytes is a void *, there is no way for the debugger to know how to print it (void * can point to anything; how to interpret it is totally up to you) so it just prints out the pointer itself.
If you want to print the data from the debugger directly, you can tell it how to interpret the pointer and print it out. If you know that the data blob is n bytes long, you can run the following command:
p/x *(uint8_t (*)[<n>])data.bytes
where <n> is replaced with the literal length of the data (e.g. uint8_t (*)[8])). *(uint8_t (*)[<n>])data.bytes tells the debugger to reinterpret data.bytes as an array of n bytes (giving it the length so it knows how much data to read from memory) while p/x tells it to print the hex values of the bytes it finds.
How do you print something to the console of Xcode, and is it possible to view the Xcode console from the app itself?
Thanks!
How to print:
NSLog(#"Something To Print");
Or
NSString * someString = #"Something To Print";
NSLog(#"%#", someString);
For other types of variables, use:
NSLog(#"%#", someObject);
NSLog(#"%i", someInt);
NSLog(#"%f", someFloat);
/// etc...
Can you show it in phone?
Not by default, but you could set up a display to show you.
Update for Swift
print("Print this string")
print("Print this \(variable)")
print("Print this ", variable)
print(variable)
#Logan has put this perfectly. Potentially something worth pointing out also is that you can use
printf(whatever you want to print);
For example if you were printing a string:
printf("hello");
3 ways to do this:
In C Language (Command Line Tool) Works with Objective C, too:
printf("Hello World");
In Objective C:
NSLog(#"Hello, World!");
In Objective C with variables:
NSString * myString = #"Hello World";
NSLog(#"%#", myString);
In the code with variables, the variable created with class, NSString was outputted be NSLog. The %# represents text as a variable.
#Logan said it perfectly. but i would like to add an alternative here,
if you want to view logs from just your application then you can make
a custom method that keeps saving the log to a file in documents
directory & then you can view that log file from your application.
There is one good advantage for developers of the app after the app has been released & users are downloading it. Because your app will be able to send logs & crash reports to the developers (of course with the permissions of the device user !!!) & it'll be the way to improve your application.
Let me know (To other SO users), if there is another way of doing the same thing. (Like default Apple feature or something)
Let me know if it helps or you want some more idea.
You can also use breakpoints. Assuming the value you want is defined within the scope of your breakpoint you have 3 options:
print it in console doing:
po some_paramter
Bare in mind in objective-c for properties you can't use self.
po _someProperty
po self.someProperty // would not work
po stands for print object.
Or can just use Xcode 'Variable Views' . See the image
I highly recommend seeing Debugging with Xcode from Apple
Or just hover over within your code. Like the image below.
In some environments, NSLog() will be unresponsive. But there are other ways to get output...
NSString* url = #"someurlstring";
printf("%s", [url UTF8String]);
By using printf with the appropriate parameters, we can display things this way. This is the only way I have found to work on online Objective-C sandbox environments.
In Swift with Xcode you can use either print() or NSLog().
print() just outputs your text. Nothing more.
NSLog() additionally to your text outputs time and other useful info to debug console.
I loaded libqrencode library in my cocoa project but I'm not sure how to use it exactly. I have a text field in which you type a text, and once done you click a button and I log that text with NSLog. Now I want to encode that text to be able to use it later and generate a QRcode out of it, so in the manual it's saying to use this format
QRcode* QRcode_encodeString (const char * string,
int version,
QRecLevel level,
QRencodeMode hint,
int casesensitive
)
I am not sure how to use that in my method to log the results as well
- (IBAction)GenerateCode:(id)sender {
NSString *urlText = [[NSString alloc] initWithFormat:#"%#", [_urlField stringValue]];
NSLog(#"The url is %#", urlText);
}
You need to get from an NSString instance to a const char *. This has been answered several times on SO, but here's one. Once you do that, you can call QRCode_encodeString() directly and pass whatever you desire for the arguments.
If you need more specifics, you'll have to try something, post your code, and describe how it's not working for you so we can help you more directly without just writing it for you.
I'm getting an EXC_BAD_ACCESS error, and It's because of this part of code. Basically, I take an input and do some work on it. After multiple inputs, it throws the error. Am I doing something wrong with my memory here? I'd post the rest of the code, but it's rather long -- and I think this may be where my problem lies (It's where Xcode points me, at least).
-(IBAction) findShows: (id) clicked
{
char urlChars[1000];
[self getEventURL: urlChars];
NSString * theUrl = [[NSString alloc] initWithFormat:#"%s", urlChars];
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:theUrl]];
int theLength = [data length];
NSString *content = [NSString stringWithUTF8String:[data bytes]];
char eventData[[data length]];
strcpy(eventData, [content UTF8String]);
[self parseEventData: eventData dataLength: theLength];
[whatIsShowing setStringValue:#"Showing events by this artist"];
}
When a crash occurs, there will be a backtrace.
Post it.
Either your program will break in the debugger, and the call stack will be in the debugger UI (or you can type 'bt
With that, the cause of the crash is often quite obvious. Without that, we are left to critique the code.
So, here goes....
char urlChars[1000];
[self getEventURL: urlChars];
This is, at best, a security hole and, at worst, the source of your crash. Any time you are going to copy bytes into a buffer, there should be some kind of way to (a) limit the # of bytes copied in (pass the length of the buffer) and (b) the # of bytes copied is returned (0 for failure or no bytes copied).
Given the above, what happens if there are 1042 bytes copied into urlChars by getEventURL:? boom
NSString * theUrl = [[NSString alloc] initWithFormat:#"%s", urlChars];
This is making some assumptions about urlChars that will lead to failure. First, it assumes that urlChars is of a proper %s compatible encoding. Secondly, it assumes that urlChars is NULL terminated (and didn't overflow the buffer).
Best to use one of the various NSString methods that create strings directly from the buffer of bytes using a particular encoding. More precise and more efficient.
NSData *data = [NSData dataWithContentsOfURL:[NSURL URLWithString:theUrl]];
I hope this isn't on the main thread... 'cause it'll block if it is and that'll make your app unresponsive on slow/flaky networks.
int theLength = [data length];
NSString *content = [NSString stringWithUTF8String:[data bytes]];
char eventData[[data length]];
strcpy(eventData, [content UTF8String]);
This is about the least efficient possible way of doing this. There is no need to create an NSString instance just to then turn it into a (char *). Just grab the bytes from the data directly.
Also -- are you sure that the data returned is NULL terminated? If not, that strcpy() is gonna blow right past the end of your eventData buffer, corrupting the stack.
[self parseEventData: eventData dataLength: theLength];
[whatIsShowing setStringValue:#"Showing events by this artist"];
What kind of data are you parsing that you really want to parse the raw bytes? In almost all cases, such data should be of some kind of structured type; XML or, even, HTML. If so, there is no need to drop down to parsing the raw bytes. (Not that raw data is unheard of -- just odd).
The bytes you get from [content UTF8String] could conceivably be different in number from the value of [data length]. Try using strncpy() instead and see if that still crashes. (It's also possible that getEventURL: sometimes fails to return a string in the format expected, but that's impossible to tell without the source to that method.)
Is it possible that the string contained in urlChars sometimes comes back non-NULL-terminated? You might want to try zeroing out the array, for example using bzero.
Additionally, there are a bunch of techniques for debugging EXC_BAD_ACCESS. Since you're doing a lot of pure C string manipulation, the usual method of turning on NSZombieEnabled may or may not help you (though I recommend turning it on regardless). Another technique you can try is recovering a previous stack frame using GDB. See my previous answer to a similar question if you're interested.
In my opinion the code is too complex. Do not resort to plain C arrays and strings unless you absolutely have to, they are harder to get right. (It’s no rocket science, but if you play with guns all the time, you will shoot yourself in the foot sooner or later.) Even if you insist on parsing plain C strings, isolate the code using the function interface:
// Callers have to mess with char*.
- (void) parseEventData: (char*) data {…}
// Callers can stay in the Objective-C land.
- (void) parseEventData: (NSString* or NSData*) data {
char *unwrappedData = …;
…
}
I’d certainly think twice before I used strcpy in my code. And I think you are leaking theUrl (although that should not cause EXC_BAD_ACCESS in this case). As for the bug itself, you might be hanging on parts of urlChars or eventData and when those stack-based variables disappear, you cause the segfault?
I want to print out the Text contents of a NSTextView using the NSLog function in Objective-C. The code I have so far is:
NSString *s=[updateSource textStorage];
NSLog(s);
All I get is the error:
[NSConcreteTextStorage getCharacters:range:]: selector not recognized [self = 0x43f4b0]
Use [updateSource string] instead. [updateSource textStorage] is not an NSString, but rather an NSTextStorage.
It's not the cause of your problem, but you should be using NSLog(#"%#",s); to log your string. The first argument of NSLog should always be a format string, and not the value you're trying to log.
(if you don't, your app will likely crash if the value contains percent characters)