I know that this question is a possible duplicate, but even after looking at some Google tutorials and questions even on this forum none of them gives me a decent answer about this subject.
I have:
NSString *str = #"text";
And I would like to do something like:
char cstring [512] = str;
(this only shows what I want to do, after looking at Apple's NSString Class Ref I didn't even think about using it).
Up to now I have:
char command [512] = [[NSString stringWithFormat:#"text"] cStringUsingEncoding:NSUTF8StringEncoding];
Still, with that I get errors.
Any solution?
try const char *command = [str UTF8String];
A c string is returned as a pointer, not as an array of characters. To use it, you can change your variable to a pointer.
const char *command = [theString cStringUsingEncoding:NSUTF8StringEncoding];
Since you want the UTF8 encoding, you can use the UTF8String convenience method.
const char *command = [theString UTF8String];
If you need the data to be stored in a character array, you can use the getCString:maxLength:encoding: method, passing the array as the buffer. This will allow you to store the string directly to the buffer, and will tell you if the buffer is too small.
char command[512];
if(![theString getCString:command maxLength:sizeof(command)/sizeof(*command) encoding:NSUTF8StringEncoding]) {
NSLog(#"Command buffer too small");
}
Related
i got the following char array in Objective-C (Xcode):
char *incomeMessage;
NSString *str = [[NSString alloc] initWithBytes:data.bytes length:data.length encoding:NSUTF8StringEncoding];
incomeMessage = [str UTF8String];
NSLog(#"%c", incomeMessage[0]);
NSLog(#"%c", incomeMessage[1]);
NSLog(#"%c", incomeMessage[2]);
NSLog(#"%c", incomeMessage[3]);
NSLog(#"%c", incomeMessage[4]);
NSLog(#"%c", incomeMessage[5]);
For example I get some results like this in console:
"3
2
6
1
8
4"
Now i want to replace the char in incomeMessage[2] by 4:
incomeMessage[2] = '4';
But then it gives me the error:
EXC_BAD_ACCESS
Do you have an idea, how to solve the problem?
According to the reference documentation, UTF8String returns a read-only (const char*) reference to the string data.
The reference material goes on to note:
This C string is a pointer to a structure inside the string object,
which may have a lifetime shorter than the string object and will
certainly not have a longer lifetime. Therefore, you should copy the C
string if it needs to be stored outside of the memory context in which
you use this property.
So I'd suggest following their advice and creating a copy of the array and then performing your modifications against that.
For example: http://ideone.com/mhjwZW
You might have better luck with something like:
NSString* str = [[NSString alloc] initWithBytes:data.bytes length:data.length encoding:NSUTF8StringEncoding];
char* incomeMessage = malloc([str lengthOfBytesUsingEncoding:NSUTF8StringEncoding] + 1);
strcpy(incomeMessage, [str UTF8String]);
//now you can change things
incomeMessage[2] = '4';
//do this when you're done
free(incomeMessage);
Although, is there any particular reason why you want to use a C-string/character array as opposed to an NSMutableString? I think you might find replaceCharactersInRange:withString: a better approach generally. See also: stringByReplacingCharactersInRange:withString:.
i got the following char array in Objective-C (Xcode)
You don't, you know. All you have is a pointer. You have not set aside any actual memory; there is no array there.
incomeMessage = [str UTF8String];
All you've done in that line is repoint the pointer incomeMessage at your string's UTF8String. A string's UTF8String is immutable. Note this passage in the docs:
you should copy the C string if it needs to be stored outside of the memory context in which you use this property.
So basically, if you want to write into an array of char, your first task should be to make an array of char.
I was wondering if there is a simple way to compare a const char with an NSString, or do I have to convert the const char to an NSString before doing do?
I have been looking through Apple docs but struggling to find an answer to my question.
Either
NSString *str = #"string";
const char *myChar = "some string";
if (strcmp(myChar, str.UTF8String))
or
[str isEqualToString:[NSString stringWithUTF8String:myChar]];
The core foundation route is also an option:
CFStringRef myStr = CFSTR("some chars");
bool result = CFStringCompareWithOptions(myStr, ((__bridge CFStringRef)str),CFRangeMake(0,CFStringGetLength(myStr)), kCFCompareCaseInsensitive);
Better still:
[str isEqualToString: #(myChar) ];
This is no worse than a cast, which you're bound to need since the types are incommensurable.
You are comparing two different things, so you have to convert one to the other. Which way you convert is up to you.
I have an NSString object and want to convert it into a std::string.
How do I do this in Objective-C++?
NSString *foo = #"Foo";
std::string bar = std::string([foo UTF8String]);
Edit: After a few years, let me expand on this answer. As rightfully pointed out, you'll most likely want to use cStringUsingEncoding: with NSASCIIStringEncoding if you are going to end up using std::string. You can use UTF-8 with normal std::strings, but keep in mind that those operate on bytes and not on characters or even graphemes. For a good "getting started", check out this question and its answer.
Also note, if you have a string that can't be represented as ASCII but you still want it in an std::string and you don't want non-ASCII characters in there, you can use dataUsingEncoding:allowLossyConversion: to get an NSData representation of the string with lossy encoded ASCII content, and then throw that at your std::string
As Ynau's suggested in the comment, in a general case it would be better to keep everything on the stack instead of heap (using new creates the string on the heap), hence (assuming UTF8 encoding):
NSString *foo = #"Foo";
std::string bar([foo UTF8String]);
As noted on philjordan.eu it could also be that the NSString is nil. In such a case the cast should be done like this:
// NOTE: if foo is nil this will produce an empty C++ string
// instead of dereferencing the NULL pointer from UTF8String.
This would lead you to such a conversion:
NSString *foo = #"Foo";
std::string bar = std::string([foo UTF8String], [foo lengthOfBytesUsingEncoding:NSUTF8StringEncoding]);
I have been able to find methods like -[NSString stringWithCString:encoding:] but they do not seem to play well when the cstring is a pointer.
First up, don't use initWithCString, it has been deprecated.
Couple of ways you can do this:
const *char cString = "Hello";
NSString *myNSString = [NSString stringWithUTF8String:cString];
If you need another encoding like ASCII:
const *char cString = "Hello";
NSString *myNSString = [NSString stringWithCString:cString encoding:NSASCIIStringEncoding];
If you want to see all the string encodings available, in Xcode, hold command + option then double click on NSASCIIStringEncoding in the above code block.
You will be able to see where Apple have declared their enumeration for the string encoding types. Bit quicker than trying to find it in the documentation.
Some other ones you might need:
NSASCIIStringEncoding
NSUnicodeStringEncoding // same as NSUTF16StringEncoding
NSUTF32StringEncoding
Checkout Apple's NSString Class Reference (encodings are at the bottom of the page)
With modern Objective-C (since Xcode 5 at least) you can just do:
char const* cString = "Hello";
NSString *myNSString = #(cString);
stringWithCString:encoding: creates an NSString from a given C string. To create a C string from an NSString, use either the UTF8String method (generally preferred) or cStringUsingEncoding: (if you need an encoding other than UTF-8).
I'm trying to use a function that has the following signature to sign a HTTP request:
extern void hmac_sha1(const unsigned char *inText, int inTextLength, unsigned char* inKey, const unsigned int inKeyLength, unsigned char *outDigest);
And this is the method I wrote to use it:
- (NSString *)sign: (NSString *)stringToSign {
NSString *secretKey = #"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
const unsigned char *inText = (unsigned char *)[stringToSign UTF8String];
int inTextLength = [stringToSign length];
unsigned char *inKey = (unsigned char *)[secretKey UTF8String];
const unsigned int inKeyLength = (unsigned int)[secretKey length];
unsigned char *outDigest;
hmac_sha1(inText, inTextLength, inKey, inKeyLength, outDigest);
NSString *output = [NSString stringWithUTF8String:(const char *)outDigest];
return output;
}
The problem is I'm sure this is not the way I'm supposed to do this casting, as inside this hmac_sha1 function I get a EXC_BAD_ACCESS exception.
Since I am new to Objective-C and have close to no experience in C (surprise!) I don't really know what to search for. Any tips on how I can start solving this?
Thanks in advance!
BTW, I got the reference for this function here in stackoverflow.
It looks like the problem is not with the casting, but with outDigest. The fifth argument to hmac_sha1 should point to an already allocated buffer of size 20 bytes (I think).
If you change the line that says
unsigned char *outDigest;
to say
#define HMACSHA1_DIGEST_SIZE 20
void *outDigest = malloc(HMACSHA1_DIGEST_SIZE);
That should get you past the crash inside hmac_sha1.
Then you've got the problem of converting the data at outDigest into an NSString. It looks like hmac_sha1 will put 20 bytes of random-looking data at outDigest, and not a null terminated UTF-8 string, so stringWithUTF8String: won't work. You might want to use something like this instead if you have to return an NSString:
NSString *output = [[NSString alloc] initWithBytesNoCopy:outDigest
length:HMACSHA1_DIGEST_SIZE
encoding:NSASCIIStringEncoding
freeWhenDone:YES];
I don't think NSString is really the right type for the digest, so it might be worth changing your method to return an NSData if you can.
This wasn't part of your question but it's a bug nonetheless, you shouldn't use -length to get the byte count of an UTF8 string. That method returns the number of Unicode characters in the string, not the number of bytes. What you want is -lengthOfBytesUsingEncoding:.
NSUInteger byteCount = [stringToSign lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
Also be aware that the result does not account for a terminating NULL character.
Are you sure you don't need to allocate some memory for outDigest before calling hmac_sha1? Since you pass in a pointer, rather than a pointer to a pointer, there's no way that the memory can be allocated inside the routine.