How to check a character array is null in objective C?
char hName[255];
- (void)setHost {
phent = gethostbyaddr((const char*)&haddr, sizeof(int), AF_INET);
if(phent){
strncpy(hName,phent->h_name,255);
}
-(void) getHost {
NSString *str = [[NSString alloc] initWithBytes:tTemp.hName
length:sizeof(tTemp.hName) encoding:NSASCIIStringEncoding];
}
I have a character array named hName[255]. I will assign values to this array at some point of my project. Now i need to check if the hName[255] contains null value. i tried some methods. I get a string str from that array and check if it is equal to #""; It failed. Then i check the length of the string str. Even if the array contains no values it will return 255. How can i check the array contains null value. Any help? Thanks in advance!!
You might need to elaborate a bit on your question. For example, to just check if a pointer is null is pretty simple :
char *my_chars;
...
if (! my_chars)
NSLog(#"Woo, my_chars is null");
else
NSLog(#"my_chars is at 0x%08x", my_chars);
because null is just 0 :)
However, it doesn't look like that's your problem. you've created an array of characters like so
char my_chars[255];
so my_chars is not going to be null.
However, as outis says in his answer, you've just allocated it and not zeroed the contents so you have no idea what's in those 255 bytes! Out of the three options he suggests I'd personally go with this one :
char my_chars[255];
memset(my_chars, 0, sizeof(my_chars));
now, you have an array of 255 zeroes :) This is pretty easy to check to see if it's null :
if (0 == strlen(my_chars))
NSLog(#"It's not null but it is an empty string!");
else
NSLog(#"my_chars contains a valid string which is %i chars long", strlen(my_chars));
Hope that helps.
Sam
First thing, note that the word "null" is overloaded. You can have null pointers and null (empty) strings, and there's the null character ('\0', equal to 0 when converted to an int:((int)'\0') == 0). There are also uninitialized variables, which may or may not be null. I'm guessing you're talking about an uninitialized character array, used as a c-string.
Most likely, hName is being allocated on the stack (I can't tell without seeing more of the source code), which means it's not zero-initialized. Practically speaking, hName will hold whatever data was last stored in the region of memory that hName occupies. You'll need to initialize it yourself.
char hName[255] = {0};
// or
memset(hName, 0, sizeof(hName));
// or, if you have bzero
bzero(hName, sizeof(hName));
Also note that since hName is declared as an array rather than a pointer, sizeof(hName) is the number of characters it stores.
void test() {
char *name1 = "";
char name2[255];
// All the following lines will be true
strlen(name1) == 0;
sizeof(name2) == 255
0 <= strlen(name2) && strlen(name2) < 255;
// pointers are 4 or 8 bytes on most machines these days
sizeof(name1) == 4 || sizeof(name1) == 8;
}
Edit (addressing code sample):
The length of str in getHost is 255 because you tell it to have that length when you copy from tTemp.hName. NSStrings can contain nulls, though you may have difficulty printing them and any characters following.
It's not clear from the code sample if hName is a global (globals are bad) or a property. Similarly, the scope of the other variables, such as haddr and tTemp, is unclear. Some of those should be parameters to the methods.
The name "setHost" should be reserved for a setter–one of a pair of methods ("accessors", in Objective-C parlance) that get and set a property. They return and take (respectively) a type that's notionally the type of the property. In this case, NSString* makes the most sense; best to use an NSString in your code and switch to (via NSString's cStringUsingEncoding: or UTF8String). The partner to -(void)setHost:(NSString*) would be -(NSString*)host.
Once you make the switch to NSString (and use stringFromCString:withEncoding:), you can simply examine its length or compare it to #"" to check for an empty string.
#interface MyHost : NSObject {
NSString *name_;
...
}
/* post ObjC 2.0 */
#property(retain) NSString* name;
/* pre ObjC 2.0 */
-(NSString*)name;
-(void)setName:(NSString*);
/* any ObjC version */
-(int)setHostFromAddress:(MyAddress*)addr;
...
#end
#implementation MyHost
/* post ObjC 2.0 */
#synthesize name = name_;
-(int)setHostFromAddress:(MyAddress*)addr {
struct hostent *phost;
phost = gethostbyaddr(addr.address, addr.length, addr.type);
if (phost) {
self.name = [NSString stringWithCString:phost->h_hname encoding:NSASCIIStringEncoding];
}
return h_errno;
}
/* pre ObjC 2.0 */
-(NSString*)name {
return name_;
}
-(NSString*)setName:(NSString*)nom {
[name_ release];
name_ = [nom retain];
}
-(int)setHostFromAddress:(MyAddress*)addr {
struct hostent *phost;
phost = gethostbyaddr([addr address], [addr length], [addr type]);
if (phost) {
[self setName:[NSString stringWithCString:phost->h_hname encoding:NSASCIIStringEncoding]];
}
return h_errno;
}
...
#end
Related
I've searched Stackoverflow and other sites, but I can't seem to find this answer.
In Apple Text Editor source, they have at least one routine that does some apparently strange logical ANDing between two non-boolean variables. Casting them as Bools CAN be done, but doesn't make much sense. I'm learning Swift and much less familiar with Objective-C, but for the life of me, I can't figure out how they are trying to achieve the goal stated as "Build list of encodings, sorted, and including only those with human readable names."
Here is the code:
/* Return a sorted list of all available string encodings.
*/
+ (NSArray *)allAvailableStringEncodings {
static NSMutableArray *allEncodings = nil;
if (!allEncodings) { // Build list of encodings, sorted, and including only those with human readable names
const CFStringEncoding *cfEncodings = CFStringGetListOfAvailableEncodings();
CFStringEncoding *tmp;
NSInteger cnt, num = 0;
while (cfEncodings[num] != kCFStringEncodingInvalidId) num++; // Count
tmp = malloc(sizeof(CFStringEncoding) * num);
memcpy(tmp, cfEncodings, sizeof(CFStringEncoding) * num); // Copy the list
qsort(tmp, num, sizeof(CFStringEncoding), encodingCompare); // Sort it
allEncodings = [[NSMutableArray alloc] init]; // Now put it in an NSArray
for (cnt = 0; cnt < num; cnt++) {
NSStringEncoding nsEncoding = CFStringConvertEncodingToNSStringEncoding(tmp[cnt]);
if (nsEncoding && [NSString localizedNameOfStringEncoding:nsEncoding]) [allEncodings addObject:[NSNumber numberWithUnsignedInteger:nsEncoding]];
}
free(tmp);
}
return allEncodings;
}
The line in question contains the "&&." Any guidance would be appreciated.
Objective-C is a strict superset of C, so the same rules for logical
operators apply. In contrast to Swift, which is much more strict with
types, the logical operators in C take arbitrary scalar operands.
(The boolean type bool did not even exist in early versions of C,
it was added with the C99 standard.)
The C standard specifies (see e.g. http://port70.net/~nsz/c/c11/n1570.pdf, which is a draft of the C11 standard):
6.5.13 Logical AND operator
Constraints
2 Each of the operands shall have scalar type.
Semantics
3 The && operator shall yield 1 if both of its operands compare
unequal to 0; otherwise, it yields 0. The result has type int.
In your case, in
if (nsEncoding && [NSString localizedNameOfStringEncoding:nsEncoding])
the left operand has type NSUInteger (which can be unsigned long
or unsigned int, depending on the platform), and the right
operand has type NSString *, which is a pointer type. Therefore
the above expression is equivalent to
if (nsEncoding != 0 && [NSString localizedNameOfStringEncoding:nsEncoding] != 0)
where the zero in the right operand is the null pointer constant
which is usually written as NULL, or nil for Objective-C pointers:
if (nsEncoding != 0 && [NSString localizedNameOfStringEncoding:nsEncoding] != nil)
Some more information how this relates to Swift
Cocoa/Cocoa Touch Objective-C methods which return an object pointer
usually return nil to indicate an error
(compare Handling Error Objects Returned From Methods
in the "Error Handling Programming Guide"). So
[NSString localizedNameOfStringEncoding:nsEncoding] != nil
would mean "no localized name for the encoding could be determined".
The Swift equivalent would be a method returning an optional string,
and you could check the success with
NSString.localizedNameOfStringEncoding(nsEncoding) != nil
However, this does not compile, and here is the reason why: If you option-click on the Objective-C localizedNameOfStringEncoding method
in Xcode to show its declaration then you'll see
+ (NSString * _Nonnull)localizedNameOfStringEncoding:(NSStringEncoding)encoding
Here _Nonnull indicates that the method is not expected to return
nil. This kind of nullability annotations were introduced to
improve the mapping of Objective-C methods to Swift, see for example
"Nullability and Objective-C" in the Swift Blog.
Because of this _Nonnull annotation, the method is imported to Swift
as
public class func localizedNameOfStringEncoding(encoding: UInt) -> String
So testing the return value in Objective-C can be done but makes no
sense because the method always returns a non-nil value.
In Swift the compiler assumes that the return value is never nil
and returns a non-optional String.
The translation of that if-statement to Swift would therefore just be
if nsEncoding != 0 {
// ...
}
Is it possible to replace a character from a c string after converting it from NSString via the UTF8string method?
For example take the code below. It is to format a string with particular rule.
- (NSString *)formatString:(NSString *)input {
if (input.length==0) {
return #"";
}
//code to determine rule
....
....
// substitute output format with input characters
if (rule) {
input = [input substringFromIndex:prefix.length];
char *string = (char *)[rule UTF8String];
int repCount = 0;
for (int i=0; i<rule.length; i++) {
if (string[i] == '#') {
if (repCount < input.length)
string[i] = [input characterAtIndex:repCount++];//bad access
else
string[i] = ' ';
}
}
NSMutableString *output = [NSMutableString stringWithCString:string encoding:NSUTF8StringEncoding];
...
... //do something with the output
return output;
} else {
return input;
}
}
Initially string[0] has '#' and it should get replaced with the character in the input. This is not happening.
In a word, NO. That buffer doesn't belong to you so leave it alone.
A couple of issues:
You are casting UTF8String, which returns a const char *, to char *. UTF8String is, by definition, returning a read-only string and you should use it as such. (You really should use casts sparingly, if at all. Certainly never use casts to override const qualifiers for variables.)
If you want to perform this C-string manipulation, you have to copy the string to your own buffer. For example, use getCString or getCharacters methods (but only after you've created a buffer to receive them, and remember to add a character for the NULL terminator).
By the way, you're also returning characterAtIndex, which is a unichar (which can be larger than 8-bits), and using it in your char * buffer (8-bits per character). I'd be wary about mixing and matching those without being very careful. It is best to pick one and stick with it (and unichar offers a little more tolerance for those non-8-bit characters).
Perhaps you check for this earlier, but you're setting string to be those characters after the prefix, and then proceed to check the next rule.length number of characters. But, as far as I can tell, you have no assurances that string actually has that many characters left in it. You should test for that, or else that will also cause problems.
Personally, I'd retire this whole C-string algorithm and employ the appropriate NSString and/or NSMutableString methods to do whatever replacement you wanted, e.g. stringByReplacingCharactersInRange, stringByReplacingOccurrencesOfString, or the equivalent NSMutableString methods, replaceCharactersInRange or replaceOccurrencesOfString.
I'm getting a value back from an Objective C library (it's Firebase, but that doesn't really matter) of type id. The documentation states that this value will be an NSNumber for both boolean and numeric results. I want to take a different action based on whether or not this result corresponds to a boolean.
I know this is possible because printing out the class of the result via NSStringFromClass([value class]); gives "__NSCFBoolean" for booleans, but I'm not really sure how to correctly structure the comparison.
The objCType method gives information about the type of the data contained in the
number object:
NSNumber *n = #(1.3);
NSLog(#"%s", [n objCType]); // "d" for double
NSNumber *b = #YES;
NSLog(#"%s", [b objCType]); // "c" for char
The possible values are documented in
"Type Encodings"
in the "Objective-C Runtime Programming Guide".
Since BOOL is defined as unsigned char, it is reported as such by this method.
This means that you cannot distinguish it from a NSNumber object containing any char.
But it is sufficient to check between "boolean" and "numeric":
if (strcmp([obj objCType], #encode(BOOL)) == 0) {
// ...
} else if (strcmp([obj objCType], #encode(double)) == 0) {
// ...
} else {
// ...
}
To develop a calculator in xcode I am using a c class for converting infix to postfix expression and its evaluation. But I have an NSString in my View controller class and I need to pass this NSString to a C class where the conversion and evaluation happens. How can I do this?
I think you need to convert NSString to cString. It can be done by
[str UTF8String]
Assuming you have an NSMutableArray containing NSString objects, and you want to convert this to a C array containing C strings, you need to allocate memory for a C array of char * of suitable size (e.g., count of the NSMutableArray + maybe 1 extra if you want a NULL terminator for the array, otherwise you need to pass along the array's element count everywhere). Then for each element of the NSMutableArray, populate the corresponding index in the C array with the C string returned by UTF8String of the NSString object.
Note that the C strings returned by UTF8String are “autoreleased”, so if you need to keep them around for longer than the current autorelease context, you need to duplicate them (e.g., strdup) and then free them (and the C array, which you need to free in any case) after you're done with them.
For example:
#include <Foundation/Foundation.h>
#include <stdio.h>
#include <string.h>
void printCStringsInArray(const char **arr) {
int i = 0;
while (*arr) { printf("%d: %s\n", i++, *arr++); }
}
int main () {
const char **carr;
#autoreleasepool {
NSMutableArray *arr;
const char **p;
arr = [NSMutableArray arrayWithObjects:#"11", #"+", #"12", nil];
p = carr = malloc(sizeof(*carr) * ([arr count] + 1));
for (NSString *s in arr) {
*p++ = strdup([s UTF8String]);
}
*p = NULL; // mark the end of carr
}
printCStringsInArray(carr);
{ // free the C strings and array
const char **p = carr;
while (*p) { free(*p++); };
free(carr);
}
return 0;
}
This prints:
0: 11
1: +
2: 12
edit: Of course if you just want to call some C function taking individual strings as const char * from your otherwise Objective-C code, you don't need to do anything this complicated, just convert each string on the fly with UTF8String and use the NSMutableArray for iteration, etc. But then one might wonder what it is that you can do with C strings that you couldn't do directly with NSStrings.
It's easy because you can use NSString's UTF8String method, but you have to handle memory allocation: allocate a block to contain the string.Choose if to wrap it into a NSData so that it will be released automatically when out of the pool block, or if to free it manually:
#autoreleasepool
{
NSArray* strings= #[ #"Hello", #"Hey" ]; // Your strings
// This could be also a NSMutableArray if you need it
NSMutableArray* cstrings=[NSMutableArray new]; // C strings
for(NSString* string in strings) // Iterate over all the strings
{
const char* temp=[string UTF8String]; // Get C string
const unsigned long length=strlen(temp); // Allocate memory for it
char* string=(char*)malloc((length+1)*sizeof(char));
strcpy(string,temp); // Copy it
// Store it inside NSData
[cstrings addObject: [NSData dataWithBytesNoCopy: string length: (length+1)*sizeof(char)]];
}
}
I'm really new to Objective C and am trying to write a program to go through the collatz conjecture. When I run the program, it stops after the first scanf and comes up with "EXC_BAD_ACCESS". Here's my code:
int original,i;
NSString *PrintFull;
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
NSLog(#"Collatz Conjecture:");
NSLog(#"Print full results?");
scanf("%s",PrintFull);
NSLog(#"What number should we go up to?");
scanf("%d", &original);
while (original <= 100) {
NSLog(#"\n\n%d", original);
i = original;
while (i != 1) {
if (i % 2) {
i = (i*3)+1;
} else {
i = (i/2);
}
if ([PrintFull isEqualToString:#"yes"]) {
NSLog(#"%d",i);
}
}
original++;
}
}
What am I doing wrong here?
scanf does not work with with object types such as NSString. Please see SO post - Using scanf with NSStrings.
scanf's arguments after the format string should point to already allocated objects. In this case you've just declared a pointer and passed it in without setting it. scanf will try to write to this location, but since the pointer contains a garbage value, the application crashes.
scanf is from the C library 'stdio.h', meaning it doesn't know about NSStrings, which are from the Objective-C 'Foundation' framework.
The following should solve these problems
int original,i;
- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
NSLog(#"Collatz Conjecture:");
NSLog(#"Print full results?");
char inputBuffer[80];
scanf("%s", inputBuffer);
NSString *printFull = [NSString stringWithCString:inputBuffer encoding:NSUTF8Encoding];
First, you have to initialize and alloc the NSString. Second, scanf can't handle NSString.
Also notice, that class names begin with a capital letter and class instances with a small one.