I'm learning the basics of objective-C by Reading 'Objective C For Dummies'.
I'm using XCode 4.4, and I'm trying to get some simple code to work. This question has been posed online before. However - the code doesn't seem to compile with the new version of XCode.
At issue seems to be the line NSLog (#"Here is some amazing text! %i",c); This throws an 'Expected Expression' Error. Per the previous form posting, I have disabled automatic reference checking in preferences and this still fails.
#include <stdio.h>
int main(int argc, const char * argv[])
{
//declare variables
int a;
int b;
int c;
//set the variables
a = 2;
b = 3;
//Perform the computations
c = a % b;
//Output the results
NSLog (#"Here is some amazing text! %c",c);
return 0;
}
Add #import <Foundation/Foundation.h> at the top, and change the NSLog to this:
NSLog (#"Here is some amazing text! %d",c);
Because %c doesn't mean "a variable called c", but rather a char. %d means an int, which is what c is.
You forgot to include the Foundation header:
#import <Foundation/Foundation.h>
Sidenote: The format specifier should be %d.
Related
I have an extremely simple code snippet to get the application support directory of a use. Problem is, it doesn't work! I get the values in a CFArrayRef (because I want to use C code since I will use this snippet in a C++ application later) and try to get the first string in there using CFArrayGetValueAtIndex(). I convert the returned void* to a char* using a C-style cast and attempt to output it using printf(), but nothing gets outputted! Can anyone explain why? I've looked online through the documentation for CFArray and tried to google the issue, but came up with nothing. Any help would be really appreciated!
Code:
#include <Foundation/Foundation.h>
#include <typeinfo>
int main (int argc, const char * argv[])
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
CFArrayRef array = \
(CFArrayRef)NSSearchPathForDirectoriesInDomains(
NSApplicationSupportDirectory,
NSUserDirectory,
YES);
char* string = (char*)CFArrayGetValueAtIndex(array, 0);
printf("string: %s\n", string);
[pool drain];
}
Why are you converting it to a char*? The resulting array from NSSearchPathForDirectoriesInDomains() contains CFStringRefs, not char*s.
Try using CFShow((CFTypeRef)CFArrayGetValueAtIndex(array, 0));
I am new to the objective c and i write the code according to a reference book.
but something went wrong and I don't know why.
#import <Foundation/Foundation.h>
int main (int argc, const char * argv[])
{
if (argc==1){
NSLog(#"you need to provide a file name");
return (1);
}
FILE *wordFile = fopen(argv[1], "r");
char word[100];
while(fgets(word , 100, wordFile)){
word[strlen(word)-1] = '\0';
NSLog(#"the length of the %s is %lu", word, strlen(word));
}
fclose(wordFile);
return 0;
}
the tool indicates that the while part went wrong, EXC_BAD_ACCESS.
Any idea?
It compiles and runs fine on my machine. But imagine you have an empty line in your file. Then strlen(word) will return zero. Hence word[strlen(word)-1] = '\0'; will try to set some memory which might not be valid since word[-1] might not be a valid memory cell, or a memory cell that you can legally access.
Oh, and by the way, it has nothing to do with objective-c. This is mostly (but for the NSLog call) pure ansi C.
Update: I'm working with the GNU-runtime on Linux. The problem does not occur on MacOS with the Apple-runtime.
Update 2: I compiled the GNU-runtime on MacOS and build the example with it. The error does not occur on MacOS with the GNU-runtime. I would say the problem is the glibc (since backtrace and backtrace_symbols are glibc extensions).
When printing a backtrace in a GCC compiled Objective-C app using backtraceand backtrace_symbols, I don't get any Objective-C symbols. Only the filenames, addresses and C-symbols appear.
I compiled with -g and linked with -rdynamic.
My test app:
void _printTrace()
{
void *addr[1024];
int aCount = backtrace(addr, 1024);
char **frameStrings = backtrace_symbols(addr, aCount);
for (int i = 0; i < aCount; i++) {
printf("%s\n", frameStrings[i]);
}
free(frameStrings);
}
#interface TheObject
+ (void)_printTrace;
+ (void)printTrace;
#end
#implementation TheObject
+ (void)_printTrace
{
_printTrace();
}
+ (void)printTrace
{
[self _printTrace];
}
#end
void printTrace()
{
[TheObject printTrace];
}
int main(int argc, char **argv)
{
printTrace();
return 0;
}
and it's output:
./test.bin(_printTrace+0x1f) [0x8048e05]
./test.bin() [0x8048e60]
./test.bin() [0x8048e8b]
./test.bin(printTrace+0x34) [0x8048ec5]
./test.bin(main+0xf) [0x8048eda]
/lib/libc.so.6(__libc_start_main+0xe5) [0xb7643bb5]
./test.bin() [0x8048b51]
Is there a way to let the Objective-C symbols appear in this backtrace?
dladdr() only reports global and weak symbols. But all Objective-C function symbols are local:
$ readelf -s so_backtrace
Symbol table '.dynsym' contains 29 entries:
…
Symbol table '.symtab' contains 121 entries:
Num: Value Size Type Bind Vis Ndx Name
…
49: 08048a01 13 FUNC LOCAL DEFAULT 14 _c_TheObject___printTrace
50: 08048a0e 47 FUNC LOCAL DEFAULT 14 _c_TheObject__printTrace
…
You can verify that local symbols are never returned by looking at the GNU libc source code yourself. backtrace_symbols() is defined in sysdeps/generic/elf/backtracesyms.c. It relies on _dl_addr(), which is defined in elf/dl-addr.c, to provide it with the symbol names. That ultimately calls determine_info(). If it can, it uses the the GNU hash table, which does not include local symbols by design:
49 /* We look at all symbol table entries referenced by the hash
50 table. */
…
60 /* The hash table never references local symbols so
61 we can omit that test here. */
If the GNU hash table isn't present, it falls back to standard hash table. This includes all the symbols, but the determine_info() code filters out all but the global symbols and weak symbols:
90 if ((ELFW(ST_BIND) (symtab->st_info) == STB_GLOBAL
91 || ELFW(ST_BIND) (symtab->st_info) == STB_WEAK)
To symbolicate the Objective-C function addresses, you would have to perform the look-up yourself and not filter out the local function symbols. Further, you would have to demangle the Objective-C function symbols to restore _c_TheObject___printTrace to +[TheObject _printTrace].
GNUstep's NSException implementation doesn't use backtrace, instead it uses libbfd (binary file descriptor). I think the function that actually does the work is called static void find_address, which you can view here. Using this trivial example, I get the results that follow.
#include <Foundation/Foundation.h>
#interface Test : NSObject {}
+ (void) test;
#end
#implementation Test
+ (void) test
{
Class GSStackTrace = objc_getClass("GSStackTrace");
id stack = [GSStackTrace currentStack];
for (int i = 0; i < [stack frameCount]; i++)
{
NSLog (#"%#", [[stack frameAt:i] function]);
}
}
#end
int main(int argc, char **argv)
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
[Test test];
[pool release];
return 0;
}
Output (when compiled with debug symbols):
2010-10-18 14:14:46.188 a.out[29091] +[GSStackTrace currentStack]
2010-10-18 14:14:46.190 a.out[29091] +[Test test]
2010-10-18 14:14:46.190 a.out[29091] main
2010-10-18 14:14:46.190 a.out[29091] __libc_start_main
You may be able to pick apart GSStackTrace. It is a “private” class (that's why I need to use objc_getClass, you'll also get lots of unrecognised selector warnings), but it seems to contain all the code necessary to read Objective-C class names.
Tested on Ubuntu 9.04 with GNUstep configured with --enable-debug (so that GSFunctionInfo is included in the build).
I expect you'll need to ask the ObjC run time about the addresses to get symbol information. The addresses returned from backtrace() could probably be passed to something like object_getClass() to get the class, for example. I haven't tried any of this but it's where I'd look next in this case.
Again I'm trying to wrap my head around Objective-C and Cocoa. So I've posted some code below. What I'm trying to do is basically use the function "th_brk_line" by entering text into a UITextField then displaying it into a UITextView once it has been processed by "th_brk_line". I've posted the external function definition and the typedef for "thchar_t" as well for clarity,I hope. I guess what I'm trying to accomplish is understanding how this C function would be used in Objective-C as well as a working example with UITextField and UITextView. This is an exercise mainly for my understanding the concept with Objective-C and Cocoa.
Thanks Again!
typedef unsigned char thchar_t;
extern int th_brk_line(const thchar_t *in, thchar_t *out, size_t n, const char *delim);
int main () {
char line [1024];
char bline [1024];
while (fgets (line, sizeof line, stdin)) {
if (line [strlen (line) - 1] == '\n')
line [strlen (line) - 1] = '\0';
th_brk_line (line, bline, 1024, "|");
printf ("% s \n'", bline);
}
return 0;
}
At a high level, you need to put your text field and your text view on the screen and some sort of button that you press to make the text in the field appear in the view having been processed. You can use the keyboard's return key to do this.
On the assumption that you have done the above, calling a C function from Objective-C is easy. It's exactly the same as calling it from C. The tricky bit is that your function expects a C string as input and gives you a C string as output. Look in to NSString -UTF8String and NSString +stringWithCString:encoding:
Why does the compiler give me the following error message on the provided code: "initializer element is not constant". The corresponding C/C++ code compiles perfectly under gcc.
#import <Foundation/Foundation.h>
const float a = 1;
const float b = a + a; // <- error here
int main (int argc, const char * argv[]) {
NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
// insert code here...
NSLog(#"Hello, World!");
[pool drain];
return 0;
}
That code will only compile correctly if the const float statements appear somewhere other than the file scope.
It is part of the standard, apparently. It is important that all file-scope declared variables are initialised with constant expressions, not expressions involving constant variables.
You are initialising the float 'b' with the value of another object. The value of any object, even if it is a const qualified, is not a constant expression in C.
#dreamlax is correct, you can't have a const declaration whose initialization depends upon another (const) variable. If you need one to depend on the other, I suggest creating a variable that you can treat as a constant and initialize it only once. See these SO questions for details:
Defining a constant in objective-c
Constants in Objective C
I don't have Xcode on my machine here so I can't try my example,
But can you try
#define A (1)
#define B (A + A)
const float a = A;
const float b = B;