Mixing C functions in an Objective-C class - c

Question

I am writing an Objective-C class but it uses an API written in C. This is mostly fine as mixing C calls with Objective-C calls causes few problems.
However one of the API call requires a call back method (example):
success = CFHostSetClient(host, MyCFHostClientCallBack, &context);
Where MyCFHostClientCallBack is a C function defined like this:
static void MyCFHostClientCallBack(CFHostRef host, CFHostInfoType typeInfo, const CFStreamError *error, void *info);
Can/How do I call an Objective-C method in place of this?
Can/Should I mix C functions with my Objective-C calls?
How do I mix C functions with Objective-C methods?

Answer 1

Mixing C and Objective-C methods and function is possible, here is a simple example that uses the SQLite API within an iPhone App: (course site)
Download the Zip file (09_MySQLiteTableView.zip)
C functions need to be declared outside of the #implementation in an Objective-C (.m) file.
int MyCFunction(int num, void *data)
{
//code here...
}
#implementation
- (void)MyObjectiveCMethod:(int)number withData:(NSData *)data
{
//code here
}
#end
Because the C function is outside of the #implementation it cannot call methods like
[self doSomething]
and has no access to ivars.
This can be worked around as long as the call-back function takes a userInfo or context type parameter, normally of type void*. This can be used to send any Objective-C object to the C function.
As in the sample code, this can be manipulated with normal Objective-C operations.
In addition please read this answer: Mixing C functions in an Objective-C class

Answer 2

To call Objective-C code from a C callback I would use something like:
void * refToSelf;
int cCallback()
{
[refToSelf someMethod:someArg];
}
#implementation SomeClass
- (id) init
{
self = [super init];
refToSelf = self;
}
- (void) someMethod:(int) someArg
{
}

Answer 3

Can/How do I call an Objective-C method in place of this?
You cannot.
Can/Should I mix C function in with my Objective-C call?
Yes. Write a C function and use that as the callback to the CF function.
How do I mix C function with Objective-C methods?
You can set self as the info pointer in your context structure. That will be passed to the callback. Then, in the callback, cast the info pointer back to id:
MyClass *self = (id)info;
You can then send self messages. You still can't directly access instance variables, though, since a C function is outside of the #implementation section. You'll have to make them properties. You can do this with a class extension. (Contrary to what that document says, you would not declare the extension inside #implementation, but in the same file with it, generally right above it.)

Answer 4

What I've always found helpful in this situation is to make an Obj-C wrapper on top of the C API. Implement what you need to using C functions, and build an Objective-C class (or two) on top of it, so that's all the outside world will see. For example, in the case of a callback like this, you might make a C function that calls Obj-C delegate methods on other objects.

Answer 5

.m call function inside .c:
CrifanLib.h
#ifndef CrifanLib_h
#define CrifanLib_h
#include <stdio.h>
void fileModeToStr(mode_t mode, char * modeStrBuf);
#endif /* CrifanLib_h */
CrifanLib.c
#include "CrifanLib.h"
#include <stdbool.h>
void fileModeToStr(mode_t mode, char * modeStrBuf) {
// buf must have at least 10 bytes
const char chars[] = "rwxrwxrwx";
for (size_t i = 0; i < 9; i++) {
// buf[i] = (mode & (1 << (8-i))) ? chars[i] : '-';
bool hasSetCurBit = mode & (1 << (8-i));
modeStrBuf[i] = hasSetCurBit ? chars[i] : '-';
}
modeStrBuf[9] = '\0';
}
called by Objective-C's .m:
#include “CrifanLib.h"
#interface JailbreakDetectionViewController ()
#end
#implementation JailbreakDetectionViewController
…
char* statToStr(struct stat* statInfo){
char stModeStr[10];
fileModeToStr(statInfo->st_mode, stModeStr);
...
}
...
done.