### Rebinding Symbols with fishhook

Source: https://github.com/facebook/fbretaincycledetector/blob/main/rcd_fishhook/README.md

Example demonstrating how to rebind 'close' and 'open' system calls using fishhook. This involves defining replacement functions and then calling rebind_symbols. Ensure fishhook.h/c are added to your project.

```Objective-C
#import <dlfcn.h>

#import <UIKit/UIKit.h>

#import "AppDelegate.h"
#import "fishhook.h"
 
static int (*orig_close)(int);
static int (*orig_open)(const char *, int, ...);
 
int my_close(int fd) {
  printf("Calling real close(%d)\n", fd);
  return orig_close(fd);
}

int my_open(const char *path, int oflag, ...) {
  va_list ap = {0};
  mode_t mode = 0;
 
  if ((oflag & O_CREAT) != 0) {
    // mode only applies to O_CREAT
    va_start(ap, oflag);
    mode = va_arg(ap, int);
    va_end(ap);
    printf("Calling real open('%s', %d, %d)\n", path, oflag, mode);
    return orig_open(path, oflag, mode);
  } else {
    printf("Calling real open('%s', %d)\n", path, oflag);
    return orig_open(path, oflag, mode);
  }
}

int main(int argc, char * argv[])
{
  @autoreleasepool {
    rebind_symbols((struct rebinding[2]){{"close", my_close, (void *)&orig_close}, {"open", my_open, (void *)&orig_open}}, 2);
 
    // Open our own binary and print out first 4 bytes (which is the same
    // for all Mach-O binaries on a given architecture)
    int fd = open(argv[0], O_RDONLY);
    uint32_t magic_number = 0;
    read(fd, &magic_number, 4);
    printf("Mach-O Magic Number: %x \n", magic_number);
    close(fd);
 
    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
  }
}
```

--------------------------------

### Import FBRetainCycleDetector

Source: https://github.com/facebook/fbretaincycledetector/blob/main/README.md

Import the necessary header file to use the FBRetainCycleDetector library.

```objc
#import <FBRetainCycleDetector/FBRetainCycleDetector.h>
```

--------------------------------

### Configure Detector with Filters

Source: https://github.com/facebook/fbretaincycledetector/blob/main/README.md

Create a configuration object with custom filters to exclude specific retain cycles. This is useful for known non-leaking cycles.

```objc
NSMutableArray *filters = @[
  FBFilterBlockWithObjectIvarRelation([UIView class], @"_subviewCache"),
];

// Configuration object can describe filters as well as some options
FBObjectGraphConfiguration *configuration = 
[[FBObjectGraphConfiguration alloc] initWithFilterBlocks:filters 
                                     shouldInspectTimers:YES];
FBRetainCycleDetector *detector = [[FBRetainCycleDetector alloc] initWithConfiguration:configuration];
[detector addCandidate:myObject];
NSSet *retainCycles = [detector findRetainCycles];
```

--------------------------------

### Find Retain Cycles

Source: https://github.com/facebook/fbretaincycledetector/blob/main/README.md

Instantiate the detector, add an object to analyze, and find retain cycles. The result is a set of arrays, where each array represents a retain cycle.

```objc
FBRetainCycleDetector *detector = [FBRetainCycleDetector new];
[detector addCandidate:myObject];
NSSet *retainCycles = [detector findRetainCycles];
NSLog(@"%@", retainCycles);
```

--------------------------------

### Configure Detector to Skip Timers

Source: https://github.com/facebook/fbretaincycledetector/blob/main/README.md

Initialize the detector with a configuration that disables inspection of NSTimers, preventing them from being flagged as retain cycles.

```objc
FBObjectGraphConfiguration *configuration = 
[[FBObjectGraphConfiguration alloc] initWithFilterBlocks:someFilters 
                                     shouldInspectTimers:NO];
FBRetainCycleDetector *detector = [[FBRetainCycleDetector alloc] initWithConfiguration:configuration];
```

--------------------------------

### Hook Associated Objects Manager

Source: https://github.com/facebook/fbretaincycledetector/blob/main/README.md

Integrate FBAssociationManager early in the application's lifecycle to track associated objects, enabling the detector to identify cycles involving them.

```objc
#import <FBRetainCycleDetector/FBAssociationManager.h>

int main(int argc, char * argv[]) {
  @autoreleasepool {
    [FBAssociationManager hook];
    return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
  }
}
```

--------------------------------

### Find Retain Cycles with Max Length

Source: https://github.com/facebook/fbretaincycledetector/blob/main/README.md

Customize the maximum length of retain cycles to detect. A larger value increases detection scope but may slow down the process.

```objc
FBRetainCycleDetector *detector = [FBRetainCycleDetector new];
[detector addCandidate:myObject];
NSSet *retainCycles = [detector findRetainCyclesWithMaxCycleLength:100];
```

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