Calling C function with array pointer and int pointer from Swift

Question

I am creating a C lib plus a wrapper for easy use in Swift. The C function takes two parameters, an array pointer and an int pointer:

int crgetproclist(struct kinfo_proc *proc_list, size_t *count) {
    int err = 0;
    size_t length = 0;
    
    static const int name[] = { CTL_KERN, KERN_PROC, KERN_PROC_ALL, 0 };
    
    // Call sysctl with a NULL buffer to get proper length
    err = sysctl((int *)name, (sizeof(name) / sizeof(*name)) - 1, NULL, &length, NULL, 0);
    //if (err) return [-1];
    
    // Get the actual process list
    err = sysctl((int *)name, (sizeof(name) / sizeof(*name)) - 1, proc_list, &length, NULL, 0);
    //if (err) return [-1];
    
    *count = length / sizeof(struct kinfo_proc);
    
    for (int i = 0; i < *count; i++) {
        struct kinfo_proc proc = proc_list[i];
        proc = proc;
    }
       
    return 1;
}

I call that function from my Swift wrapper:

var data: [Process] = []

override func viewDidLoad() {
    super.viewDidLoad()
    
    let proc_list: UnsafeMutablePointer<kinfo_proc> = UnsafeMutablePointer<kinfo_proc>.allocate(capacity: 500)
    var count: size_t = 0
    let result = crgetproclist(proc_list, &count)
    
    var foobar: [Process] = []
    
    if (result == 1) {
        for i in 1..<count {
            var proc: kinfo_proc = proc_list[i]
            var process = Process(proc: proc)
            foobar.append(process)              // <---- works
            self.data.append(process)           // <---- EXC_BAD_ACCESS ????
        }
        
        self.data.sort(by: {
            (a: Process, b: Process) -> Bool in
            return a.name.lowercased() < b.name.lowercased()
        })
        
        self.myTable.reloadData()
    }
}

class Process: NSObject {
    
    var _proc: kinfo_proc
    var pid: pid_t
    var name: String
    var icon: NSImage?
    var isAlive: Bool = false
    var uid: uid_t = 0
    
    init(proc: kinfo_proc) {
        self._proc = proc
        self.pid = proc.kp_proc.p_pid
        self.name = String(cString: crgetprocname(pid))
        self.uid = crgetuid(pid)
        
        super.init()
    }
}

Questions

1. How to correctly create and pass an UnsafeMutablePointer to the C function? I hard coded capacity: 500 which works, but how to do it correctly without a hardcoded capacity?

2. When I try to append it to my class variable array data it runs into a EXC_BAD_ACCESS, but when I append it to foobar which is the same type it works. Why? How to assign it to a class variable without memory error?

Answer 1

I can only answer the first part of your question: In order to determine the necessary allocation count for the process list you must allow that crgetproclist() is called with a NULL argument (in the same way as sysctl() can be called with a NULL argument for oldp to get the needed buffer size):

int crgetproclist(struct kinfo_proc *proc_list, size_t *count) {
    int err = 0;
    size_t length;

    static const int name[] = { CTL_KERN, KERN_PROC, KERN_PROC_ALL };
    
    if (proc_list == NULL) {
        // Call sysctl with a NULL buffer to get proper length
        length = 0;
        err = sysctl((int *)name, (sizeof(name) / sizeof(*name)), NULL, &length, NULL, 0);
    } else {
        // Get the actual process list
        length = *count * sizeof(struct kinfo_proc);
        err = sysctl((int *)name, (sizeof(name) / sizeof(*name)), proc_list, &length, NULL, 0);
    }
    if (err) return -1;
    *count = length / sizeof(struct kinfo_proc);
    return 1;
}

Now you can call that function from Swift twice: First to determine the allocation count, and then again to retrieve the process list:

var count: size_t = 0
crgetproclist(nil, &count)

let procList = UnsafeMutablePointer<kinfo_proc>.allocate(capacity: count)
if crgetproclist(procList, &count) == 1 {
    for i in 0..<count {
        let proc = procList[i]
        // ...
    }
}
procList.deallocate()

Note also that you can implement the function easily in pure Swift:

func getProcessList() -> [kinfo_proc]? {
    var name : [Int32] = [ CTL_KERN, KERN_PROC, KERN_PROC_ALL ]
    var length = size_t()
    sysctl(&name, UInt32(name.count), nil, &length, nil, 0)
    let count = length / MemoryLayout<kinfo_proc>.size
    var procList = Array(repeating: kinfo_proc(), count: count)
    let result = sysctl(&name, UInt32(name.count), &procList, &length, nil, 0)
    guard result == 0 else { return nil } // Some error ...
    return Array(procList.prefix(length / MemoryLayout<kinfo_proc>.size))
}