Detecting if iOS app is run in debugger

I set up my application to either send debugging output to console or a log file. Now, I'd like to decide with in the code whether

• it is run in the debugger (or simulator) and have thus a console window where I would like to read the output directly or if
• there is no console window and thus, the output should be redirected to a file.

Is there a way to determine if the app runs in the debugger?

Solution 1:

There's a function from Apple to detect whether a program is being debugged in the Technical Q&A 1361 (entry in Mac library and entry in iOS library; they are identical).

Code from the Technical Q&A:

#include <assert.h>
#include <stdbool.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/sysctl.h>

static bool AmIBeingDebugged(void)
    // Returns true if the current process is being debugged (either 
    // running under the debugger or has a debugger attached post facto).
{
    int                 junk;
    int                 mib[4];
    struct kinfo_proc   info;
    size_t              size;

    // Initialize the flags so that, if sysctl fails for some bizarre 
    // reason, we get a predictable result.

    info.kp_proc.p_flag = 0;

    // Initialize mib, which tells sysctl the info we want, in this case
    // we're looking for information about a specific process ID.

    mib[0] = CTL_KERN;
    mib[1] = KERN_PROC;
    mib[2] = KERN_PROC_PID;
    mib[3] = getpid();

    // Call sysctl.

    size = sizeof(info);
    junk = sysctl(mib, sizeof(mib) / sizeof(*mib), &info, &size, NULL, 0);
    assert(junk == 0);

    // We're being debugged if the P_TRACED flag is set.

    return ( (info.kp_proc.p_flag & P_TRACED) != 0 );
}

Also pay attention to this note at the end of the Q&A:

Important: Because the definition of the kinfo_proc structure (in <sys/sysctl.h>) is conditionalized by __APPLE_API_UNSTABLE, you should restrict use of the above code to the debug build of your program.

Solution 2:

It is possible to instruct the debugger to set environment variables when it launches a process it is about to debug. This can be done in Xcode by going to the menu item Product->Edit Scheme. Then under the Debug scheme's Arguments tab add a new environment variable. The variable should be named "debugger" with the value "true". Then the following code snippet can be used to determine if the debugger launched your process:

NSDictionary* env = [NSProcessInfo processInfo].environment;

if ([env[@"debugger"] isEqual:@"true"]) {
    NSLog(@"debugger yes");
}
else {
    NSLog(@"debugger no");
}

Solution 3:

For those who are looking for a simpler solution - this works perfectly:

func isDebuggerAttached() -> Bool {
    return getppid() != 1
}

Solution 4:

The simplest solution actually is

_isDebugging = isatty(STDERR_FILENO);

It isn't exactly the same as telling whether the app is running under debugger, but good enough (even better?) to determine whether the log should be written to disk.