Q: How do I determine if I'm being run under the debugger?

A: How do I determine if I'm being run under the debugger?
The code in Listing 1 shows the best way to do this.

#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 );
}

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.

https://developer.apple.com/library/mac/qa/qa1361/_index.html