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

#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 );
}

1. 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.