When writing programs that should be easy to debug, it is often useful to know if you are running with a debugger attached. In Linux, there is no reliable way of doing this, but you can make a clever hack that is quite okay. gdb and other debuggers work by using ptrace, but in Linux, only one process at a time can ptrace another. So, the trick is to try to ptrace ourselves, and if it fails, we can assume that we are running in a debugger. (Or in strace, for that matter.) Doing this properly requires starting a new thread, however. It's not very hard, but it's a bit tricky getting everything right, and Ihave the feeling I'm doing this over and over again.

And there never seems to be any good code to find on Google, either. So, as a courtesy to the rest of the world, here is a code snippet:

#include <pthread.h>
#include <stdlib.h>
#include <stdbool.h>
#include <sys/ptrace.h>

static void*
test_trace(void* ignored)
{
	return (void*)ptrace(PTRACE_TRACEME, 0, NULL, NULL);
}

bool
is_debugger_attached(void)
{
	pthread_attr_t attr;
	void* result;
	pthread_t thread;

	pthread_attr_init(&attr);
	pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
	if (pthread_create(&thread, &attr, test_trace, NULL) != 0) {
		pthread_attr_destroy(&attr);
		return false;
	}
	pthread_attr_destroy(&attr);
	if (pthread_join(thread, &result) != 0) {
		return false;
	}

	return result != NULL;
}