loop.c

#include <assert.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <sys/wait.h>

#include "types.h"
#include "bpf.h"

int
loop(FILE *log, const struct bpf *bpf, const struct opts *opts)
{
	unsigned cur;
	int rs;
	char err[256];
	unsigned prev = 0;

	assert(bpf);
	assert(log);
	assert(opts);

	fprintf(log, "monitoring for packets on %s interface\n", opts->ifname);
	while (1) {
		(void)read(bpf->fd, bpf->buf, bpf->len); /* blocks until there are packets */

		rs = bpf_gstats(bpf->fd, &cur, err, sizeof err);
		if (rs) {
			fprintf(stderr, "failed to get interface stats: %s\n", err);
			return 1;
		}
		if (cur < prev) {
			fprintf(log, "overflow detected\n");
			prev = cur;
			continue;
		}
		if (cur - prev < opts->count) {
			continue;
		}

		pid_t pid;

		fprintf(log, "packet count reached "
				"(prev=%u cur=%u)\n", prev, cur);
		prev = cur;

		pid = fork();
		if (pid == -1) {
			perror("fork");
			return 1;
		}
		else if (pid == 0) {
			fprintf(log, "child process started\n");
			(void)execve(opts->path, opts->argv, NULL);
			perror("execve");
			_exit(1);
		}
		else {
			int status;

			rs = waitpid(pid, &status, 0);
			if (rs == -1) {
				perror("waitpid");
				return 1;
			}
			if (WIFSIGNALED(status)) {
				int sig = WTERMSIG(status);
				const char *desc = strsignal(sig);
				fprintf(log, "child process terminated by signal: %d (%s)\n", sig, desc);
			}
			else {
				fprintf(WEXITSTATUS(status) ? stderr : log,
						"child process exited with exit code %d\n",
						WEXITSTATUS(status));
			}
		}
	}
}