/code/C/init.c

/* unchristened custom init by Pegasus Epsilon <pegasus@pimpninjas.org> */
#include <unistd.h>     /* fork(), exec(), getuid() */
#include <sys/types.h>  /* wait(), getuid() */
#include <sys/wait.h>   /* wait() */
#include <sys/prctl.h>  /* prctl() */
#include <stdlib.h>     /* exit() */
#include <stdarg.h>     /* va_list, va_start(), vprintf(), va_end() */
#include <stdio.h>      /* vprintf(), putchar(), fflush(), perror() */
#include <errno.h>      /* errno */
#include <string.h>     /* strcpy() */
/* specify your stage2 here */
#define STAGE2 "/sbin/stage2"
/* if we fail to start the above, then we start the below instead. */
#define SHELL "/bin/sh"
#define MYNAME "init"
/* possible names:
 * big bang
 * prime mover
 * minute disturbance
 * quantum foam
 * ...
 */
/* let us create a puts/printf hybrid */
void say (const char *restrict const fmt, ...) {
	va_list args;
	va_start(args, fmt);
	vprintf(fmt, args);
	putchar('\n');
	fflush(stdout);
	va_end(args);
}
int spawn (const char *path, char *const name) {
	if (fork()) {
		int status;
		if (0 > wait(&status) && ECHILD != errno) perror(name);
		return status;
	} else {
		execl(path, name, NULL);
		perror(name); /* on success, "%s: Success\n" is printed. */
		exit(0);
	}
}
int main (int argc, char **argv) {
	(void)(argc); /* stfu gcc */
	/* we care about being root */
	if (getuid()) {
		say("must be superuser");
		exit(0);
	}
	/* we don't care about being PID1 - it's a silly thing to check
	 * either we are PID1 and we will reap processes, or something
	 * is misconfigured - but that's not our problem.
	 */
	if (fork()) { /* parent - become death */
		int status;
		strcpy(argv[0], "death");
		prctl(PR_SET_NAME, argv[0]);
		/* by default, SIGCHLD is set to SIG_IGN
		 * this means that wait() should wait for all
		 * children to terminate and then fail with ECHILD
		 */
		while (0 < wait(&status));
		if (ECHILD != errno) perror(MYNAME);
		else say("%s exiting", MYNAME);
	} else { /* child - become life */
		strcpy(argv[0], "life");
		prctl(PR_SET_NAME, argv[0]);
		/* since we are the child, our parent above
		 * will not terminate until we do.
		 * infinite loop means we should not terminate
		 * until something kills us
		 */
		for (;;) {
			spawn(STAGE2, "stage2");
			spawn(SHELL, "rescue");
		}
	}
}
download