Wait for another process

wait4pid.c

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <sys/ptrace.h>

const char *name;

void usage()
{
	printf("usage: %s pid\n", name);
	exit(2);
}

void report_error()
{
	perror(name);
	exit(1);
}

int main(int argc, char *argv[])
{
	name = argv[0];
	if (argc < 2)
		usage();

	pid_t pid = atoi(argv[1]);
	if (pid <= 0)
		usage();

	if (ptrace(PT_ATTACH, pid, (caddr_t)0, 0) < 0)
		report_error();

	while (1) {
		int status;
		pid_t pid2 = waitpid(pid, &status, 0);
		if (!WIFSTOPPED(status))
			break;
		if (ptrace(PT_CONTINUE, pid, (caddr_t)1, SIGCONT) < 0) {
			ptrace(PT_DETACH, pid, (caddr_t)1, SIGCONT);
			report_error();
		}
	}

	return 0;
}