다른 프로그램의 표준 입력과 출력을 다 가로채는 방법?

$ cat calc.c
#include <stdio.h>
#include <unistd.h>
 
static int fdump(int n)
{
        FILE *fp = fopen("result.txt", "a");
        if (fp)
        {
                char buf[BUFSIZ];
                snprintf(buf, sizeof(buf), "%d\n", n);
                fputs(buf, fp);
                fclose(fp);
        }
 
        return fp?0:-1;
}
 
int main(void)
{
        int x, i=10;
        setbuf(stdout, NULL);
 
        while (--i)
        {
                printf("%d\n", i);
                scanf("%d", &x);
                fdump(x);
        }
 
        return 0;
}
 
$ gcc -g -W -Wall -o calc calc.c

$ cat popen_rw.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
 
typedef struct pipe_rw
{
        pid_t cpid;
        int pipe_r[2];
        int pipe_w[2];
} RWPIPE;
 
int pclose_rw(RWPIPE *rwp)
{
        int status, ret = 0;
 
        if (rwp)
        {
                if (rwp->cpid > 0)
                {
                        kill(rwp->cpid, SIGTERM);
 
                        do {
                                ret = waitpid(rwp->cpid, &status, WUNTRACED|WCONTINUED);
                        } while (!WIFEXITED(status) && !WIFSIGNALED(status));
                }
 
                close(rwp->pipe_r[0]);
                close(rwp->pipe_w[1]);
                free(rwp);
        }
 
        return ret;
}
 
RWPIPE *popen_rw(const char *command)
{
        RWPIPE *rwp = (RWPIPE *)malloc(sizeof(*rwp));
        if (rwp == NULL)
                return NULL;
 
        memset(rwp, 0x00, sizeof(*rwp));
        if (pipe(rwp->pipe_r) != 0 || pipe(rwp->pipe_w) != 0)
        {
                free(rwp);
                return NULL;
        }
 
        rwp->cpid = fork();
        if (rwp->cpid == -1)
        {
                free(rwp);
                return NULL;
        }
 
        if (rwp->cpid == 0)
        {
                dup2(rwp->pipe_w[0], STDIN_FILENO);
                dup2(rwp->pipe_r[1], STDOUT_FILENO);
 
                close(rwp->pipe_r[0]);
                close(rwp->pipe_r[1]);
                close(rwp->pipe_w[0]);
                close(rwp->pipe_w[1]);
 
                execl(command, command, NULL);
                printf("Error: fail to exec command - %s ..\n", command);
                exit (1);
        }
        else
        {
                close(rwp->pipe_r[1]);
                close(rwp->pipe_w[0]);
        }
 
        return rwp;
}
 
ssize_t read_p(RWPIPE *rwp, void *buf, size_t count)
{
        return read(rwp->pipe_r[0], buf, count);
}
 
ssize_t write_p(RWPIPE *rwp, const void *buf, size_t count)
{
        return write(rwp->pipe_w[1], buf, count);
}
 
int main(void)
{
        char rbuf[BUFSIZ], wbuf[BUFSIZ];
        int ret, len, n = 0;
 
        RWPIPE *rwp = popen_rw("./calc");
        if (rwp == NULL)
        {
                printf("Error: fail to open command ..\n");
                return EXIT_FAILURE;
        }
 
        while (1)
        {
                memset(rbuf, 0x00, sizeof(rbuf));
                if (read_p(rwp, rbuf, sizeof(rbuf)) < 1)
                {
                        printf("No more input..\n");
                        break;
                }
                n = atoi(rbuf);
 
                len = snprintf(wbuf, sizeof(wbuf), "%d\n", n * n);
                ret = write_p(rwp, wbuf, len);
                if (ret != len)
                {
                        printf("Write %d bytes (expected %d) ..\n", ret, len);
                        break;
                }
        }
        pclose_rw(rwp);
 
        return EXIT_SUCCESS;
}
$ gcc -g -W -Wall popen_rw.c
$ ls result.txt
ls: cannot access result.txt: No such file or directory
$ ./a.out
No more input..
$ cat result.txt
81
64
49
36
25
16
9
4
1