[Operating system]

In the following code it uses pipes and file descriptors. It opens and close the pipes but I don't understand why and when it needs to do such a thing. I know this program will read input and print, but not very clear about other things.

#include < stdio.h >

#include < stdlib.h >

#include < unistd.h >

#include < string.h >

int main()

{

char mapl[] = "qwertyuiopasdfghjklzxcvbnm"; /* for encoding letter */

char mapd[] = "1357924680"; /* for encoding digit */

int fd[2]; /* for the pipe */

char buf[80], buf2[80];

int i, j, n, returnpid;

if (pipe(fd) < 0) {

printf("Pipe creation error ");

exit(1);

}

returnpid = fork();

if (returnpid < 0) {

printf("Fork failed ");

exit(1);

} else if (returnpid == 0) { /* child */

/* close(fd[1]); child forgets to close out */

while ((n = read(fd[0],buf,80)) > 0) { /* read from pipe */

buf[n] = 0;

printf(" message [%s] of size %d bytes received ",buf,n);

sleep(3); /* add this line to delay child */

for (i = 0, j = 0; i < n; i = i+2, j++) /* skip the odd characters */

buf2[j] = buf[i];

write(fd[1],buf2,j); /* accidentally echo back new string via fd[1] */

}

close(fd[0]);

printf(" I have completed! ");

} else { /* parent */

close(fd[0]); /* close parent in */

while (1) {

printf(" please enter a message ");

n = read(STDIN_FILENO,buf,80); /* read a line */

if (n <= 0) break; /* EOF or error */

buf[--n] = 0;

printf(" message [%s] is of length %d ",buf,n);

for (i = 0; i < n; i++) /* encrypt */

if (buf[i] >= 'a' && buf[i] <= 'z')

buf[i] = mapl[buf[i]-'a'];

else if (buf[i] >= 'A' && buf[i] <= 'Z')

buf[i] = mapl[buf[i]-'A']-('a'-'A');

else if (buf[i] >= '0' && buf[i] <= '9')

buf[i] = mapd[buf[i]-'0'];

printf(" sending encrypted message [%s] to child ",buf);

write(fd[1],buf,n); /* send the encrypted string */

}

close(fd[1]);

wait(NULL);

printf(" I have completed! ");

}

exit(0);

}