I/O Redirection- How does the shell do
- The quick answer:
- We modify the file descriptor table
- We need to take a closer look at how the file descriptor table works
File Descriptor Table (FD Table)- Each Process has a file descriptor table in the kernel (in the Process Control Block)
- fork() duplicates the file descriptor table
- Draw picture of two file descriptor tables with stdin, stdout, stderr.
Modifying the FD Table- In order to redirect input/output, we need to modify the file descriptor table
- New system call:
- int dup(int oldfd)
- Makes a copy of the given fd in the first available fd table slot
- We can use close() to open up slots
Redirection Exampleredirect.c/* redirect.c - example of redirection */
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char **argv)
{
pid_t id;
int fd;
if ((fd = open("out", O_CREAT | O_WRONLY, 0644)) < 0) {
printf("cannot open out\n");
exit(1);
}
id = fork();
if (id == 0) {
/* we are in the child */
/* close stdout in child */
close(1);
dup(fd); /* replace stdout in child with "out" */
close(fd);
execl("/bin/date", "date", NULL);
}
/* we are in the parent */
close(fd);
id = wait(NULL);
return 0;
}
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