Operating Systems 2019F Lecture 4

Video

Video from the lecture given on Sept. 13, 2019 is now available.

Topics

• files
• open, read, write, close system calls
• I/O redirection
• command line arguments
• programmatic directory access

Notes

What can you do with a file normally?
* open it <--- why?
* read from
* write to
* close it <--- why?

By having open and close, we tell the OS that
 - we're going to do multiple operations on this file, and
 - save the mapping of the filename to the underlying file data
 - do permission checks now, not when we access the file

When you open a file, you get a file descriptor (a small int)

Code

filetest.c

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>

int main(int argc, char *argv[], char *envp)
{
        int fd;
        char *msg = "Hello world!\n";
        char *stdmsg = "Hello standard out!\n";

        write(1, stdmsg, 20);
        
        /* open a file */

        fd = open("test.txt", O_CREAT|O_APPEND|O_WRONLY,
                  S_IRUSR|S_IWUSR);        

        printf("Got file descriptor %d\n", fd);
        
        /* write something to the file */
        write(fd, msg, 13);

        /* close the file */
        close(fd);

        return 0;
}

dirtest.c

#include <sys/types.h>
#include <dirent.h>
#include <stdio.h>

int main(int argc, char *argv[], char *envp[])
{
        DIR *d;
        struct dirent *entry;

        if (argc > 1) {
                d = opendir(argv[1]);
        } else {
                d = opendir(".");
        }
        
        entry = readdir(d);
        while (entry) {
                printf("Entry: %s\n", entry->d_name);
                entry = readdir(d);                
        }

        closedir(d);

        return 0;
}