将未命名管道的输出重定向到c中的日志文件

Question

i want to redirect the output of an unnamed pipe to an opened log file in c but i can't seem to make it happen, my code looks like this:

close(fildes[1]); 

FILE * file = fopen(logfile, "w");
int fd = fileno(file);

if (fd == -1) {
bail_out(EXIT_FAILURE, strerror(errno));   
}

/* if (dup2(fd, fildes[0]) == -1) {
      bail_out(EXIT_FAILURE, strerror(errno));   
   } */


/* for testing purposes */
char reading_buf[3];
while(read(fildes[0], reading_buf, 3) > 0) {
  write(fd, reading_buf, 1); 
}


(void)wait(&status);
close(fildes[0]);
break;

the pipe gets filled, i've tested that with the while loop at the bottom. but when i comment out the dup2 call nothing gets redirected to the file. i think i don't fully understand dup2 and pipes.

Also: if i dup2 the reading end of the pipe and then close the original fildes[0], does the pipe get closed? or is it simply the FD that gets closed. not entirely should about that either..

Answer 1

To redirect output from a pipe to a file, somebody needs to read from the read end of the pipe and write to the file's file descriptor. It can't be done merely by duping the file descriptors.

For instance, say you have a pipe

int filedes[2];
pipe (filedes);

and a file

FILE *logfile = fopen (logfile_path, "w");
int logfd = fileno (logfile);

you could launch a child process to do the redirection using the cat command

int child = fork ();
if (child == 0) {
    // close the write end of the pipe
    close (filedes[1]);
    // make STDOUT point to the log file
    dup2 (logfd, STDOUT_FILENO);
    // make STDIN point to the read end of the pipe
    dup2 (filedes[0], STDIN_FILENO);
    // launch cat
    execlp ("cat", "cat", (char*) NULL);
    // in case of error
    exit (1);
}

Now, whatever is written to the write end of the pipe in the parent will be read by the child executing cat and written to the file.

// close the read end of the pipe, it's not needed in the parent
close (filedes[0]);
// write something
const char *msg = "Hello World!\n";
write (filedes[1], msg, strlen (msg));
// close the write end of the pipe, this will make the child read EOF
close (filedes[1]);

Don't forget to collect the zombie

wait (&status);

and close the file

fclose (logfile);

---

About your last question, when a file descriptor is duped there will be two of them pointing to the same underlying open file. So, when one of them is closed the file remains open for it can be accessed through other descriptor.

This is explained in the close man page:

If fd is the last file descriptor referring to the underlying open file description (see open(2)), the resources associated with the open file description are freed;