dup2

dup2.c

#define _POSIX_SOURCE
// If you define this macro, then the functionality from 
// the POSIX.1 standard (IEEE Standard 1003.1) is available
#include <errno.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#undef _POSIX_SOURCE
#include <stdio.h>

void print_inode(int fd) {
    struct stat info;
    // int fstat(int fd, struct stat *buf);
    // Return information about a file.

    // char *strerror(int errnum);
    // The strerror() function returns a pointer to a string that
    // describes the error code passed in the argument errnum.
    if (fstat(fd, &info) != 0)
      fprintf(stderr,"fstat() error for fd %d: %s\n",fd,strerror(errno));
    else
      printf("The inode of fd %d is %d\n", fd, (int) info.st_ino);
}

int main() {
    int fd;
    char fn[]="dup2.file";

    // creat(path, mode) <==> open(path, O_WRONLY|O_CREAT|O_TRUNC, mode)
    // O_WRONLY <==> write-only
    // O_CREAT <==> If pathname does not exist, create it as a regular file.
    // O_TRUNC <==> see doc
    // mode applies only to future accesses of the newly created file
    // S_IWUSR <==> 00200 user has write permission
    if ((fd = creat(fn, S_IWUSR)) < 0)
      perror("creat() error");
    else {
      print_inode(fd);

      // int dup2(int oldfd, int newfd);
      // 
      if ((fd = dup2(0, fd)) < 0)
        perror("dup2() error");
      else {
        puts("After dup2()...");
        print_inode(0);
        print_inode(fd);
        puts("The file descriptors are different but they");
        puts("point to the same file which is different than");
        puts("the file that the second fd originally pointed to.");
        close(fd);
      }
      // int unlink(const char *pathname);
      // unlink() deletes a name from the filesystem.
      unlink(fn);
    }
}