File Descriptors, Open Files and Inodes.

• Figure 5-2 from pate 95 of the book.
• 
• File Descriptors are in index into a per-process table.
  • As we saw in the last section, multiple file descriptors in a process can point to the same file. (newfd = fcntl(fd, F_DUPFD))
  • This happens across processes as well (a child process inherits the parent's open files )
  • Information in this table:
    • This includes flags for the file descriptor (O_CLOEXEC)
    • A pointer to the open file table.
• The file table is system wide
  • Multiple processes can access the same file at the same time.
  • This contains per file information
    • The current file offset.
    • Flags related to opening the file
    • The access mode specified in open
    • Signal related settings
    • A reference to the inode.
• The inode is a file system structure
  • Contains:
    • file type
    • permissions
    • locks applied to the file.
    • Timestamps (access, last write, creation ...)
  • It does NOT contain the file name.
• He points out
  • Two different file descriptors that return to the same file share an offset value.
    • Reads, writes and seeks in one have an effect on the other.
• The DUP family

  • #include <unistd.h>
    
    int dup(int oldfd);
    int dup2(int oldfd, int newfd);
    
    #define _GNU_SOURCE             /* See feature_test_macros(7) */
    #include <fcntl.h>              /* Obtain O_* constant definitions */
    #include <unistd.h>
    
    int dup3(int oldfd, int newfd, int flags);
           

  • Dup will make a copy of the file descriptor at the lowest numbered unused file descriptor.
    • dup(oldfd);
  • dup2(oldfd, newfd)
    • Will make a copy of oldfd at newfd.
    • If newfd is open, it will close it.
    • If oldfd is not valid, newfd is not closed, error
    • If oldfd == newfd, nothing happens.
  • In both cases, the old and new fd are the same
    • Except file descriptor flags are not shared (FD_CLOEXEC is off on the new fd)
  • dup3 is the same as dup2 except
    • You can set flags on the new fd. (O_CLOEXEC)
    • An error is returned if oldfd==newfd
    • This is linux specific.
  • Return value: valid fd on success or -1 on error.
  • The man page warns that errors related to closing newfd are lost with dup2 and dup3.