Question

This is for a Unix class.

Please fully answer the question :)

The figure that it references is on the attached picture.

I have also added the information that comes with that figure.

3.3 Assume that a process executes the following three function calls: fdl-open (path, oflags); fd2 = dup ( fd ) ; fd3-open (path, oflags); Draw the resulting picture, similar to Figure 3.9. Which descriptors are affected by an fentl on fdl with a command of F_SETFD? Which descriptors are affected by an fentl on fdl with a command of F SETFL? dup and dup2 Functions An existing file descriptor is duplicated by either of the following functions: #include int dup(int fd); int dup2 (int fd, int fd2); Both return: new file descriptor if OK, -1 on error The new file descriptor returned by dup is guaranteed to be the lowest-numbered available file descriptor. With dup2, we specify the value of the new descriptor with the fd2 argument. If fd2 is already open, it is first closed. If fd equals fd2, then dup2 returns fd2 without closing it. Otherwise, the FD_CLOEXEC file descriptor flag is cleared for fd2, so that fd2 is left open if the process calls exec The new file descriptor that is returned as the value of the functions shares the same file table entry as the fd argument. We show this in Figure 3.9. process table entry file table file status flags current file offset v-node pointer v-node table v-node informatio v data fd flags pointer fd 0: fd 1 fd 2: fd 3: i-node informatiorn current file size i_vnode Figure 3.9 Kernel data structures after dup (1) In this figure, we assume that when it's started, the process executes newfd dup ( 1 ) ; We assume that the next available descriptor is 3 (which it probably is, since 0, 1, and 2 are opened by the shell). Because both descriptors point to the same file table entry, they share the same file status flags-read, write, append, and so on-and the same current tile offset. Each descriptor has its own set of file descriptor flags. As we describe in Section 3.14, the close-on-exec file descriptor flag for the new descriptor is always cleared by the dup functions. Another way to duplicate a descriptor is with the fcntl function, which we describe in Section 3.14. Indeed, the call dup(fd); is equivalent to fcntl(fd, F_DUPFD, 0); Similarly, the call dup2 (fd, fd2) is equivalent to close (fd2); fcntl(fd, F DUPFD, fd2); In this last case, the dup2 is not exactly the same as a close followed by an fcntl. The differences are as follOwS: dup2 is an atomic operation, whereas the alternate form involves two function calls. It is possible in the latter case to have a signal catcher called between the close and the fcntl that could modify the file descriptors. (We describe signals in Chapter 10.) The same problem could occur if a different thread changes the file descriptors. (We describe threads in Chapter 11.) 1. 2. There are some errno differences between dup2 and fcntl The dup2 system call originated with Version 7 and propagated through the BSD releases. The fentl method for duplicating file descriptors appeared with System III and continued with System V. SVR32 picked up the dup2 function, and 42BSD picked up the fentÏ function and the F_DUPFD functionality. POSIX.1 requires both dup2 and the F_DUPFD feature of fentl.

Answer 1

fd1 = open(path,oflags);

fd2 = dup(fd1);

when it's duplicated the next available descriptor is 4 . That's the reason it is used.

fd3 = open(path,oflags);

Lvnedo

F_SETFD (int)

This command is used to manipulate the flags associated with a file descriptor. It sets the file descriptor fd1 flags to the value specified by arg.

F_SETFL (int)

It set the file status flags of fd1 descriptor to the value specified by argument. File access mode that are O_RDONLY, O_WRONLY, O_RDWR and file creation flags such as O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC in argument are ignored.