Chapter 10, Problem 10.6HW

Program Plan Intro

Opening and Closing files:

Using “Open” function, a process can open an existing file or generates a new file.

• This function is used to converts a filename to a file descriptor and returns the result as a descriptor number.
• The descriptor returned is always the lowest descriptor that is not presently open in the process.
• Each process in the LINUX begins life with three open files.
  • Descriptor 0 – standard input.
  • Descriptor 1 – standard output.
  • Descriptor 2 – standard error.
• The “Open” function consists of three arguments. That is “Open(filename, flags, mode)”.
  • The argument “filename” defines the name of the given file
  • The argument “flags” represents how the process plans to access the file. Some flags names are as follows.
    • O_RDONLY – it means reading only.
    • O_WRONLY – it means writing only.
    • O_RDWR – it means reading and writing.
  • The “mode” argument identifies the access permission bits of new files.

Example:

The example for open an existing file for reading is shown below:

sample1 = Open("foo1.txt", O_RDONLY, 0);

From the above “open” function,

• The filename is “foo1.txt”.
• Flag name is “O_RDONLY”.
• Mode is “0”.

Explanation of Solution

Corresponding code from given question:

Main.c:

//Header file

#include "csapp.h"

//Main function

int main()

{

  //Declare int variable

  int fd1, fd2;

/* Open the file "foo.txt" using "Open" function and store descriptor number in fd1 */

  fd1 = Open("foo.txt",O_RDONLY, 0);

/* Open the file "bar.txt" using "open" function and store descriptor number in fd2 */

    fd2 = Open("bar.txt",O_RDONLY, 0);

  //Frees up the descriptor number in "fd2"

  Close(fd2);

/* Open the file "baz.txt" using "Open" function and store descriptor number in fd2 */

  fd2 = Open("baz.txt",O_RDONLY, 0);

  //Display the descriptor number in "fd2"

  printf("fd2 = %d\n", fd2);

  //Exit the process

  exit(0);

}

Explanation:

The given code is used to returns the descriptor number for given file.

• Include the header file
• Define the main function.
  • Declare two variables “fd1” and “fd2” in “int” data type.
  • Open the file “foo.txt” using “Open” function and store its descriptor number in “fd1”.
  • Open the file “bar.txt” using “Open” function and store its descriptor number in “fd2”.
  • Frees up the descriptor number in “fd2” using “Close” function.
  • Open the file “baz.txt” using “Open” function and store its descriptor number in “fd2”.
  • Then displays the descriptor number in “fd2”.
  • Finally exit the process using “exit” function.
• Before run the program, user needs to create the three files “foo.txt”, “bar.txt” and “baz.txt”.

Reasons for displaying given output:

• The “Open” function always returned the smallest unopened descriptor.
• From the given code, first call to “Open” returns the descriptor 3 that is the descriptor 3 is in “fd1”.
• Then call to “Open” returns the descriptor 4 that is the descriptor 4 is in “fd2”.
• After that “fd2” frees up using “Close” function.
• Now, call to “Open” function returns the descriptor 4 in “fd2”. Therefore, the output of the given code is “fd2 = 4”.

Sample Output

fd2 = 4