IC221 Lab 6: Basic kerenel-based I/O with file descriptors

Background

Up 'til now, we've been using the C standard library routines fopen, fclose, fscanf and fprintf for I/O. We know from last class, that these standard libary routines must, at some point, use system calls to do thier job. The basic I/O system calls are open, close, read and write. Whereas C standard library routines use FILE* (file stream pointer) objects to reference connections to files, system calls use file descriptors. How they fit into the kernel's I/O architecture was explained last class.

In this lab, you'll implement the basic wc functionality using system calls for I/O. Your simple wc function will take a list of files and print the total number of bytes in all the files put together.

Basic I/O operations with file streams vs. file descriptors

Operation/concept	C standard library	system call
reference to file connection	file stream ponter (FILE*)	file descriptor (int)
open a file connection	FILE *fopen(const char *filename, const char *mode);	int open(const char *path, int oflag, /* mode_t mode */);
close file connection	int fclose(FILE *stream);	int close(int fildes);
read from file connection	int fscanf(FILE *restrict stream, const char *restrict format, ...);	ssize_t read(int fildes, void *buf, size_t nbyte);
write to file connection	int fprintf(FILE *restrict stream, const char *restrict format,/* args*/ ...);	ssize_t write(int fildes, const void *buf, size_t nbyte);
predefined file connections	stdin, stdout, stderr	STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO (0, 1 and 2, respectively)

Questions to answer: 25 points

Create a lab06 dirctory. Copy these questions into a file ans.txt (in your lab06 directory) and add the answers to the questions. Do this before the rest of the lab!

• 1. Do a man -s2 open. Describe open's return value.

• 2. Do a man -s2 open. Here's a quote:

  Values for oflag are constructed by  a  bitwise-inclusive-OR
  of  flags  from  the  following  list, defined in <fcntl.h>.
  ...
  	      

  "Bitwise-inclusive-OR" is a single "|". So a|b is the bitwise-OR of a and b. This means that the ith bit of the result is the OR of the ith bits of a and b. Given this, read the man page and explain the differences between these three calls to open:

  open("foo",O_WRONLY);
  open("foo",O_WRONLY|O_APPEND);
  open("foo",O_WRONLY|O_CREAT);
  	    

• 3. As described in lecture, stdin, stdout and stderr are file descriptors 0, 1, and 2, respectively. You can also use the constants STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO, which are just long, descriptive names for 0, 1 and 2.

  Read the man page for dup2.

  • a. Is this a system call? How can you tell?
  • b. After reading the DESCRIPTION from the manpage, describe the net effect of the following two-statement sequence:

    int fd = open("foo",O_RDONLY);
    dup2(fd,STDIN_FILENO);
    		  

• 4. Do a man -s2 close and describe one circumstance that could cause a call to close to fail.

• 5. Here's the prototype for write:

  ssize_t write(int fildes, const void *buf, size_t nbyte);
  	    

  and consider the following call to write, which is a correct call:

  write(2,"hi\n",3);
  	    

  • a. Where is this output going?
  • b. Why must the third argument be a 3?

• 6. Do a man -s2 read. Look at the return value. Note that if end-of-file is encountered and nothing is read in, the return value is 0. This is different from fscanf! Here's the prototype:

   ssize_t read(int fildes, void *buf, size_t nbyte);
  	    

  Explain why the folllowing two function calls are OK

  char c;                  char B[20];
  read(0,&c,1);            read(0,B,20);
  		

  but this call is not:

  char *p;
  read(0,p,1);
  		

• 7. Do a man -s2 dup. If the call

  int fd = dup(1);
  	    

  ... is made, what would change in the process's file descriptor table, and what would change in the system open file table entry that file descriptor 1 refers to? HINT: You might want to look at the Class 9 notes to remind yourself about these kernel structures.

Part 1: 25 Points

Create a C source code file named lab06.c that compiles to a program mywc that reads a character/byte at a time from standard input using the read system call, and writes out the number of characters/bytes read 'til end of file. You may use fprintf to produce the output. Here's an example run:

bash$ mywc < /usr/bin/gs
8304168

Part 2: 20 Points

The time utility takes a shell-command string, executes it, and returs the number of seconds of "user" time, "system" time, and actual "wall-clock" time required to execute. Use it to time your solution to Part 1, i.e.

bash$ time mywc < /usr/bin/gs
	

and record the number in ans.txt. Now modify your lab06.c so that your mywc calls to read attempt to read 100 characters/bytes at a time into a 100-character array. Rerun the timing command, and add the new time to ans.c along with an explanation for the difference in the times. NOTE: pay close attention to read's return value in order to get this right!

Part 3: 20 Points

Now modify your lab06.c so that your mywc accepts a command-line argument. With no arguments, the program runs as before, with an argument, however, it treats the argument as a file name and reads from that file instead of standard input. Note that you need to spit out error messages and exit if the filename is bad. Read the man page for open in order to do that. Here are some example runs:

bash$ mywc < /usr/bin/gs
8304168
bash$ mywc /usr/bin/gs
8304168
bash$ mywc lkjlkjk
mywc: Error!  File "lkjlkjk" not found!
	

Part 4: 10 Points

Now modify your lab06.c so that your mywc accepts an arbitrary number of command-line arguments, each of which are file names. Your mywc program should return the total combined number of characters/bytes in all the files listed on the command line. If none are listed, return the number of bytes read from standard input, as before. Here are some example runs:

bash$ mywc < /usr/bin/gs
8304168
bash$ mywc /usr/bin/gs /etc/hosts /usr/include/math.h 
8315730
bash$ mywc /usr/bin/gs /etc/hosts lkjlkjk /usr/include/math.h
mywc: Error!  File "lkjlkjk" not found!
	

Submission

Submit the lab06 directory via the usual submit script.