Lecture 20 – Oct 28th, 2019


  1. Log in to clyde.
  2. $ cp -r ~steve/ex/linenumbers .


  1. Modify linenumbers.c such that it reads a line from stdin using fgets(3) and writes to stdout using fprintf(3) (you’ll see why we’re using fprintf(3) rather than printf(3) shortly) except that each line should be prefixed with its line number as in this example.
    $ ./linenumbers < Makefile
    1: CC := clang
    2: CFLAGS := -Wall -std=c11 -g -fsanitize=address,undefined
    3: LDFLAGS := -fsanitize=address,undefined
    5: .PHONY: all clean
    7: all: linenumbers
    8: clean:
    9: 	$(RM) linenumbers

    You may assume a line is no longer than 1024 characters for now.

  2. Modify linenumbers.c such that if at least one argument is given (so argc > 1), the first argument is treated as the file to read rather than stdin. I.e., open the file using fopen(3). Make sure to handle the case where fopen(3) returns NULL by printing out an appropriate message using perror(3) and exiting with EXIT_FAILURE.

    If no arguments are given, then behave as before, namely reading from stdin. You’ll probably want code that looks like

    FILE *input = stdin;
    if (argc > 1) {
      input = fopen(/* ... */);
      if (!input) {
        // Handle the error.

    Here’s an example of the error handling.

    $ ./linenumbers "does not exist"
    does not exist: No such file or directory
    $ echo $?

    Make sure you fclose(3) the input when you’re done with it.

  3. Modify linenumbers.c such that if no arguments are given, it behaves as in part 1. If one argument is given, it behaves as in part 2. If two arguments are given, the output is written to the file named by the second argument rather than to stdout. And if more than two arguments are given, it prints out a usage message and exits with EXIT_FAILURE.

    Be careful that you don’t accidentally overwrite a file you care about.

    $ ./linenumbers one two three
    Usage: ./linenumbers [INPUT [OUTPUT]]

    Make sure you fclose(3) the output when you’re done with it.

  4. It’s traditional that if - is passed as a file name for input, then the input should come from stdin and if - is passed as a file name for output, then the output should go to stdout. Implement this functionality, using strcmp(3) from string.h.

    $ echo -e "Hello world\nHow are you?" | ./linenumbers - output.txt
    $ cat output.txt
    1: Hello world
    2: How are you?
  5. Modify linenumbers.c to remove the restriction on line lengths. You can use strchr(3) to search for a newline. If it doesn’t exist, output what you have so far, and then keep reading with fgets(3) until you reach the end of the line and output it using fputs(3).
  6. Modify linenumbers.c one final time to make sure that you handle error return values (which you really should have been doing all along) from each of the standard library functions: fopen(3) and fgets(3) return NULL on error; fputs(3) and fclose(3) return EOF on error; and fprintf(3)` returns a negative number on error.