C no evil

Lab exercises: Due January 29th by midnight

The goals for this assignment are:

Working with stdout and stderr
Working with environment variables
Working with C strings

To get started, click on this Github Link.

1. Stack diagram I

Consider the following program.

int main() {
  const char* str = "unicorn";

  const char* m = NULL;
  for (const char* p = str; *p != '\0'; p++) {
    if (*p == 'c') m = p;
  }
  // Draw the stack here
  printf("%s\n", m);
  return 0;
}

Fill in a table that shows how the values of *p and *m change each iteration of the loop.

Loop Iteration

NA (NULL)

Draw the state of the stack (e.g. a stack diagram) after the loop completes. You do not need to show all intermediate steps.

2. Stack diagram II

Draw the stack for the following program. Be careful: strtok modifies the variable line by replacing the delimiters with the null character \0.

int main() {
  char line[1024]; // max of 1023 characters can be read from the line
  strcpy(line, "ls -l -a");

  int n = 0;
  char* list[128]; // max of 128 words can be put in the list
  char* token = strtok(line, delims);
  while (token)
  {
      list[n++] = token;
      token = strtok(NULL, delims);
  }
  list[n] = NULL;
  // Draw the stack here
  return 0;
}

3. stdout, stderr

Write a program, outerr.c, that outputs the even integers between 0 and 10 to standard out and the odd integers between 1 and 10 to standard error.

$ ./outerr
0
1
2
3
4
5
6
7
8
9
$ ./outerr > even.txt 2> odd.txt
$ cat even.txt
0
2
4
6
8
$ cat odd.txt
1
3
5
7
9

4. Environment

Write a program, myenv.c, that loops through the process’s environment variables and prints the value for PATH. Use the code below, based on pointer arithmetic, as a starting point.

int main(int argc, char** argv, char** envp)
{
  while(*envp)
  {
    printf("%s\n",*envp++);
  }
}

$ ./myenv
PATH=/home/alinen/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
$ env | grep PATH
PATH=/home/alinen/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

The shell command env prints the environment

The paths above were truncated for brevity. Yours will be much longer!

5. Submission

Both team members should submit their own code.

5.1. Submit your work to Github

Add and check in your program using git and then push your changes to Github. Run the following command inside your labs-USERNAME directory.

$ cd labs-USERNAME
$ git add .
$ git commit -m "Descriptive message"
$ git push

Run git status to check the result of the previous git command. Check the Github website to make sure that your program uploaded correctly.