OPERATING SYSTEMS LABORATORY II - C Review I

IMPORTANT: If gcc command does not work on your own system, it should be installed first. Below are the commands for installing gcc on various Linux distributions; you just have to run the command (assuming you have a working internet connection) for your OWN distribution ONLY ONCE.

1. Pardus: sudo pisi it gcc
2. Ubuntu, Kubuntu, Debian, etc: sudo apt-get install build-essential
3. Fedora, Scientific Linux, etc: su -c ``yum install gcc''

Each of these commands will require the user to enter password.

1. Using argc and argv as command line arguments.
   • http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code5.c code5.c

     #include <stdio.h>
     int main(int argc, char *argv[ ])
     {
      int i;
      for (i=0; i < argc; i++)
      printf("command line argument [%d] = %s \n",i, argv[i]);
     }
     

   • Analyze the code.
   • Execute the code. What is the output and why?
   • Execute as

      ./code5 your name your surname your age
     

   • Describe the functionalities of argc and argv.
2. Arrays, Pointers and Dynamic Memory Allocation
   1. Pointer: A variable that contains the address of a variable. http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code6.c code6.c

      #include <stdio.h>
      int main(int argc, char *argv[])
      {
        int x = 1, y = 2, z[10];
        int *ip; /* ip is a pointer to int */
        ip = &x; /* ip now points to x */
        printf("The address of pointer 'ip' is %p \n",&ip);
        printf("The thing that pointer 'ip' contains inside is %p \n",ip);
        printf("The thing that pointer 'ip' points is %d \n",*ip);
        printf("The address of variable 'x' is %p \n",&x);
        printf("The value of variable 'x' is %d \n",x);
        y = *ip; /* y has the value of x now */
        *ip = 0; /* x is 0 now */
        ip = &z[0]; /* ip now points to z[0] */
        return 0;
      }
      

      • Analyze the code.
      • Execute the code. What is the output and why?
      • Exercise: Modify the code above that ;
        • creates two 'double' type pointers,
        • puts numbers in,
        • prints out the values inside the pointers (address info.),
        • prints out the values that pointers point.
   2. In C, there is a strong relationship between pointers and arrays, strong enough that pointers and arrays should be discussed simultaneously. The pointer version of any code will in general be faster (Why?). http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code7.c code7.c
      • Analyze the code.
      • Execute the code several times. What is the output and why? Observe the changes in the addressing scheme.
   3. Dynamic Memory Allocation: Allocating memory at runtime.
      • Malloc; http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code8.c code8.c
        • Analyze the code.
        • What is the function of 'malloc'.
      • Realloc; http://siber.cankaya.edu.tr/OperatingSystems/cfiles/code9.c code9.c
        • Analyze the code.
        • What is the function of 'realloc'.
        • What is happening by the assignment? 'ip = tmp;'
      • For other memory related functions take a look at man pages. e.g., man malloc,
        • What is the difference between malloc and calloc? (We will discuss later)
        • What is brk()? (We will discuss later)