CISC2005 Exercise 03

1. Prerequisites

1. Successful installation of *nix terminal (e.g., WSL, cygwin) or a *nix operating system (e.g., Linux, Mac).
   

For Windows system, I recommend you to install WSL. https://learn.microsoft.com/en-us/windows/wsl/install

1. Successful installation of a C compiler, GCC is recommended.
   
2. Successful installation of a text editor, VIM or Emacs is recommended.
   

1. Tasks

In this section, students are required to execute the following codes, and try to understand the behaviour. For each program, students should capture a screenshot of the successful execution, and answer attached questions briefly. In the submission file, please attach the execution screenshot and the explanation of each question in sequence.

Q1: Execute the code several times, and explain why the variable pid is consistent/inconsistent.

#include <stdio.h>

#include <sys/types.h>

#include <unistd.h>

int main() {

pid_t pid;

int i = 5;

printf("Hello EveryOne :), the value of i is %d: This statement before fork()\n", i);

pid = fork();

printf("The value of pid is %d. Thsi statement after fork(), Goodbye :)\n", pid);

return 0;

}

Q2: please explain the result of the variable myVariable in output.

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

int main(int argc, char *argv[]) {

int myVariable = 100;

printf("hello world (pid:%d)\n", (int)getpid());

printf("Value of myVariable before fork is %d\n", myVariable);

int rc = fork();

if (rc < 0) {

// fork failed; exit`

fprintf(stderr, "fork failed\n");

exit(1);

}

else if (rc == 0) {

myVariable = 200;

printf("The changed value of myVariable from child is %d\n", myVariable);

} else {

myVariable = 300;

printf("The changed value of myVariable from parent is %d\n", myVariable);

}

return 0;

}

Q3: Please explain the frequency of each level printed in the output.

#include <stdio.h>

#include <sys/types.h>

#include <unistd.h>

int main(){

pid_t pid1, pid2, pid3;

pid1 = 0; pid2 = 0; pid3 = 0;

pid1 = fork();

if (pid1 == 0) {

pid2 = fork();

pid3 = fork();

} else {

pid3 = fork();

if (pid3 == 0)

pid2 = fork();

}

if ((pid1 == 0) && (pid2 == 0))

printf("level1\n");

if (pid1 != 0)

printf("level2\n");

if (pid2 != 0)

printf("level3\n");

if (pid3 != 0)

printf("level4\n");

return 0;

}

Q4: Please explain that if the wait() function can be replaced by waiting for a while using a for-loop? Why?

#include <stdio.h>

#include <sys/types.h>

#include <sys/wait.h>

int main() {

int status;

pid_t pid;

pid = fork();

if (pid == -1)

printf("\n ERROR child not created.");

else if (pid == 0) {

printf("\n I'm the child!");

exit(0);

} else {

pid_t TCpid;

TCpid = wait(&status); //wait returns a exiting status of child process, as well as the child’s pid

printf("\n I'm the parent!");

printf("\n The child with pid = %d terminated with a status = %d \n", TCpid, status);

}

return 0;

}

Q5: Is it possible the “Child Complete” printed before execution of subprocess? What does wait(NULL) exactly mean?

#include <stdio.h>

#include <sys/types.h>

#include <unistd.h>

#include <stdlib.h>

int main() {

pid_t pid;

pid = fork();

if (pid < 0) {

fprintf(stderr, "Fork Failed");

exit(-1);

} else if (pid == 0) {

char ** args;

args = malloc(3 * sizeof(char*));

args[0] = "ls";

args[1] = "-l";

execv("/bin/ls", args);

} else {

wait(NULL);

printf("Child Complete\n");

exit(0);

}

}

Q6: Please try three different commands using myShell and capture the screenshot of execution. (e.g., ls, pwd.)

#include <stdio.h>

#include <string.h>

#include <unistd.h>

#include <stdlib.h>

int count(char* buffer) {

int count = 0;

char* argument;

argument = strtok(buffer, " \n");

while(argument != NULL) {

count++;

argument = strtok(NULL, " \n");

}

return count;

}

int main() {

char buffer[512];

char* path = "/bin/";

while(1) {

printf("myShell>");

fgets(buffer, 512, stdin);

int pid = fork();

if (pid != 0) {

wait(NULL);

} else {

int no_of_args = count(buffer);

char** array_of_strings = malloc((sizeof(char*)* (no_of_args+1)));

int i = 0;

char* ptr;

ptr = strtok(buffer, " \n");

while(ptr != NULL) {

array_of_strings[i] = (char*)malloc((sizeof(char)*strlen(ptr)));

strcpy(array_of_strings[i], ptr);

ptr = strtok(NULL, " \n");

i++;

}

char* prog = malloc((sizeof(char)*(no_of_args+1)));

prog = strcat(strcpy(prog, path), array_of_strings[0]);

int rv = execv(prog, array_of_strings);

}

}

return 0;

}

Q7: After executing the following code, a new file named myFile.txt is generated. Is the content in myFile.txt will be consistent? Why?

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <fcntl.h>

#include <errno.h>

#include <sys/wait.h>

int main(int argc, char *argv[]){

printf("hello world (pid:%d)\n", (int)getpid());

int fd = open("myFile.txt", O_CREAT|O_RDWR);

if(fd == -1 ) {

printf("Unable to open the file\n exiting....\n");

return 0;

}

int rc = fork();

if (rc < 0) {

fprintf(stderr, "fork failed\n");

exit(1);

}

else if (rc == 0) {

printf("hello, I am child (pid:%d)\n", (int)getpid());

char myChar='a';

write(fd, &myChar,1);

printf("writing a character to the file from child\n");

}

else {

printf("hello, I am parent of %d (pid:%d)\n",

rc, (int)getpid());

char myChar='b';

write(fd, &myChar,1);

printf("writing a character to the file from parent\n");

}

return 0;

}