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CISC2005 Exercise 03
- Prerequisites
- 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
- Successful
installation of a C compiler, GCC is recommended.
- Successful
installation of a text editor, VIM or Emacs is recommended.
- 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;
}
