PESIT Bangalore South Campus Hosur road, 1km before Electronic City, Bengaluru -100 Department of Information Sciences and Engineering

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1 INTERNAL ASSESSMENT TEST 2 Solutions 1. Explain the working of the waitpid() API with the help of a program. The program needs to take 2 command line arguments: the first argument should be used as the exit status by he child process. The second argument, whenever passed, is just an indication that the child process has to dump core. When no argument is passed, the child process should wait for any of the following signals: STOP, CONTINUE or TERMINATE. The parent should report the corresponding termination status. (10 Marks) /* Program to show the working of waitpid */ #include <sys/wait.h> #include <stdlib.h> #include <unistd.h> #include <stdio.h> int main(int argc, char *argv[]) pid_t cpid, w; int status; if ((cpid = fork()) == -1) perror ("fork failure"); */ if (cpid == 0) /* Child process */ printf("child PID is %ld\n", (long) getpid()); if (argc == 1) pause(); /* Wait for signals _exit(atoi(argv[1])); else /* Code executed by parent */ do B.E 6 th Semester Page 1 of 1

2 w = waitpid(cpid, &status, WUNTRACED WCONTINUED); if (w == -1) perror("waitpid"); exit(exit_failure); if (WIFEXITED(status)) printf("exited, status=%d\n", WEXITSTATUS(status)); else if (WIFSIGNALED(status)) printf("killed by signal %d\n", WTERMSIG(status)); else if (WIFSTOPPED(status)) printf("stopped by signal %d\n", WSTOPSIG(status)); else if (WIFCONTINUED(status)) printf("continued\n"); while (!WIFEXITED(status) &&!WIFSIGNALED(status)); exit(exit_success); Explain the waitpid API and the significance of the different values for the first argument and the third argument. B.E 6 th Semester Page 2 of 1

3 2. With the help of a C program, explain how the UNIX Operating System supports file and record locking. (10 Marks) /* USP Locking Example */ #include <fcntl.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> int main (int argc, char **argv) int fd; struct flock lock; if (argc < 2) printf ("Usage: %s FileName\n", argv [0]); printf ("opening %s\n", argv [1]); /* Open a file descriptor to the file. */ if ((fd = open (argv [1], O_WRONLY)) == -1) perror ("Error in opening file"); printf ("locking\n"); /* Initialize the flock structure. */ memset (&lock, 0, sizeof(lock)); lock.l_type = F_WRLCK; /* Place a write lock on the file. */ if (fcntl (fd, F_SETLKW, &lock) == -1) perror ("Error in locking file"); B.E 6 th Semester Page 3 of 1

4 printf ("locked; hit Enter to unlock... "); /* Wait for the user to hit Enter. */ getchar (); printf ("unlocking\n"); /* Release the lock. */ lock.l_type = F_UNLCK; if (fcntl (fd, F_SETLKW, &lock) == -1) perror ("Error in unlocking file"); close (fd); return 0; Explain the locking concept. Also, explain how this single program needs to be executed to demonstrate file locking. B.E 6 th Semester Page 4 of 1

5 3. Write short notes on the following: a. APIs related to symbolic links (5 Marks) Symbolic links symlink - make a new name for a file Synopsis #include <unistd.h> int symlink(const char *oldpath, const char *newpath); Description symlink() creates a symbolic link named newpath which contains the string oldpath. readlink - read value of a symbolic link Synopsis #include <unistd.h> ssize_t readlink(const char *path, char *buf, size_t bufsiz); Description readlink() places the contents of the symbolic link path in the buffer buf, which has size bufsiz. readlink() does not append a null byte to buf. It will truncate the contents (to a length of bufsiz characters), in case the buffer is too small to hold all of the contents. b. Exit handlers. (5 Marks) Explain the use of atexit() function to register exit handlers, their purpose and their execution. B.E 6 th Semester Page 5 of 1

6 4. With the help of a relevant C program, explain the working of setjmp and longjmp APIs.. (10 Marks) Explain the functionality of setjmp and longjmp APIs. Then the program. #include <stdio.h> #include <setjmp.h> static jmp_buf buf; void second(void) printf("second\n"); // prints longjmp(buf,1); // jumps back to where setjmp was called // making setjmp now return 1 void first(void) second(); printf("first\n"); // does not print int main() if (! setjmp(buf) ) first(); // when executed, setjmp returns 0 else // when longjmp jumps back, setjmp returns 1 printf("main\n"); // prints B.E 6 th Semester Page 6 of 1

7 return 0; Explain the nonlocal goto concept using setjmp and longjmp. B.E 6 th Semester Page 7 of 1

8 5. Keeping two related processes in mind, explain with the help of a relevant diagram, how the UNIX kernel supports processes. (10 Marks) Explain the diagram in detail. B.E 6 th Semester Page 8 of 1

9 6. With a relevant code snippet, bring out the differences between the working of the fork() and vfork() APIs. (10 Marks) Explain the following program using either fork or vfork and bring out the differences. Sample code. #include <unistd.h> /* Symbolic Constants */ #include <sys/types.h> /* Primitive System Data Types */ #include <errno.h> /* Errors */ #include <stdio.h> /* Input/Output */ #include <sys/wait.h> /* Wait for Process Termination */ #include <stdlib.h> /* General Utilities */ int glob = 6; int main (void) int var; pid_t pid; var = 88; printf ( Before vfork\n ); if ((pid = vfork()) < 0) err_sys ( vfork error ); else if (pid == 0) glob ++; var ++; _ printf ( pid = %d, glob = %d, var = %d\n, getpid(), glob, var); B.E 6 th Semester Page 9 of 1

10 7a. With the help of a relevant diagram, explain the memory layout of a C program. (6 Marks) Command-line arguments and environment variables Stack Heap Uninitialized data (bss) Initialized data Text B.E 6 th Semester Page 10 of 1

11 7b. Explain briefly the working of the alloca() API. (4 Marks) alloca function Has the same calling sequence as malloc(); however, instead of allocating memory from the heap, memory is allocated from the stack frame of the current function. What is the advantage? We don t need to free this memory. It goes away automatically when the function returns. The alloca function increases the size of the stack frame. Disadvantage: Some systems can t support alloca; it s impossible to increase the size of the stack frame after the function has been called. B.E 6 th Semester Page 11 of 1