Select the statements below which accurately describe the operation of this system. This system hangs without producing output.

Transcription

1 Consider the following code fragment. int fd[2]; char ch; pipe(fd); write(fd[1],"ab",2); if (fork() == 0) { read(fd[0],&ch,1); write(1,&ch,1); else{ read(fd[0],&ch,1); write(1,&ch,1); Select the statements below which accurately describe the operation of this system. The string ab or the string ba is output. The only output is the single character a. This system hangs without producing output. The string ab is output; there is no possibility that ba is output. None of the above. 1

2 Suppose the file named /tmp/datafile (protected mode 0666) contains the following 26 bytes of character data: abcdefghijklmnopqrstuvwxyz A program executes the following code fragment: char ch, ch1; fd=open("/tmp/datafile",o_rdwr); read(fd,&ch,1); lseek(fd,2,seek_cur); fd1=open("/tmp/datafile",o_rdwr); read(fd1,&ch1,1); if (ch==ch1) printf("equal"); else printf("not equal"); exit(0); What is the output by this program? equal not equal Something other than equal or not equal Cannot be determined from this code fragment 2

3 Suppose a UNIX process with PID 108. Further suppose that the process successfully uses one of the exec system calls to execute the sh shell. Which of the following best describes the PID of the executing sh process? The process has PID 109. The process has PID 108. It cannot be determined from the given information. None of the above. 3

4 Suppose the file named /tmp/datafile (protected mode 0666) contains the following 26 bytes of character data: abcdefghijklmnopqrstuvwxyz A program executes the following code fragment: char ch;... fd=open("/tmp/datafile",o_rdwr); read(fd,&ch,1); read(fd,&ch,1); if (fork()==0){ fd=open("/tmp/datafile",o_rdwr); read(fd,&ch,1); write(fd,&ch,1); wait(null); write(fd,&ch,1); Which of the following statements best describes the contents of datafile after the execution of this code? abcdefghijklmnopqrstuvwxyz abcccfghijklmnopqrstuvwxyz abccbfghijklmnopqrstuvwxyz aabdefghijklmnopqrstuvwxyz None of the above 4

5 char buf[5]; sharedfd=open("test",o_rdwr); sharedfdlock.l_type=f_rdlck; sharedfdlock.l_start=0; sharedfdlock.l_whence=seek_set; sharedfdlock.l_len=0; fcntl(sharedfd,f_setlkw, &sharedfdlock); i=0; while (read(sharedfd,&buf[i],1)!=0) i++; buf[i-1]= \0 ; sharedfdlock.l_type=f_unlck; fcntl(sharedfd,f_setlk, &sharedfdlock); lseek(sharedfd,0,seek_set); sharedfdlock.l_type=f_wrlck; fcntl(sharedfd,f_setlkw, &sharedfdlock); for (i=0;i<strlen(buf);i++){ buf[i]=buf[i]+1; write(sharedfd,&buf[i],1); sharedfdlock.l_type=f_unlck; fcntl(sharedfd,f_setlk, &sharedfdlock); Assuming that test initially contains aaa, can the contents of test be determined if two processes execute the code above simultaneously and no other processes access the file during this time? If so, what is the content of the file? 5

6 Jane wants to allow Jim to view the contents of any of the files in the project directory of her home directory. However, the system administrator will not create a group for Jim and Jane. Design a way for Jane to allow Jim to view these files, without allowing any other users to view the files. 6

7 Consider the following code fragment. int x; /* Procedure declarations */ main(){ int id[2]; /* INITIALIZATION CODE*/ fd=open("afile",o_rdwr); id[0]=fork(); if (id[0]==0){ /* CHILD 1 CODE */ else { id[1]=fork(); if (id[1]==0) { /* CHILD 2 CODE */ else { /* PARENT CODE */ Assume that afile is accessible for read and write by all processes created by the execution of this code, and assume that afile is initially empty. Give code for parent, child 1, and chold 2, so that the parent writes (only) a to the file, child 1 writes only b to the file, and child 2 writes only c to the file, and the final contents of the file is abc. Your solution may specify code that must be executed before any fork() and may define functions. 7

8 Consider the following two code fragments: /* FRAGMENT 1 */ #define MIN(x,y) (x < y)? x : y... int a, b, c, x, y;... a = 1; b = 2; x = 3; y = 4; c = MIN(a,b); /* FRAGMENT 2 */ int MIN(x,y) int x, y; { return ((x < y)? x : y);... int a, b, c, x, y;... a = 1; b = 2; x = 3; y = 4; c = MIN(a,b); Which of the following best describes the execution of these two codes The two fragments are not functionally equivalent. In identical system contexts, FRAGMENT 1 will be faster than FRAG- MENT 2. After FRAGMENT 1 executes, c will have value 3. After FRAGMENT 2 executes, c will have value 3. None of the above statetments are true. More than one of the above statements are true. 8