TDC368 UNIX and Network Programming Camelia Zlatea, PhD Week 6:  Inter-Process Synchronization  Signals  Lock-Files

UNIX Network Programming – TDC Page 2 Spring 2003 Discussion  File Redirectation  Inter-process Communication via pipe –Pipe synchronization –Deadlock? »A deadlock situation can occur when two processes open respectively the two ends of a pipe for writing –Starvation? »Blocking reader / writer for unbounded interval of time –Errors »“Broken Pipe”? SIGPIPE –Process Type »Related process(named or unamed pipes) »Unrelated process (only named pipes)  Pipe vs. File for inter-process communication?

UNIX Network Programming – TDC Page 3 Spring 2003 I/O redirection - ex: sort result #include int main(void) { int fd1, fd2; switch (fork()) { case -1: perror("fork"); break; case 0: if ((fd1 = open("test.dat", O_RDONLY))==1) || (fd2 = open("results", O_WRONLY|O_CREAT|O_TRUNC,0644))==-1) { perror("open failure"); _exit(errno); } /* set standard input from "test.dat" */ if (dup2(fd1,0)==-1) _exit(errno); /* set standard output to "results" */ if (dup2(fd2,1)==-1) _exit(errno); close(fd1); close(fd2); execl("sort","sort"); perror("execl"); exit(errno); } return 0; }

UNIX Network Programming – TDC Page 4 Spring 2003 Simulating sleep by using alarm and pause #include void wakeup() { ; }; unsigned int sleep ( unsigned int timer ) { if (sigset(SIGALRM, wakeup)==-1) { perror("sigset"); return 1; } (void)alarm( timer ); (void)pause(); return 0; }

UNIX Network Programming – TDC Page 5 Spring 2003 Example: A Producer-Consumer relation int main(void) { int fd, n, i; pid_t pid, ppid; char buf[1]; if((fd=open("/tmp/data_file", O_APPEND|O_CREATE, 0640)) <0) exit(1); sigset(SIGTERM,SIG_IGN);/* signal */ ; sigset(SIGINT,SIG_IGN); /* signal */ pid=fork(); switch (pid) { case -1: { perror(“FORK”); exit(1); } case 0: /* child process - Producer */ sigset(SIGUSR1,wakeup); sighold(SIGUSR1); /* block / hold signals SIGUSR1 until sigpause*/ for (i=0; i<=100; i++) { /* sleep a random amount of time */ n = (int)(getpid()%256); srand((unsigned)n); sleep(rand() %5); /* writes a character in file */ sprintf(buf,"%d",i); write(fd, buf,sizeof(buf)); fprintf(stderr,"Producer PID=%d value = %d\n",getpid(), i); ppid=getppid(); kill(ppid, SIGUSR2); sigpause(SIGUSR1);/* pause(); until SIGUSR! received*/ } break; void wakeup() { ; };

UNIX Network Programming – TDC Page 6 Spring 2003 Example: A Producer-Consumer relation default: /* -parent code - Consumer */ sigset(SIGUSR2,wakeup); sighold(SIGUSR2); /* block / hold signals SIGUSR2 until sigpause*/ for (i=0; i<=100; i++) { /* sleep a random amount of time */ n = (int)(getpid()%256); srand((unsigned)n); sleep(rand() %5); sigpause(SIGUSR2); /* pause(); */ /* reads a character from file */ read(fd, buf,sizeof(buf)); fprintf(stderr,"Consumer PID=%d value=%d\n",getpid(),atoi(buf)); kill(pid, SIGUSR1) } break; } exit(0); }

UNIX Network Programming – TDC Page 7 Spring 2003  Provide a mechanism to share data among processes  Provide a mechanism to synchronize reading and writing of shared files.  Applicable for regular files  A process imposes a lock on a file(or record) so that other processes can’t modify the file(record) until it is unlocked by the process. UNIX File/Record Locking

UNIX Network Programming – TDC Page 8 Spring 2003 UNIX File/Record Locking  write lock –exclusive lock –processes are prevented from setting any overlapping read/write locks on the locked region of the file  read lock –shared lock –processes are prevented from setting any overlapping write locks on the locked region of the file

UNIX Network Programming – TDC Page 9 Spring 2003  mandatory lock –enforced by OS kernel –example: chmod a+l file_name  an erroneous situation: A process sets a mandatory lock on a file and never unlocks it. The file cannot be accessed by any other process until the owner process is either killed or system is re-booted UNIX File/Record Locking

UNIX Network Programming – TDC Page 10 Spring 2003  advisory lock –processes can still use read or write to access the shared file unless cooperation procedure is followed  processes using advisory locks to share a file must cooperate: –try to set a lock on the record to be accessed (read or write) –if fails to set the lock then: »wait and test_again or »do_work and test_again –if succeeds to set the lock »accesses the locked file/record »releases the lock UNIX File/Record Locking

UNIX Network Programming – TDC Page 11 Spring 2003 System V 4 and POSIX.1 #include int fcntl(int fd, int cmd,struct flock fl); cmd: –F_SETLK »sets a file lock; »do not block if failure –F_SETLKW »sets a file lock »blocks until succeeds –F_GETLK »tests if a file or record is locked and by whom UNIX File/Record Locking

UNIX Network Programming – TDC Page 12 Spring 2003 System V 4 and POSIX.1 struct flock { shortl_type; /* lock type */ shortl+whence; /* rec. ref. */ off_tl_start; /* rec. offset */ off_tl_len; /* rec.length */ pid_tl_pid; /* process ID*/ }; UNIX File/Record Locking

UNIX Network Programming – TDC Page 13 Spring 2003 System V 4 and POSIX.1 /* lock type */ F_RDLCK - read/shared lock F_WRLCK - write/exclusive lock F_UNLCK - unlock a record/file /* record reference */ SEEK_CUR - current file pointer SEEK_SET - byte 0 of file SEEK_END - EOF UNIX File/Record Locking

UNIX Network Programming – TDC Page 14 Spring 2003 Simulating Semaphores with Lock Files int Pflag(char *file_name, int ntries, int nsec) { int fd; i=0; while ((fd = create(file_name,0)) ==-1 && errno ==EACCES) /* EACCES - file exists, no write permission on it */ if (++i <ntries) sleep(nsec); else return 0; close(fd); /* lock_file created with 0 bytes and no permissions */ if (fd == -1) return 0; else return 1; }

UNIX Network Programming – TDC Page 15 Spring 2003 Simulating Semaphores with Lock Files int Vflag(char *file_name) { int i; i = unlink(file_name); if (i==0) return 1; /* success, lock file removed */ if (i==-1) return 0; /* failure, lock file was not removed */ }

UNIX Network Programming – TDC Page 16 Spring 2003 Process Synchronizations using Lock Files int main(void) { pid_t p; int i; q, status, nbytes; int fd; char buf[80]; /* parent process opens date file Shared Data File */ if ((fd = open("/tmp/data_file", O_RDONLY|O_CREATE, 0666))<0) { perror("open"); exit(1); } /* parent forks 3 new processes */ for (i=0; i<4; i++) { if (p=fork()) == 0) break; } switch (p) { case 0:/* a child process */ if (Pflag("/tmp/lock_file", 5,10)) { if (nbytes=read(fd,buf,sizeof(buf)))>0) { fprintf(stderr,"PID=%d data=%s\n", getpid(), buf); sleep(10); /* releases the resource - shared file */ Vflag("/tmp/lock_file"); } else fprintf(stderr, "Cannot access data file\n"); break;

UNIX Network Programming – TDC Page 17 Spring 2003 Process Synchronizations using Lock Files case -1:/* cannot fork processes */ perror("fork"); exit(1); break; default:/* parent */ while ((q=wait(&status))>0) ; break; } exit(0); }