Discussion of Assignments 2. Line buffered vs. full buffered I/O. Some often encountered issues in the submissions.
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1 3 4 Discussion of Assignment 1 Discussion of Assignments 1 and 2 Accompanying Tutorial to Operating Systems Course Alexander Holupirek, Stefan Klinger Database and Information Systems Group Department of Computer & Information Science University of Konstanz Summer Term 2009 Using System Calls for Unbuffered I/O Sample solution Same remarks on intermixing unbuffered and standard I/O Submission example: Line buffered vs. full buffered I/O Same remarks on test setups Performance graphs Hot vs. cold cache In general: Comment your code! 1 2
2 7 8 Line buffered vs. full buffered I/O Discussion of Assignments 2 Demo Rudimentary shell: Process Creation Zombie creation fork(2) and exit(3) plus missing wait(2) Describe what you are (not) doing to create a zombie. Problem description & how to get rid of them 5 Process chaining Correct usage of fork(2), exec(3) and waitpid(2) wait(2) introduces a problem here. Status of newly created process Correct usage of WIF... macros to determine process termination. Examples: Army of zombies, Fork bomb Some often encountered issues in the submissions Concepts/concepts.zombies.txt Some often encountered issues in the submissions static int exitcode = -3; // the latest exit Code 23 static char * lfunction = " none "; // the last started program 109 /** Execute any none shell command **/ 110 void execute ( char ** argv ) 111 { 112 pid_t pid = fork (); 113 lfunction = argv [0]; if( pid == 0){ // we are in the child process 116 int x = execvp (* argv, argv ); 117 exit (x); 118 } else { 119 wait (& exitcode ); // wait till the child finishes and store its exit code for later retrieval 120 } 121 } lfunction is a pointer to a string constant. No error handling for fork(2), execvp(3), wait(2). exec(2) does not return exit code of the executed program! In fact, if exec(2) returns at all (remember process chaining!) this indicates an error and -1 is returned with the global errno set to specify the error.
3 11 12 Patch osh. c : 53: osh_final. c : 52: ,14 4 # include < string.h > /* memset */ 5 # include <sys / mount.h> 6 # include <sys / param.h> 7 +# include <sys / wait.h> 8 # include < unistd.h > /* fork */ 9 # include " misc.h" # define PROMPT "[ osh ] >> " static void welcome_ message ( void ); 14 + static void execute ( char ** argv ); 15 static void cmd_help ( int argc, char ** argv ); 16 static void cmd_quit ( int argc, char ** argv ); 17 + static void cmd_ zombie ( int argc, char ** argv ); 18 + static void cmd_ status ( int argc, char ** argv ); /* status / exit code of last external command executed. */ 21 + static int status = -1; 23 /* Structure for shell command. */ 24 struct cmd_ entry { -30,9 static cmd_t cmds [] = { 26 "\t\t\ tprints this help message " }, 27 { " quit ", cmd_quit, " quit " \ 28 "\t\t\ texit the shell " }, 29 + { " zombie ", cmd_zombie, " zombie " \ 30 + "\t\t\ tcreates a zombie process " }, 31 + { " status ", cmd_status, " status " \ 32 + "\t\t\ tprints status code " \ 33 + "of last external command executed " }, 34 { NULL, NULL, NULL } 35 }; 37 +/** Create a zombie ( should be avoided in productional code ). */ 38 + static void 39 + cmd_zombie ( int argc, char ** argv ) 40 +{ 41 + ( void ) argc ; 42 + ( void ) argv ; 43 + pid_t pid ; if (( pid = fork ()) == -1) 46 + err ( errno, " fork failed."); 47 + if ( pid == 0) { 48 + /* Newly created child process. */ 49 + sleep (1); 50 + exit (0); 51 + } else { 52 + /* We are the parent and should wait (2) for our children However, we explicitly do _not_ wait to turn our 54 + children to zombies, i. e., never fetch their termination 55 + status from process control block kept in kernel. */ 56 + } 57 +} /* Examines status information ( stored by function execute ( char ** a 60 + * in global variable status */ 62 + static void 63 + cmd_status ( int argc, char ** argv ) 64 +{ 65 + ( void ) argc ; 66 + ( void ) argv ; if ( WIFEXITED ( status )) 69 + printf (" normal termination, exit status = %d\n", 70 + WEXITSTATUS ( status )); 71 + else if ( WIFSIGNALED ( status )) 72 + printf (" abnormal terminiation, signal number = %d%s\n", 73 + WTERMSIG ( status ), 74 +# ifdef WCOREDUMP 75 + WCOREDUMP ( status )? "( core file generated )" : ""); 76 +# else 77 + ""); 78 +# endif 79 + else if ( WIFSTOPPED ( status )) 80 + printf (" child stopped, signal number = %d\n", 81 + WSTOPSIG ( status )); 82 +} 10
4 /* Execute a program on user request. Spawns a new process * Termination status of last issued and terminated command 86 + * is stored in global variable status. May be examined 87 + * using internal status command ( see cmd_ status ) */ 89 + static void 90 + execute ( char ** argv ) 91 +{ 92 + pid_t pid ; if (( pid = fork ()) == -1) 95 + err ( errno, " fork failed."); 96 + else if ( pid == 0) { 97 + /* Newly created child process chaining a new command. */ 98 + int i; 99 + if ((i = execvp ( argv [0], argv )) == -1) err ( errno, " exec failed."); } fprintf ( stderr, " chained process takes forked id: %d\n", pid ); /* Wait for our children and store their exit status. */ if ( waitpid ( pid, & status, 0) == -1) err ( errno, " waitpid failed."); 106 +} -75,10 main ( void ) 112 memset ( line, 0, sizeof ( line )); 113 while ( getline ( PROMPT, line, sizeof ( line ))!= NULL ) { 114 argv = get_args ( line, & argc ); if ( argc > 0) if ( argc > 0) { 117 for ( cmd = cmds ; cmd - > func!= NULL ; cmd ++) if ( strcmp ( argv [0], cmd -> name ) == 0) if ( strcmp ( argv [0], cmd -> name ) == 0) { 120 cmd -> func (argc, argv ); break ; } if (cmd -> func == NULL ) /* No internal command has been found Trying to launch external. */ execute ( argv ); } 128 free_args ( argv ); 129 } 130 return (0); Examine termination status Macros to examine termination status POSIX.1 macros Integer status returned by wait(2) and waitpid(2) is implementation defined. Certain bits indicate exit status (for a normal return) Other bits indicate the signal number (for an abnormal termination) A bit may be set if a core file has been generated. POSIX.1 specifies that the termination status is to be looked at using various macros that are defined in <sys/wait.h>.
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