Lecture 10 CSCI 510 - Fork

• Readings:
  • Lec 1: 2.1, 2.2, 12.5 [syllabus][vi-basics][unix-basics]
  • Lec 2: 12.5(strings), 3.3(stat)
  • Lec 3: 3.1-3.3(stat) man ar
  • Lec 4: 4.4-4.5, 3.18(chown) man 5 ar
  • Lec 5: 12.4, 4.63 man make
  • Lec 6: man cc, man cpp, /usr/include/stdio.h
  • Lec 7: man cpp again, dumpaddr.c, printargs.c, printenv.c
  • Lec 8: man gdb, [gdb quick reference], 5.10.1-5.10.2, 5.1-5.2
  • Lec 9:
  • Lec 10: 5.3-5.6,
• Test Plan : Closed notes, closed book, function prototypes
• STDIO Review: Some stdio functions
  • the FILE struct from stdio.h
    • ...
  • FILE *fopen(char *path, char *mode)
  • int fclose(FILE *fp)
  • char *fgets(char *s, int n, FILE *stream)
  • printf(char *format, ...)
  • fprintf(FILE *fp, char *format, ...)
  • sprintf(char *buf, char *format, ...)
  • scanf(char *format, ...)
  • fscanf(FILE *fp, char *format, ...)
  • sscanf(char *buf, char *format, ...)
  • getc, putc, getchar, putchar
  • gets, fgets, puts, fputs
  • fflush(FILE* fp)
• Performance issues with Stdio
  • cat with read and write one byte at a time
  • cat with read and write one buffer at a time
  • cat with getchar and putchar
  • Performance
• Buffering I/O
  • three types of buffering
    1. unbuffered
    2. fully buffered
    3. line oriented
  • int setvbuf(FILE *stream, char *buf, int type, size_t size)
• Memory Layout / Environment Revisted
  • Memory layout of running process from last time
  • argv style list of strings of the form "name=value"
  • extern char **environ;
• Environment manipulation functions Revisited
  • char *getenv(name)
  • setenv(name, value, overwrite)
  • void unsetenv(name)
  • int putenv (string)
• Notes on Environment Functions
  • putenv- removes old entry if it exists
  • setenv-does not remove old entry if overwrite = 0
  • What happens when we try to stuff in a longer new value?
  • What happens when we try to add a new name?
• fork
• Standard fork Skeleton

        if((p = fork()) < 0){
  
        }else if (p == 0) {
  
        }else{
  
        }
   

• Fork example: parentChildren.c
• What's copied?
  • all variables, data space, heap stack etc.
  • per process open file table
  • signal table
  • ...
• Properties Inherited:
  • open files, IDs, cwd, root directory, umask, signal mask and table, environment, shared memory segments
• Properties Different:
  • return value of fork, PID, PPID, times, file locks, pending signals
• Fork and IO example

• Fork example: forkstdio.c

• vfork
• Exec family

       #include 
   
       int execl(const char *path, const  char  *arg0,  ...,  const
       char *argn, char * /*NULL*/);
   
       int execv(const char *path, char *const argv[]);
   
       int execle(const char *path, const char  *arg0,  ...,  const
       char *argn, char * /*NULL*/, char *const envp[]);
   
       int execve(const char *path, char *const argv[], char *const
       envp[]);
   
       int execlp(const char *file, const char  *arg0,  ...,  const
       char *argn, char * /*NULL*/);
   
       int execvp(const char *file, char *const argv[]);
  

• wait

       #include 
       #include 
   
       pid_t wait(int *stat_loc);
  
       pid_t waitpid(pid_t pid, int *stat_loc, int options);   
  
       int waitid(idtype_t idtype, id_t id, siginfo_t  *infop,  int options);
  

• Notes on Environment Functions
  • putenv- removes old entry if it exists
  • setenv-does not remove old entry if overwrite = 0
  • What happens when we try to stuff in a longer new value?
  • What happens when we try to add a new name?
• Fork example: parentChildren.c
• What's copied?
  • all variables, data space, heap stack etc.
  • per process open file table
  • signal table
  • ...
• Properties Inherited:
  • open files, IDs, cwd, root directory, umask, signal mask and table, environment, shared memory segments
• Properties Different:
  • return value of fork, PID, PPID, times, file locks, pending signals
• Fork and IO example

     
  #include 
  #include 
  #include 
  
  main(){
     int fd;
     pid_t pid;  /* the return value from fork */
     char buf[15]; /* the buffer */
     char *before,*child,*parent, *common;
     
     before = "Before the fork\n";
     child  = "child after the fork\n";
     parent  = "parent after the fork\n";
     common  = "common code after the fork\n";
     if((fd= open("junk",O_WRONLY|O_CREAT, 0777)) == NULL)
         fatal("ForkIO Test Open of junk Failed");
  
     write(fd, "Before the fork\n", strlen(before));
  
     if((pid = fork()) < 0){
        fatal("ForkStdio Test fork failed");
     }else if (pid == 0) {
        write(fd, child, strlen(child));
     }else{
        write(fd, parent, strlen(parent));
     }
     sleep(1);
     write(fd, common, strlen(common));
  }
  
  fatal(char *msg)
  {
    fprintf(stderr,"Fatal Error: %s", msg);
    exit(1);
  }
  
     
  

• Fork example: forkstdio.c

  
  
  #include 
  #include 
  
  main(){
  
     FILE *fp;   
     pid_t pid;  /* the return value from fork */
     char buf[150]; /* the buffer */
  
     if((fp= fopen("junk","w+")) == NULL)
         fatal("ForkStdio Test Open of junk Failed");
     setvbuf(fp, buf, _IOFBF, 20);
  
     fprintf(fp, "Before the fork");
  
     if((pid = fork()) < 0){
       fatal("ForkStdio Test fork failed");
     }else if (pid == 0) {
       fprintf(fp, "child after the fork");
       sleep(1);
     }else{
       fprintf(fp, "PARENT AFTER THE FORK");
       sleep(1);
     }
     sleep(1);
     fprintf(fp, "Common Code after the fork, pid val=%u\n",(unsigned short) pid);
  }
  
  fatal(char *msg)
  {
    fprintf(stderr,"Fatal Error: %s", msg);
    exit(1);
  }
   

• vfork
• Exec family
• wait

[Readings:]

[Assignment:]