Lecture 8 CSCE 510 - Processes

• Test Sept 27 (old versions available next time?)
• gdb - Gnu Debugger
  • Unix debugger: breakpoints, stepping through code
  • compile with -g option
  • gdb program [core]
  • b line
  • b function
  • run [arglist]
  • bt
  • c
  • n
  • s
  • help
  • quit
• tsearch trees
• Example: tsearch.ex.c
• Processes: What exactly is a process?
  • current status of execution (context)
  • resources allocated
  • credentials - which user and group
  • current working directory
  • memory space of executing program
• UIDs, GIDs
  • /etc/password
  • /etc/group
  • root = user ID 0 ( superuser)
  • man -k userid
  • man -k groupid
  • man -k group | grep name
• Process IDs
  • pid
  • ps
  • fixed PIDs
    1. process 0 - scheduler
    2. process 1 - init
    3. process 2 -
• PID syscalls
  • getpid()
  • getppid()
• Process Terminology
  • parent, child
  • orphan
  • exit status of a child check by parent using system call wait
  • zombie
• Group IDs: getgid(), getpgid()
  • supplementary groups
  • setgroups(int ngroups, int *gidset)
  • int getgroups(int gidsetsize, int grouplist[])
• Effective, real, saved UID
  • int setuid(uid_t euid)
• set uid,gid on exec
• Memory layout
  • arguments to main
  • environment
  • stack
  • heap
  • bss -
  • initialized data
  • text segment
• Dumping pointers

  dumpaddr(char *label, void *address){
     fprintf(stdout,"%p %s\n", address, label);
  }
  
  dumpaddr("Global addresses global", &global);
  
  

• Environment
  • argv style list of strings of the form "name=value"
  • extern char **environ;
• Example: printargs.c

  /* This example shows how to access command line arguments */
   
  #include 
  main(int argc,char *argv[]) {
     int i;
     for(i=0; i < argc; ++i)
  	printf("argv[%d]=""%s""\n", i, argv[i]);
  }
  

• Example: printenv.c

  /* This example shows how to access environment variables */
  
  #include 
  extern char **environ;
  main(){
      char **p;
     for(p=environ; *p != NULL; ++p){
       fprintf(stderr,"%s\n", *p);
     }
  }
  
  

• Environment manipulation functions
  • 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?
• int getrusage(int who, struct rusage *usage)
  • struct rusage sys/reource.h page 106
• Creating new processes: fork
• Standard fork Skeleton

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

• Fork example: parentChildren.c

  #include 
  #include 
  
  main(){
    int pid, pid2, status;
    if ((pid = fork()) < 0)
      fatal("Cannot fork");
  
    else if (pid == 0){   /* Child code */
      printf("Child code\n");
      if ((pid = fork()) < 0)fatal("Cannot fork");
      else if (pid == 0){   /* Child of Child code */
        printf("Child of Child code\n");
        sleep(1);
        exit(30);
  
      }else {   /* Parent of Child code */
        printf("Parent of Child code\n");
        sleep(5);
        exit(31);
      }
  
    }else{               /* Parent Code */
      while( pid2 = wait(&status) > 0){
        printf("\n pid=%d \n return value %d \n status = %4x, status shifted=%d\n",
  	     pid, pid2, status, status>>8);
      }
    }
  }
  
  
  fatal(char *s){
    fprintf(stderr,"Fatal Error %s\n", s);
    exit(0);
  }