UNIX Processes

• UNIX provides a PGID that is used to group processes together.
• The PGID of a process may be obtained with
  int getpgrp(void)
• A process initially belongs to its parent's group, but new groups may be established with
  int setpgid(pid_t pid, pid_t pgid)
  Sets the process group ID of the process with ID pid to pgid. If pid is equal to 0, the process ID of the calling process is used.
• Groups may have a control terminal which is the first tty device opened by the group leader. Initially the group leader is the shell, and the login device is the control terminal. A new group that is started is not attached to the previous control terminal (if any). Interupt, quit and hangup signals from a terminal go to all process associated with that terminal. For example all processes that start from one given terminal belong to the same group. Then if the terminal goes down it's possible to find all its processes.

• The program_path is the name of the file to execute.
• The program_path must be absolute, i.e. start with /.
• argi are the arguments to the program, which in C are accessible through the parameters to main.
• Conventionally arg0 is the name of the program.
• A NULL argument ends the list.

• A process may terminate by explicit use of the exit system call.
• This call is used implicitly when a process terminates normally.

• wait suspends the process until a child process terminates and sends a SIGCHLD signal.
• wait returns the id of that process, or -1 immediately if there are no children.
• Exit status information (see below) is returned to the parent in *status_pointer.
• To run a process in background (& in C and Bourne shells) the parent does not execute wait.

• RLIMIT_CPU - CPU time in seconds
• RLIMIT_FSIZE - Maximum filesize
• RLIMIT_DATA - max data size
• RLIMIT_STACK - max stack size
• RLIMIT_CORE - max core file size
• RLIMIT_RSS - max resident set size
• RLIMIT_NPROC - max number of processes
• RLIMIT_NOFILE - max number of open files
• RLIMIT_MEMLOCK - max locked-in-memory address space
• RLIMIT_AS - address space (virtual memory) limit

struct rlimit {
    rlim_t    rlim_cur;
    rlim_t    rlim_max;
};
          

• SIGSEGV for data limits.
• SIGXFSZ for file limits.
• SIGXCPU for CPU time.

struct rusage {
    struct timeval ru_utime; /* user time used */
    struct timeval ru_stime; /* system time used */
    long ru_maxrss;          /* maximum resident set size */
    long ru_ixrss;           /* integral shared memory size */
    long ru_idrss;           /* integral unshared data size */
    long ru_isrss;           /* integral unshared stack size */
    long ru_minflt;          /* page reclaims */
    long ru_majflt;          /* page faults */
    long ru_nswap;           /* swaps */
    long ru_inblock;         /* block input operations */
    long ru_oublock;         /* block output operations */
    long ru_msgsnd;          /* messages sent */
    long ru_msgrcv;          /* messages received */
    long ru_nsignals;        /* signals received */
    long ru_nvcsw;           /* voluntary context switches */
    long ru_nivcsw;          /* involuntary context switches */
};
          

struct timeval {
    long    tv_sec;         /* seconds */
    long    tv_usec;        /* and microseconds */
};