The Process - Part II

Process Attributes  Each UNIX process is associated with a number of attributes which help the system control the running and scheduling of processes, maintain the security of the file system and so on –process id –environment –effective user-id –privileges

The process-id  A process can obtain its own process-id by using the getpid() function call.

Process group and process group-ids  Unix allows processes to be placed into groups.  For example processes are connected by pipes from the command line, they are typically placed into a process group. whoawk ‘(print $1)’Sort -u

Process groups  Process groups are useful when you want to handle a set of processes as a whole using IPC mechanism called signals.  Each process group is denoted by a process group-id of type pid_t.  Usage: #include pid_t getpgrp (void);

Changing process group  Usage #include int setpgid (pid_t pid, pid_t pgid);

Session and session-ids  Each process group belongs to a session.  A session is about a process’s connection to a connecting terminal.  All processes explicitly or implicitly created after a user logs in belong to a session related to their current terminal.  A session is a collection of a single foreground process group using the terminal and one or more background processes.

Session-id  A session is identified by a session-id of type pid_t.  A process can obtain its current session-id with a call to getsid as follows:  Usage #include pid_t getsid(pid_t pid);

Session-d continued  If getsid is supplied a value of 0 then it returns the session-id of the calling process otherwise the session-id of the process identified by pid is returned.  The idea of a session is useful with background or daemon process.  A daemon process is simply a process which does not have a controlling terminal.

Session continued  An example of a daemon process is a cron, which executes commands at specific times and dates.  A daemon can set itself to be in a session without a controlling terminal by calling the setsid system call, and moving itself into a new session.

Session id  Usage #include pid_t setsid(void);

The environment variable  The environment of a process is simply a collection of null-terminated strings.  A programmer can make direct use of the environment of a process by adding an extra parameter envp to the parameter list of the main function within a program. –main(int argc, char **argv, char **envp)‏ { // do something }

To modify the environment from within a process /* setmyenv.c set environment for program */ main()‏ { char *argv[2], *envp[3]; argv[0] = “showmyenv”; argv[1] = (char *)0; envp[0] = “foo=bar”; envp[1] = “bar=foo”; envp[2] = (char *)0; execve(“./showmyenv”, argv, envp); perror(“execve failed\n”); }

Getting specific variable  The getenv system call can be used to get specific variables in the environment.  Usage #include char *getenv(const char *name); main()‏ { printf(“PATH = %s\n”, getenv(“PATH”); } A similar putenv(“NEWVARIABLE = value”); exists for modifying the env.

Obtaining the user- and group-ids

Setting the e-user- and group-ids