1Logging in to a UNIX Systeminit (Process ID 1 created by the kernel at bootstrap)

spawns getty for every terminal device

invokes our login shell

terminaldevice driver

user at a terminal

fd 0, 1, 2

login reads password entry (getpwnam()), asks for user's password (getpass()) and validates it; changes ownership of our terminal device, changes to our UID and changes to our home directory. Sets additional environment variables (HOME, SHELL, USER, LOGNAME, PATH)

fd 0, 1, 2

Login shell (bash)fd

0, 1

, 2

getty opens terminal device, sets file descriptors 0, 1, 2 to it, waits for a user name, usually sets some environment variable (TERM)

invokes login when user name entered

1Command Execution

current shell (bash)

executes built-in commands(echo, kill, pwd,...) orshell scripts invoked by the . (dot) command: . shell-script

sub-shell (bash) executes a shell script or calls exec() to execute a program

variables defined in the parent are not passed (use the export command in parent to pass variables). Variables defined or changed in a sub-shell does not affect variables of the parent !

fork() (creates a new shell process)

wait

or

continue if command isexecuted in the background

sub-shell terminates after script orprogram execution

1

stack (caller info., return address,

local variables)(read/write)

Typical Memory Layout

text (code & constants, read only)

command-line arguments, environment variables

initialized

uninitialized

Heap(dynamic memory allocation)

data(read/write)

char a, *p="string";int v[10];

int f(char c){

int x, y;static long i=1;...return x;

}

}read from program file

}initialized to 0

x, y, c

p, i

a, v

"string", f

Low address

1

User process

Starting and Terminating a C Program

UNIX kernel

Program file

C start-uproutine

main()

call

user functions

call

returnreturn

exit()does not return

standard I/Ocleanup

call

retu

rn

_exit()

_exit()

_exit()

exec - the only way a program is executed by the kernel

1

current processsigprocmask()

kernel

user interrupt (CTRL-C)

sigpending()

kill()

raise(), abort()

process

Signal Mask, Pending SignalsEach process has a signal mask:

set of signals currently blocked from delivery to the processsignal mask can be manipulated by sigprocmask()

if set is not NULL => signal mask is modified depending on the value of howif oset is not NULL => current signal mask is returned

sigpending() returns the set of signals that are blocked and currently pending for the processsigset_t is a data type to represent a signal set.

int sigprocmask(int how,sigset_t *set, sigset_t *oset) int sigpending(sigset_t *set)