Ch4%20Linux%20System%20Programming%20%20%20%20

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Title: Ch4%20Linux%20System%20Programming%20%20%20%20

1
Ch4 Linux System Programming Process
IPC

Jianjian SONG
Software Institute, Nanjing University
Oct, 2004

2
Content

Process process environment
Process Control
Process identifier, fork, exec
Process relationship
Signal
Inter-process communication (IPC)
Pipe, FIFO
semaphore, shared memory, message queue
Daemon
Thread

3
1. Process Process Environment

What is a process?
Program and process
Process an address space with one or more
threads executing within that address space, and
the required system resources for those threads.
(SUSv3)

4
The startup of a process

System call fork
Process resources
struct task_struct
System space stack
System call exec
The entry of C programs

5
System stack
6
The entry of C programs

crt0.o
cc/ld
main function
function prototype
int main(int argc, char argv)

7
The termination of a process

Five ways of terminating a process
Normal termination
Return from main function
Call exit function
Call _exit function
Abnormal termination
Call abort function
Terminated by a signal

8
exit _exit functions

Function prototype
include ltstdlib.hgt
void exit(int status)
include ltunistd.hgt
void _exit(int status)
Exit status
Difference
_exit is corresponding to a system call, while
exit is a library function.
_exit terminate a process immediately.

9
Exit handler

atexit function
Register a function to be called at normal
program termination.
Prototype
include ltstdlib.hgt
int atexit(void (function)(void))
on_exit function
Example

10
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11
The memory map of a C program

Text segment (code segment)
Data segment
Initialized data
Uninitialized data
Heap
Stack

12
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13
Command line arguments

main function
int main(int argc, char argv)
Example
The implementation of echo(1)
Command line option
Standard usage
getopt function

14
getopt function

Prototype
int getopt(int argc, char const argv, const
char optstring)
extern char optarg
extern int optind, opterr, optopt
Question
getopt(argc, argv, iflr)
Program example (P104, in BLP)

15
Environment variables

Environment table
Environment pointer
extern char environ

16
putenv getenv functions

Get, set or unset an environment variable
include ltstdlib.hgt
char getenv(const char name)
int putenv(char string)
int setenv(const char name, const char value,
int overwirte)
void unsetenv(const char name)

17
Shared object

Shared object
Dynamic link
Advantages and disadvantages
Example
How to create a static and shared library?
How to use (link) a static or shared library?

18
Memory allocation

Allocate and free dynamic memory.
include ltstdlib.hgt
void malloc(size_t size)
void calloc(size_t nmemb, size_t size)
void free(void ptr)
void realloc(void ptr, size_t size)

19
2. Process control

Process identifier
System call fork
The simple synchronization between parent and
child process
The exec function family
Example
The implementation of a simple shell

20
Process identifier
21
fork

fork create a child process
include ltsys/types.hgt
include ltunistd.hgt
pid_t fork(void)
returned value
pid of child (in the current (parent) process),
0 (in child process),
-1 (failure)

22
A simple example
include ltstdio.hgt include ltsys/types.hgt include
ltunistd.hgt / fork??????????(??????)/ void
main() printf(Hello, world!\n) fork() pri
ntf(bye\n)
23
The usage of fork

Code example
if ( (pidfork()) lt 0)
/ error handling /
else if (pid 0)
/ child /
else
/ parent /
Program example
Program8-1 in APUE

24
File sharing

?????????????????????????????????????????????

25
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26
vfork clone functions

vfork
include ltsys/types.hgt
include ltunistd.hgt
pid_t vfork(void)
clone
include ltsched.hgt
int clone(int (fn)(void ), void child_stack,
int flag, void arg)

27
The simple synchronization between parent and
child process

The relationship between parent and child process
wait and waitpid functions

28
The relationship between parent and child process

The parent terminates before the child
Orphan process
The child terminates before the parent
SIGCHLD signal
Handled by wait/waitpid in parent
Not handled by wait/wait in parent -gt zombie

29
wait waitpid functions

Wait for process termination
Prototype
include ltsys/types.hgt
include ltsys/wait.hgt
pid_t wait(int status)
pid_t waitpid(pid_t pid, int status, int
options)
Example

30
Race condition

int main(void)
pid_t pid
if ( (pid fork()) lt 0)
err_sys("fork error")
else if (pid 0)
charatatime("output cccccccccccc from
child\n")
else
charatatime("output pppppppppp from
parent\n")
exit(0)

31
An improvement

int main(void)
pid_t pid
TELL_WAIT()
if ( (pid fork()) lt 0)
err_sys("fork error")
else if (pid 0)
WAIT_PARENT() / parent goes first /
charatatime("output cccccccccccc from
child\n")
else
charatatime("output pppppppppp from
parent\n")
TELL_CHILD(pid)
exit(0)

32
The exec family of functions

Replace the current process image with a new
process image.
???????????????????
pid, ppid, uid, gid, working directory, root
directory
euid/egid?
????????

33
Functions prototypes

include ltunistd.hgt
extern char environ
int execl(const char path, const char arg,
...)
int execlp(const char file, const char arg,
...)
int execle(const char path, const char arg,
..., char const envp)
int execv(const char path, char const argv)
int execvp(const char file, char const
argv)
include ltunistd.hgt
int execve(const char filename, char const
argv, char const envp)

34
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35
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36
exec and opened file descriptor

close-on-exec bit of a file descriptor
Set by fcntl function
fcntl(fd, F_SETFD, 0) /????, ???????? exec
???? /
fcntl(fd, F_SETFD, 1) /???????? exec ??? /

37
Using fork and exec together

Two ways of using fork
The parent process duplicates itself, and then
two different pieces of codes are executed in
parent process and child process.
A process want to execute another program.
Example
The implementation of system function
int system(const charcmdstring)

38
Example a simple shell

printf(" ") / print prompt /
while (fgets(buf, MAXLINE, stdin) ! NULL)
bufstrlen(buf) - 1 0 / replace
newline with null /
if ( (pid fork()) lt 0 )
err_sys(fork error)
else if ( pid 0 ) / child /
execlp(buf, buf, (char ) 0)
fprintf(stderr, "couldn't execute s",
buf)
exit(127)
if ( (pid waitpid(pid, status, 0)) lt 0 )
/ parent /
err_sys(waitpid error)
printf(" ")

39
3. Process relationship

???????
???
ps, pstree??

40
Startup login (1)

41
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42
Startup login (2)

Network login

43
Process group session

Process group
The set of one or more process(es).
getpgrp/setpgid functions
Session
The set of one or more process group(s).
setsid function
Controlling terminal
Why are they introduced?
Job control

44
Process group session (contd)
45
Process group session (contd)
46
4. Signal

The concept of signals
The signal function
Send a signal
kill, raise
The alarm and pause functions
Reliable signal mechanism

47
The concept of signals

Signal
Software interrupt
Mechanism for handling asynchronous events
Having a name (beginning with SIG)
Defined as a positive integer (in ltsignal.hgt)
How to produce a signal
????,????,kill(2)??,kill(1)??,????,...

48
Signals in Linux/UNIX
?? ??
SIGABRT ??????(??abort???????)
SIGALRM ??(alarm)
SIGFPE ??????(??0,?????)
SIGHUP ????
SIGILL ??????
SIGINT ?????(Clt-C)
SIGKILL ??(????????)
SIGPIPE ????????????
SIGQUIT ?????(Clt-\)
SIGTERM ??(?kill?????????????)
SIGUSR1 ??????
SIGUSR2 ??????
49
Signals in Linux/UNIX (contd)
?? ??
SIGSEGV ??????(???)
SIGCHLD ????????
SIGCONT ???????
SIGSTOP ??(????????)
SIGTSTP ?????(Clt-Z)
SIGTTIN ????????????
SIGTTOUT ????????????
50
Signal handling

????
???????
SIGKILL, SIGSTOP
????????
????????
????

51
The signal function

Installs a new signal handler for the signal with
number signum.
include ltsignal.hgt
typedef void (sighandler_t)(int)
sighandler_t signal(int signum, sighandler_t
handler)
(Returned Value the previous handler if
success, SIG_ERR if error)
The handler parameter
a user specified function, or
SIG_DEF, or
SIG_IGN

52
The signal function (contd)

Program example
static void sig_usr(int)
int main(void)
if (signal(SIGUSR1, sig_usr) SIG_ERR)
err_sys("can't catch SIGUSR1")
if (signal(SIGUSR2, sig_usr) SIG_ERR)
err_sys("can't catch SIGUSR2")
for ( )
pause()

53
Send a signal

kill(2) send signal to a process
include ltsys/types.hgt
include ltsignal.hgt
int kill(pid_t pid, int sig)
(Returned Value 0 if success, -1 if failure)
raise(3) send a signal to the current process
include ltsignal.hgt
int raise(int sig)
(Returned Value 0 if success, -1 if failure)

54
alarm pause functions

alarm set an alarm clock for delivery of a
signal
include ltunistd.hgt
unsigned int alarm(unsigned int seconds)
(Returned value 0, or the number of seconds
remaining of previous alarm)
pause wait for a signal
include ltunistd.hgt
int pause(void)
(Returned value -1, errno is set to be EINTR)

55
alarm pause functions (contd)

Program example
The implementation of the sleep function
unsigned int sleep1(unsigned int nsecs)
if ( signal(SIGALRM, sig_alrm) SIG_ERR)
return(nsecs)
alarm(nsecs) / start the timer /
pause() /next caught signal wakes us
up/
return(alarm(0) ) /turn off timer, return
unslept time /

56
Possible problems

Problems related to time
Race condition
Interrupted system calls
Reentrancy

57
Reliable signal mechanism

Weakness of the signal function
Signal block
signal mask
Signal set
sigset_t data type
Signal handling functions using signal set
sigprocmask, sigaction, sigpending, sigsuspend

58
signal set operations

include ltsignal.hgt
int sigemptyset(sigset_t set)
int sigfillset(sigset_t set)
int sigaddset(sigset_t set, int signum)
int sigdelset(sigset_t set, int signum)
(Return value 0 if success, -1
if error)
int sigismember(const sigset_t set, int signum)
(Return value 1 if true, 0 if
false)

59
sigprocmask function

?????(???)???????
include ltsignal.hgt
sigprocmask(int how, const sigset_t set,
sigset_t oldset)
(Return Value 0 is success, -1 if failure)
??how??????????
SIG_BLOCK ?set???????????(??)
SIG_UNBLOCK ????????set????(??)
SIG_SETMASK ????????set????
?? SIGKILL, SIGSTOP

60
sigpending function

??????????
include ltsignal.hgt
sigpending(sigset_t set)
(Returned Value 0 is success, -1 if failure)
Example
critical.c (Prog10-11 in APUE)

61
sigaction function

?????(???)????????????
include ltsignal.hgt
sigaction(int signum, const struct sigaction
act, struct sigaction oldact)
(Returned Value 0 is success, -1 if failure)
struct sigaction????????
handler_t sa_handler / addr of signal handler,
or SIG_IGN, or SIG_DEL /
sigset_t sa_mask / additional signals to block
/
int sa_flags / signal options /

62
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63
sigsuspend function

?sigmask????????,???????????????????,?????
include ltsignal.hgt
sigsuspend(const sigset sigmask)
(Returned value -1, errno is set to be EINTR)
sigsuspend?pause

64
signal function review

?sigaction??signal??
Sigfunc signal(int signo, handler_t func)
struct sigaction act, oact
act.sa_handler func
sigemptyset(act.sa_mask)
act.sa_flags 0
if (signo SIGALRM)
ifdef SA_INTERRUPT
act.sa_flags SA_INTERRUPT / SunOS /
endif
else
ifdef SA_RESTART
act.sa_flags SA_RESTART / SVR4, 44BSD /
endif
if (sigaction(signo, act, oact) lt 0)
return(SIG_ERR)
return(oact.sa_handler)

65
Example solution of race condition

int main(void)
pid_t pid
TELL_WAIT()
if ( (pid fork()) lt 0)
err_sys("fork error")
else if (pid 0)
WAIT_PARENT() / parent goes first /
charatatime("output cccccccccccc from
child\n")
else
charatatime("output pppppppppp from
parent\n")
TELL_CHILD(pid)
exit(0)

static sigset_t newmask, oldmask, zeromask
static void sig_usr(int signo) / one handler
for SIGUSR1, SIGUSR2 /
sigflag 1 return
void TELL_WAIT()
if (signal(SIGUSR1, sig_usr) SIG_ERR)
err_sys("signal(SIGINT) error")
if (signal(SIGUSR2, sig_usr) SIG_ERR)
err_sys("signal(SIGQUIT) error")
sigemptyset(zeromask)
sigemptyset(newmask)
sigaddset(newmask, SIGUSR1)
sigaddset(newmask, SIGUSR2)
/ block SIGUSR1 and SIGUSR2, and save current
signal mask /
if (sigprocmask(SIG_BLOCK, newmask, oldmask) lt
0)
err_sys("SIG_BLOCK error")

void WAIT_PARENT(void)
while (sigflag 0)
sigsuspend(zeromask) / and wait for parent
/
sigflag 0
/ reset signal mask to original value /
if (sigprocmask(SIG_SETMASK, oldmask, NULL) lt
0)
err_sys("SIG_SETMASK error")
void TELL_CHILD(pid_t pid)
kill(pid, SIGUSR1) / tell child we're done
/

68
5. Inter-process communication

IPC Inter-Process Communication
IPC mechanisms
shared file
signal
pipe, FIFO (named pipe), message queue,
semaphore, shared memory
socket

69
IPC illustrations
70
Simple Client-Server or IPC model
71
The concept of pipe

Pipe
Pipe mechanism in a shell
e.g. cmd1 cmd2
Pipe is half-duplex
???????????????
??????
????(FIFO)

72
The pipe function

The pipe function create a pipe
include ltunistd.hgt
int pipe(int filedes2)
(Returned value 0 if success, -1 if failure)
A pipe First In, First Out
filedes0read, filedes1 write

73
A pipe in a single process
74
A pipe between the parent child
75
The coordinationof pipe read write

????,??PIPE_BUF??????????????
????????,
????,??????????????,read??0,?????????
????,????????SIGPIPE

76
Examples

pipe1.c

77
?????????????

??shell????
cmd1 cmd2
??? cmd gt file
????
??cmd1?
if (fd1 ! STDOUT_FILENO)
if (dup2(fd1, STDOUT_FILENO) ! STDOUT_FILENO)
err_sys(dup2 error to stdout)
??cmd2?
if (fd0 ! STDIN_FILENO)
if (dup2(fd0, STDIN_FILENO) ! STDIN_FILENO)
err_sys(dup2 error to stdin)

78
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79
Examples

pipe2.c

80
pipe Application(1)solution of race condition

int main(void)
pid_t pid
TELL_WAIT()
if ( (pid fork()) lt 0)
err_sys("fork error")
else if (pid 0)
WAIT_PARENT() / parent goes first /
charatatime("output cccccccccccc from
child\n")
else
charatatime("output pppppppppp from
parent\n")
TELL_CHILD(pid)
exit(0)

static int pfd2
void TELL_WAIT()
if (pipe(pfd) lt 0)
err_sys("pipe error")
void WAIT_PARENT(void)
char c
if (read(pfd0, c, 1) ! 1)
err_sys("read error")
if (c ! 'p')
err_quit("WAIT_PARENT incorrect data")
void TELL_CHILD(pid_t pid)
if (write(pfd1, "p", 1) ! 1)
err_sys("write error")

82
pipe Application(2) shell

shpipe.c

83
popen pclose functions

popen, pclose process I/O
include ltstdio.hgt
FILE popen(const char command, const char
type)
int pclose(FILE stream)

84
Applications of popen pclose

????
??????????????

85
Home work popen???

?pipe, fork??popen

86
FIFO named pipe

???????
???
???
?????
?????????
mkfifo(1), mkfifo(3), mknod(1), mknod(2)

87
??FIFO

mkfifo make a FIFO special file (a named pipe)
include ltsys/types.hgt
include ltsys/stat.hgt
int mkfifo(const char pathname, mode_t mode)
(Returned value 0 if success, -1 if failure)
Examples
fifo1.c (Ch12 in BLP)
list a fifo (ls lF)

88
?open????FIFO

Review open system call
int open(const char pathname, int flags)
flags parameter
?????????
O_RDONLY, O_WRONLY, O_RDWR
O_NONBLOCK
consideration
??/??
??

89
FIFO??????

??FIFO????
?????(????O_NONBLOCK),???????????????????FIFO???,
??????FIFO?????????????????
?????O_NONBLOCK,??????????????????,??????????????
?FIFO,??open????? -1,errno??ENXIO?
??FIFO????
same as pipe

90
????

shell???fifo???
cat lt my_fifo
echo a string to fifo gt my_fifo
fifo2.c (Ch12 in BLP)

91
FIFO???(1)

?FIFO?????
?

92
FIFO???(2)

C/S????
?client.c, server.c

93
System V IPC

IPC objects
???(semaphore set)
????(message queue)
????(shared memory)
shell??
ipcs, ipcrm

94
System V IPC?????

???????
??IPC?????
??IPC????????(key_t key)
key????????IPC_PRIVATEftok??
????????????
?????
?????
struct ipc_perm

95
SV IPC System Calls Overview
?? ???? ??? ????
????IPC??,???IPC??? msgget semget shmget
IPC?? ??/????,?????,??/?????? msgsnd/ msgrcv semop shmat/ shmdt
IPC????/??????,??IPC msgctl semctl shmctl
96
Semaphore

??????
?????
PV??(??)

97
PV??????

procedure p(var ssamephore)
s.values.value-1
if (s.valuelt0) asleep(s.queue)
procedure v(var ssamephore)
s.values.value1
if (s.valuelt0) wakeup(s.queue)

98
Linux/UNIX??????

semaphore set
struct semid_ds
struct ipc_perm sem_perm
struct sem sem_base / ptr to first sem in
set /
time_t sem_otime / last operation time /
time_t sem_ctime / last change time /
ushort sem_nsems / count of sems in set /

99
semaphore set system calls

include ltsys/types.hgt
include ltsys/ipc.hgt
include ltsys/sem.hgt
int semget(key_t key, int nsems, int semflg)
int semop(int semid, struct sembuf sops,
unsigned nsops)
int semctl(int semid, int semnum, int cmd, ...)

100
semget get a semaphore set identifier

prototype
int semget(key_t key, int nsems, int semflg)
key parameter
?????IPC_PRIVATE
??????
ftok??
semflg parameter
??????(?9?)
IPC_CREAT, IPC_EXCL ???
examples
sem1.c

101
semop semaphore operations

prototype
int semop(int semid, struct sembuf sops,
unsigned nsops)
sops parameter
struct sembuf
unsigned short int sem_num / semaphore
number /
short int sem_op / semaphore
operation /
short int sem_flg / operation
flag /
examples
PV?????

102
semctl semaphore control operations

prototype
int semctl(int semid, int semnum, int cmd, ...)
the 4th argument
union semun
int val
struct semid_ds buf
unsigned short array
arg
cmd parameter

103
semctl semaphore control operations (contd)

cmd parameter
IPC_STAT ???????????arg.semid_ds???????
IPC_SET ????????????,??????????????arg.semid_ds??
????
IPC_RMID ???????
SETVAL ?????????semnum??????????(??arg.val)
examples
set_semvalue(???), del_semvalue(??)

104
Examples

???????
sem1.c(????)

105
Home work

??????????/?????
????

106
Message queue

????
??????????,??????????????????
First in, first out
message type ???
???
struct msqid_ds

107
message queue system calls

include ltsys/types.hgt
include ltsys/ipc.hgt
include ltsys/msg.hgt
int msgget(key_t key, int size, int flag)
int msgsnd(int msqid, const void ptr, size_t
nbytes, int flag)
int msgrcv(int msqid, void ptr, size_t nbytes,
long type, int flag)
int msgctl(int msqid, int cmd, struct shmid_ds
buf)

108
msgctl message control operations

prototype
int msgctl(int msqid, int cmd, struct shmid_ds
buf)
cmd parameter
IPC_STAT ?msqid_ds??????????????????
IPC_SET ????????????,?????????????msqid_ds??????
IPC_RMID ??????

109
Example

A C/S application
One server, several clients only one queue is
required.
Compared with FIFO implementation

110
Shared memory

????
????????????????????,????(attach)????????,????????
???????
??????????
?????????

111
shared memory system calls

include ltsys/types.hgt
include ltsys/ipc.hgt
include ltsys/shm.hgt
int semget(key_t key, int size, int flag)
void shmat(int shmid, void addr, int flag)
int shmdt(void addr)
int shmctl(int shmid, int cmd, struct shmid_ds
buf)

112
mmap/munmap system calls

mmap/munmap map or unmap files or devices into
memory

113
6. Daemon process

???daemon??
daemon???????
????
????

114
What is a daemon?
115
What is a daemon? (contd)

daemon
?????????
????, ????????? Shell ???
why daemon?

116
Concepts related to job control

Process group session
Controlling terminal
jobs, fg, bg comands
SIGINT, SIGQUIT, SIGTSTP, SIGHUP signals
A solution the problem
nohup command gtgt logfile

117
daemon?????(1)

????
????fork,??????exit
??setsid?????????
??????????????
???umask??0
???????????

118
Example

int daemon_init(void)
pid_t pid
if ( (pid fork()) lt 0)
return(-1)
else if (pid ! 0)
exit(0) / parent goes bye-bye /
/ child continues /
setsid() / become session leader /
chdir("/") / change working directory /
umask(0) / clear our file mode creation mask
/
return(0)

119
daemon?????(2)

????

120
7. Threads

What is a thread
Basic pthread functions
Thread synchronization
Thread attributes
Thread cancellation

121
What is a thread?

Thread
A single flow of control within a process.
(susv3)
Process and thread
New process when a process executes a fork call,
a new copy of the process is created with its own
variables and its own PID.
New thread When we create a new thread in a
process, the new thread of execution gets its own
stack (and hence local variables) but shares
global variables, file descriptors, signal
handlers, and its current directory state with
the process that created it.

122
Process and thread
123
POSIX thread

POSIX1003.1c
pthread library
/usr/lib/libpthread.so, /usr/lib/libpthread.a
pthread.h header file
/usr/include/pthread.h
Compiler options
gcc thread.c o thread -lpthread

124
Basic thread functions(1)

Create a new thread
include ltpthread.hgt
int pthread_create(pthread_t thread,
pthread_attr_t attr, void (start_routine)(void
), void arg)
Terminate the calling thread
void pthread_exit(void retval)

125
Basic thread functions(2)

Wait for termination of another thread
include ltpthread.hgt
int pthread_join(pthread_t th, void
thread_return)
Put a running thread in the detached state
int pthread_detach(pthread_t th)

126
Thread synchronization

????????
????????
?????????

127
POSIX4 semaphore

include ltsemaphore.hgt
int sem_init(sem_t sem, int pshared, unsigned
int value)
int sem_wait(sem_t sem)
int sem_post(sem_t sem)
int sem_destroy(sem_t sem)
int sem_trywait(sem_t sem)
int sem_getvalue(sem_t sem, int sval)

128
Example

Producer-consumer problem
thread4.c, thread4a.c (in Ch11, BLP)

129
Mutex (Mutual Exclusion)

include ltpthread.hgt
int pthread_mutex_init(pthread_mutex_t mutex,
const pthread_mutexattr_t mutexattr)
int pthread_mutex_lock(pthread_mutex_t mutex)
int pthread_mutex_unlock(pthread_mutex_t mutex)
int pthread_mutex_destroy(pthread_mutex_t
mutex)
int pthread_mutex_trylock(pthread_mutex_t mutex)

130
Example