CS 3411 Systems Programming

Transcription

1 CS 3411 Systems Programming Department of Computer Science Michigan Technological University Unix Processes

2 Today s Topics Unix Processes Creating New Processes

3 Unix Processes Process is the image of a program in execution Processes are sequential in nature Processes may reside in memory simultaneously Time multiplex the CPU(s) to get required results

4 Creating Child Processes Use fork()! As usual, read the manual page. fork() creates a child process that is a copy of the parent process, with some exceptions Execution continues just after the fork() call in both processes fork() returns the PID of the new process to the parent, and 0 to the child process The child is a copy of the parent. No memory is shared.

5 fork() #i n c l u d e <u n i s t d. h> main ( ) { f o r k ( ) ; w r i t e ( 1, " Hi \n", 4 ) ; #i n c l u d e <u n i s t d. h> main ( ) { i f ( f o r k ( ) == 0) { / C h i l d w r i t e s : / w r i t e ( 1, " Hi \n", 4 ) ; e l s e { / Parent w r i t e s : / w r i t e ( 1, "Hey \n", 5 ) ;

6 fork() #i n c l u d e <u n i s t d. h> #i n c l u d e <s t d i o. h> i n t x = 0 ; main ( ) { i f ( f o r k ( ) == 0) { / C h i l d w r i t e s : / x++; p r i n t f ( " C h i l d : x=%d\n", x ) ; f f l u s h ( s t d o u t ) ; e l s e { / Parent w r i t e s : / x++; p r i n t f ( " Parent : x=%d\n", x ) ; f f l u s h ( s t d o u t ) ;

7 fork() A forked child inherits open files of the parent The child process descriptor is a copy of the parent s process descriptor, except: Return value from fork() PID, PPID Pending signals and alarms File locks Execution times

8 File Descriptors Example #i n c l u d e <u n i s t d. h> #i n c l u d e < f c n t l. h> #i n c l u d e <s t d i o. h> main ( ) { i n t f d ; char ch1, ch2 ; f d = open ( " d a t a f i l e ",O_RDWR) ; r e a d ( fd,& ch1, 1 ) ; p r i n t f ( " I n p a r e n t : ch1 = %c \n", ch1 ) ; f f l u s h ( s t d o u t ) ; i f ( f o r k ( ) == 0) { / C h i l d / r e a d ( fd,& ch2, 1 ) ; p r i n t f ( " I n c h i l d : ch2 = %c \n", ch2 ) ; f f l u s h ( s t d o u t ) ;

9 Executing a New Binary execve() is used to execute a new program Manual page! This function executes the program it is pointed to On success, execve() does not return: The process calling execve() is completely replaced by the newly executed process On error, -1 is returned File descriptors may be set to close on exec!

10 Creating a New Process Exec is most useful when used with fork In Unix, a new process is created by first forking an existing process, then calling a variant of exec from there Most process attributes are preserved, including the PID, PPID, file locks, pending signals, execution times and open files

11 execve() Example #i n c l u d e <s t d i o. h> #i n c l u d e <e r r n o. h> #i n c l u d e < s t d l i b. h> main ( ) { char a [ 4 ], e [ 3 ] ; a [ 0 ] = " c h i l d " ; a [ 1 ] = " argument1 " ; a [ 2 ] = " argument2 " ; a [ 3 ] = NULL ; e [ 0 ] = "ENV0=v a l 0 " ; e [ 1 ] = "ENV1=v a l 1 " ; e [ 2 ] = NULL ; e x e c v e ( " c h i l d 1 ", a, e ) ; / I f we g e t here, something went wrong / p e r r o r ( " p a r e n t 1 " ) ; e x i t ( 1 ) ;

12 execve() Example #i n c l u d e <s t d i o. h> main ( argc, argv, envp ) i n t a r g c ; char a r g v [ ], envp [ ] ; { i n t i ; char ep ; p r i n t f ( " c h i l d i s r u n n i n g \n" ) ; f o r ( i = 0 ; i < a r g c ; i ++) { p r i n t f ( " a r g v [%d]=%s \n", i, a r g v [ i ] ) ; f o r ( ep = envp ; ep ; ep++) { p r i n t f ( "%s \n", ep ) ;

13 Convenience Calls To Exec execl, execlp, execle, execv, execvp are all convenience calls to execve The manual page has details!

14 More Examples #i n c l u d e < f c n t l. h> #i n c l u d e <s t d i o. h> #i n c l u d e <u n i s t d. h> #i n c l u d e < s t d l i b. h> main ( argc, a r g v ) i n t a r g c ; char a r g v [ ] ; { i n t f o r k i d, charnum ; char f d v a l [ 2 0 ] ; i f ( a r g c!= 3) { f p r i n t f ( s t d e r r, " Usage : pexec f i l e n a m e charnum \n" ) ; e x i t ( 1 ) ; i f ( ( f o r k i d = open ( a r g v [ 1 ], O_RDONLY) ) < 0 ) { f p r i n t f ( s t d e r r, " Cannot open %s \n", a r g v [ 1 ] ) ; e x i t ( 2 ) ; s p r i n t f ( f d v a l, "%d", f o r k i d ) ; / s p r i n t f! / i f ( f o r k ( ) == 0) { e x e c l ( " p c h i l d ", " p c h i l d ", f d v a l, a r g v [ 2 ], ( char ) 0 ) ; f p r i n t f ( s t d e r r, " Unable to e x e c \n" ) ; e x i t ( 3 ) ; p r i n t f ( " Parent i s a f t e r f o r k / e x e c \n" ) ;

15 More Examples #i n c l u d e <s y s / t y p e s. h> #i n c l u d e <u n i s t d. h> #i n c l u d e <s t d i o. h> #i n c l u d e < s t d l i b. h> main ( argc, a r g v ) i n t a r g c ; char a r g v [ ] ; { i n t myfd ; char gotch, v a l ; i f ( a r g c!= 3) { f p r i n t f ( s t d e r r, " Usage : p c h i l d f i l e n a m e charnum \n" ) ; e x i t ( 1 ) ; myfd = a t o i ( a r g v [ 1 ] ) ; gotch = a t o i ( a r g v [ 2 ] ) ; l s e e k ( myfd, ( o f f _ t ) gotch, SEEK_SET ) ; r e a d ( myfd, &v a l, 1 ) ; p r i n t f ( " C h i l d got c h a r %d from f d %d : %c \n", gotch, myfd, v a l ) ;

16 More Examples #i n c l u d e < f c n t l. h> #i n c l u d e <s t d i o. h> #i n c l u d e <u n i s t d. h> #i n c l u d e < s t d l i b. h> main ( argc, a r g v ) i n t a r g c ; char a r g v [ ] ; { i n t f o r k i d, charnum ; i f ( a r g c!= 3) { f p r i n t f ( s t d e r r, " Usage : p i o e x e c f i l e n a m e charnum \n" ) ; e x i t ( 1 ) ; i f ( ( f o r k i d = open ( a r g v [ 1 ], O_RDONLY) ) < 0 ) { f p r i n t f ( s t d e r r, " Cannot open %s \n", a r g v [ 1 ] ) ; e x i t ( 2 ) ; i f ( f o r k ( ) == 0) { c l o s e ( 0 ) ; dup ( f o r k i d ) ; c l o s e ( f o r k i d ) ; e x e c l ( " p i o c h i l d ", " p i o c h i l d ", a r g v [ 2 ], ( char ) 0 ) ; f p r i n t f ( s t d e r r, " Unable to e x e c \n" ) ; e x i t ( 3 ) ; p r i n t f ( " Parent i s a f t e r f o r k / e x e c \n" ) ;

17 More Examples #i n c l u d e <s y s / t y p e s. h> #i n c l u d e <u n i s t d. h> #i n c l u d e <s t d i o. h> #i n c l u d e < s t d l i b. h> main ( argc, a r g v ) i n t a r g c ; char a r g v [ ] ; { i n t myfd ; char gotch, v a l ; i f ( a r g c!= 2) { f p r i n t f ( s t d e r r, " Usage : p i o k i d charnum \n" ) ; e x i t ( 1 ) ; gotch = a t o i ( a r g v [ 1 ] ) ; l s e e k ( 0, ( o f f _ t ) gotch, SEEK_SET ) ; r e a d ( 0, &v a l, 1 ) ; p r i n t f ( " C h i l d got c h a r %d from s t d i n : %c \n", gotch, v a l ) ;