CS 3411 Systems Programming

Transcription

1 CS 3411 Systems Programming Department of Computer Science Michigan Technological University Pipe Inter-Process Communication in Unix

2 Today's Topics How to communicate between processes without using signals Creating and Using Pipes

3 How to Communicate Between Processes Have two primitive mechanisms in hand: exit/wait signals The sample program we'll be working on: Parent creates child Send child an int, x Child computes 2x and returns result Maybe a loop that keeps doing this! Parent and child share no memory; i.e. no common variables through which to communicate

4 How to Communicate Between Processes Parent and child share open le descriptors, even after fork() or fork()/exec() Proposed solution: Parent opens les, forks child Child arranges descriptors with close and dup, execs new binary Parent and child communicate via read()/write() on shared le

5 Example Code #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 <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 l i b. h> main ( ) { i n t fd, v a l, d b l v a l ; f d = open ( " commofile ", O_RDWR O_CREAT O_TRUNC, ) ; i f ( f o r k ( ) == 0) { / CHILD / r e a d ( fd, &v a l, s i z e o f ( i n t ) ) ; l s e e k ( fd, 0, SEEK_SET ) ; d b l v a l = 2 v a l ; w r i t e ( fd, &d b l v a l, s i z e o f ( i n t ) ) ; l s e e k ( fd, 0, SEEK_SET ) ; e x i t ( 0 ) ; e l s e { / PARENT / v a l = 2 ; f p r i n t f ( s t d e r r, " Asking c h i l d to d o u b l e %d\n", v a l ) ; w r i t e ( fd, &v a l, s i z e o f ( i n t ) ) ; l s e e k ( fd, 0, SEEK_SET ) ; r e a d ( fd, &d b l v a l, s i z e o f ( i n t ) ) ; f p r i n t f ( s t d e r r, " C h i l d r e p l i e d w i t h %d\n", d b l v a l ) ; w a i t (NULL ) ; e x i t ( 0 ) ;

6 Results Why did it turn out like that? Fundamental problem: Need more control over access to the shared le Specically, read from an empty le (or read when currency indicator is at EOF) should delay caller until data is available to read The Unix solution is a construct called a pipe

7 The pipe() System Call The pipe() function takes one argument in the form of an array with 2 elements The rst element is the read end of the pipe The second element is the write end of the pipe Data written to the pipe is buered by the kernel until it is read

8 Pipes Can think of a pipe as an unnamed, xed length le maintained by the kernel We also looked at the named versions! They're identical once opened - no data is to the device in either case! Pipes have separate le descriptors as well as currency indicators for reading and writing Some special properties: A read from a pipe that doesn't have sucient data to satisfy the read will block the reader until the data is available A write to a pipe that is full will delay the writer until space becomes available From a reader's perspective, EOF can only happen if there is no data in the pipe and all write descriptors on the pipe are closed From a writer's perspective, an attempt to write to a pipe without live read descriptors will result in a SIGPIPE signal being sent to the writer

9 Example Code #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 <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 l i b. h> i n t f d [ 2 ], v a l = 0, d b l v a l = 0 ; main ( ) { p i p e ( f d ) ; i f ( f o r k ( ) == 0) { / CHILD / w h i l e ( r e a d ( f d [ 0 ], &v a l, s i z e o f ( i n t ) )!= 0) { d b l v a l = 2 v a l ; w r i t e ( f d [ 1 ], &d b l v a l, s i z e o f ( i n t ) ) ; e x i t ( 0 ) ; e l s e { / PARENT / f o r ( v a l = 1 ; v a l <= 3 ; v a l ++) { f p r i n t f ( s t d e r r, " Asking c h i l d to d o u b l e %d\n", v a l ) ; w r i t e ( f d [ 1 ], &v a l, s i z e o f ( i n t ) ) ; r e a d ( f d [ 0 ], &d b l v a l, s i z e o f ( i n t ) ) ; f p r i n t f ( s t d e r r, " C h i l d r e p l i e d w i t h %d\n", d b l v a l ) ; w a i t (NULL ) ;

10 Results What now? Another problem: Pipe is essentially a construct for unidirectional transfer of information. Parent is reading its own data written into the pipe. Need one pipe for synchronized parent-to-child communication And a second pipe for synchronized child-to-parent communication When do we get EOF?

11 Example Code #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 <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 l i b. h> i n t p2c [ 2 ], c2p [ 2 ], v a l = 0, d b l v a l = 0 ; main ( ) { p i p e ( p2c ) ; p i p e ( c2p ) ; i f ( f o r k ( ) == 0) { / CHILD / c l o s e ( p2c [ 1 ] ) ; c l o s e ( c2p [ 0 ] ) ; w h i l e ( r e a d ( p2c [ 0 ], &v a l, s i z e o f ( i n t ) )!= 0) { d b l v a l = 2 v a l ; w r i t e ( c2p [ 1 ], &d b l v a l, s i z e o f ( i n t ) ) ; e x i t ( 0 ) ; e l s e { / PARENT / c l o s e ( c2p [ 1 ] ) ; c l o s e ( p2c [ 0 ] ) ; f o r ( v a l = 1 ; v a l <= 3 ; v a l ++) { f p r i n t f ( s t d e r r, " Asking c h i l d to d o u b l e %d\n", v a l ) ; w r i t e ( p2c [ 1 ], &v a l, s i z e o f ( i n t ) ) ; r e a d ( c2p [ 0 ], &d b l v a l, s i z e o f ( i n t ) ) ; f p r i n t f ( s t d e r r, " C h i l d r e p l i e d w i t h %d\n", d b l v a l ) ; c l o s e ( p2c [ 1 ] ) ; c l o s e ( c2p [ 0 ] ) ; w a i t (NULL ) ;

12 Results It works as expected this time! Let's take a look at some other examples

13 Real Examples I #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 <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 l i b. h> / We want to e x e c u t e " ps aux g r e p r o o t " / main ( ) { i n t i s P a r e n t ; i n t a p i p e [ 2 ] ; char cmd [ 2 ] [ 3 ] ; cmd [ 0 ] [ 0 ] = " ps " ; cmd [ 0 ] [ 1 ] = " aux " ; cmd [ 0 ] [ 2 ] = NULL ; cmd [ 1 ] [ 0 ] = " g r e p " ; cmd [ 1 ] [ 1 ] = " r o o t " ; cmd [ 1 ] [ 2 ] = NULL ; p i p e ( a p i p e ) ; i s P a r e n t = f o r k ( ) ; i f (! i s P a r e n t ) { / C h i l d i s g o i n g to be " g r e p r o o t " / / We want s t d i n c o n n e c t e d to our p i p e! / c l o s e ( a p i p e [ 1 ] ) ; c l o s e ( 0 ) ; dup ( a p i p e [ 0 ] ) ; c l o s e ( a p i p e [ 0 ] ) ;

14 Real Examples II e x ecvp (cmd [ 1 ] [ 0 ], cmd [ 1 ] ) ; p e r r o r ( " C h i l d e x e c : : " ) ; e x i t ( 1 ) ; e l s e { / Parent i s " ps aux " / / We want t h e s t d o u t c o n n e c t e d to p i p e / c l o s e ( a p i p e [ 0 ] ) ; c l o s e ( 1 ) ; dup ( a p i p e [ 1 ] ) ; c l o s e ( a p i p e [ 1 ] ) ; e x ecvp (cmd [ 0 ] [ 0 ], cmd [ 0 ] ) ; p e r r o r ( " Parent e x e c : : " ) ; e x i t ( 1 ) ;

15 Real Examples I #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 <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 l i b. h> / We want to e x e c u t e " s o r t < filecomm1. c g r e p f p r i n t f wc" / main ( ) { i n t a p i p e [ 2 ] ; i n t i s P a r e n t ; char cmd [ 3 ] [ 3 ] ; i n t i, l a s t C h i l d, fd, s a v e S t d o u t ; cmd [ 0 ] [ 0 ] = " s o r t " ; cmd [ 0 ] [ 1 ] = NULL ; cmd [ 1 ] [ 0 ] = " g r e p " ; cmd [ 1 ] [ 1 ] = " f p r i n t f " ; cmd [ 1 ] [ 2 ] = NULL ; cmd [ 2 ] [ 0 ] = "wc" ; cmd [ 2 ] [ 1 ] = NULL ; s a v e S t d o u t = dup ( 1 ) ; f o r ( i = 2 ; i >= 0 ; i ) { p i p e ( a p i p e ) ; i s P a r e n t = f o r k ( ) ; i f (! i s P a r e n t ) { c l o s e ( a p i p e [ 1 ] ) ; c l o s e ( 0 ) ; i f ( i!= 0) { dup ( a p i p e [ 0 ] ) ;

16 Real Examples II i f ( i == 0) { f d = open ( " filecomm1. c ", O_RDONLY) ; dup ( f d ) ; c l o s e ( a p i p e [ 0 ] ) ; e x ecvp (cmd [ i ] [ 0 ], cmd [ i ] ) ; e x i t ( 1 ) ; e l s e { i f ( i ==2) l a s t C h i l d = i s P a r e n t ; c l o s e ( a p i p e [ 0 ] ) ; c l o s e ( 1 ) ; i f ( i!=0) { dup ( a p i p e [ 1 ] ) ; c l o s e ( a p i p e [ 1 ] ) ; i f ( i ==0){ dup2 ( s a v estdout, 1 ) ; w a i t p i d ( l a s t C h i l d, NULL, 0 ) ;