CSC209H Lecture 7. Dan Zingaro. February 25, 2015

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1 CSC209H Lecture 7 Dan Zingaro February 25, 2015

2 Inter-process Communication (IPC) Remember that after a fork, the two processes are independent We re going to use pipes to let the processes communicate Just remember that any FD (file descriptor) that the parent opens before calling fork is also open in the child! This is crucial for pipes to work as an inter-process mechanism Let me briefly introduce some other IPC mechanisms...

3 Pipes (Kerrisk ) Pipe: one-way, first-in first-out communications channel Usually, one process writes into a pipe, and another process reads from the pipe int pipe(int fds[2]); You pass a pointer to two integers (i.e. an array or a malloc of two ints), and pipe fills it with two newly-opened FDs

4 Pipes... int p[2]; if (pipe(p) == -1) { perror("pipe"); exit(1); p[0] is now an FD open for reading p[1] is now an FD open for writing Whatever you send down p[1] shows up in p[0] Don t try reading from p[1] or writing to p[0]! p[1] pipe p[0]

5 Example: pipe in One Process (pipe1.c) One process talking to itself through a pipe. #include <unistd.h> #include <stdio.h> #include <stdlib.h> #define MSG_SIZE 13 char *msg = "hello, world\n"; int main(void) { char buf[msg_size]; int p[2]; if (pipe(p) == -1) { perror("pipe"); exit(1); write(p[1], msg, MSG_SIZE); read(p[0], buf, MSG_SIZE); write(stdout_fileno, buf, MSG_SIZE); return 0;

6 Example: pipe and fork (pipe2.c) Child talks, parent listens. #include <unistd.h> #include <stdio.h> #include <stdlib.h> #define MSG_SIZE 13 char *msg = "hello, world\n"; int main(void) { char buf[msg_size]; int p[2]; if (pipe(p) == -1) { perror("pipe"); exit(1); if (fork() == 0) //Child writes write(p[1], msg, MSG_SIZE); else { //Parent reads from pipe and prints read(p[0], buf, MSG_SIZE); write(stdout_fileno, buf, MSG_SIZE); return 0;

7 Example: pipe and eof (pipe3.c) The buffer is too small for the parent to read everything at once. So it reads in a loop... but parent hangs! #define BUF_SIZE 4 //Small! #define MSG_SIZE 13 char *msg = "hello, world\n"; int main(void) { char buf[buf_size]; int p[2], nbytes; if (pipe(p) == -1) { perror("pipe"); exit(1); if (fork() == 0) write(p[1], msg, MSG_SIZE); else { while ((nbytes = read(p[0], buf, BUF_SIZE)) > 0) write(stdout_fileno, buf, nbytes); return 0;

8 Closing Ends of Pipes When a process does a read on a pipe, it blocks until data is available in the pipe As long as someone has the writing end open, a read on a pipe will wait forever if the pipe is empty If no one has the writing end open, read detects eof and returns 0 Like all FDs, a process closes any open pipe FDs when it exits

9 Example: pipe and eof again (pipe4.c) The parent closes its writing end. When the child terminates, no one has the writing end open, and the parent s read returns 0. #define BUF_SIZE 4 //Small! #define MSG_SIZE 13 char *msg = "hello, world\n"; int main(void) { char buf[buf_size]; int p[2], nbytes; if (pipe(p) == -1) { perror("pipe"); exit(1); if (fork() == 0) write(p[1], msg, MSG_SIZE); else { close(p[1]); while ((nbytes = read(p[0], buf, BUF_SIZE)) > 0) write(stdout_fileno, buf, nbytes); return 0;

10 Closing Ends of Pipes... If you try to write to a pipe and no one has the reading end open, you get a signal (SIGPIPE) that terminates your program In general, each process will read or write a pipe (not both), so always close the end that it s not using If you re reading from a pipe, close p[1] before reading If you re writing to a pipe, close p[0] before writing... and make sure both ends are closed when you re finished with the pipe

11 read/write Considerations Pipes have a finite size (on Linux, 64K) If a write would overflow a pipe, the process blocks until another process empties some of the pipe with read Pipes have no message boundaries: if two writes occur and then you read, do not assume that you ll get just the first one! writes into a pipe are only guaranteed to be atomic if they are smaller than a certain number of bytes (on Linux, 4K) If two processes write to a pipe, and both write less than 4K with a write call, the pipe is guaranteed to hold all data from one process and then all data from the other If two processes write to a pipe, and one writes more than 4K with a write call, the data could be interleaved in the pipe

12 Mimicking Shell Pipelines Let s say we want to do sort <letters uniq Steps Parent creates a pipe and forks a child Child opens letters Child sets standard input to letters Child sets standard output to the writing end of the pipe Child execs sort (which now reads from letters and writes down the pipe)

13 Mimicking Shell Pipelines... Parent sets its standard input to the reading end of the pipe Now, parent s standard input receives the output from sort Parent execs uniq We haven t changed the parent s standard output!(it is the screen unless shell redirection was used)

14 Mimicking Shell Pipelines... (sortuniq1.c) int main(void) { int fd[2], pid; if (pipe(fd) == -1) { perror ("pipe"); exit(1); if ((pid = fork()) == 0) { int filefd = open("letters", O_RDONLY); dup2(filefd, STDIN_FILENO); //stdin is letters close(filefd); dup2(fd[1], STDOUT_FILENO); //stdout is pipe close(fd[0]); close(fd[1]); execl("/usr/bin/sort", "sort", (char *) NULL);

15 Mimicking Shell Pipelines... else if(pid > 0) { dup2(fd[0], STDIN_FILENO); //stdin is pipe close(fd[0]); close(fd[1]); execl("/usr/bin/uniq", "uniq", (char *) NULL); else { perror("fork"); exit(1); return 0;

16 Tracing Sortuniq if (pipe(fd) == -1) { parent standard input standard output fd[0] fd[1] keyboard screen write pipe read

17 Tracing Sortuniq... if ((pid = fork()) == 0) { int filefd = open("letters", O_RDONLY); child parent filefd fd[0] fd[1] standard output standard input fd[1] standard output standard input fd[0] letters write screen keyboard pipe read

18 Tracing Sortuniq... dup2(filefd, STDIN_FILENO); //stdin is letters parent child standard input fd[0] fd[1] standard output fd[1] standard output filefd standard input fd[0] keyboard write screen letters pipe read

19 Tracing Sortuniq... dup2(fd[1], STDOUT_FILENO); //stdout is pipe parent child standard input standard output fd[0] fd[1] filefd standard output fd[1] standard input fd[0] keyboard screen write letters pipe read

20 Tracing Sortuniq... close(fd[0]); close(fd[1]);... and then exec sort parent child standard input standard output fd[0] fd[1] filefd fd[1] fd[0] standard output standard input keyboard screen write letters pipe read

21 Tracing Sortuniq... dup2(fd[0], STDIN_FILENO); //stdin is pipe close(fd[0]); close(fd[1]);... and then exec uniq child keyboard filefd fd[1] fd[0] standard input standard output parent fd[1] fd[0] standard output standard input letters write pipe screen read

22 Bitwise Operators (King 20.1) C has six bitwise operators; they operate on bits of integers There are Two shift operators Four logical operators We re studying this now so that we understand Unix signal and FD sets later

23 Bitwise Shift Operators <<: left shift i << j shifts each bit in i to be j places to the left 0 bits are filled-in at the right >>: right shift unsigned short i, j; i = 13; /* binary */ j = i << 2; /* binary */ j = i >> 2; /* binary */

24 Bitwise and, or, not &: bit-by-bit and operator : bit-by-bit or operator ~: bitwise not operator unsigned short i, j, k; i = 21; /* binary */ j = 56; /* binary */ k = ~i; /* binary */ k = i & j; /* binary */ k = i j; /* binary */

25 Bitwise xor ^: bit-by-bit xor operator unsigned short i, j, k; i = 21; /* binary */ j = 56; /* binary */ k = i ^ j; /* binary */

26 Setting and Clearing a Bit Bits are numbered from the right, starting at b16 b15... b1 b0 To set bit b of integer n to 1: n = n (1 << b); To clear bit b of integer n to 0: n = n & ~(1 << b);

27 Testing Whether a Bit is Set Exercise: how can you test whether a particular bit is set? if (... ) { printf("bit is set");

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