Today’s topic Inter-process communication with pipes

More about files and I/O. – cin  read (0, …) – cout  write (1, …) –What happens when we use both I/O mechanisms in the same program? Check out the output of mix.cpp? –Why? – The liberty in the implementation of the C/C++ runtime library How to fix the order? fflush(0).

More on the timestamp of a file –How to get the info? Stat –See newstat.c for comparing the timestamp of two files.

Inter-Process Communication (IPC): –processes may be collaborated on tasks –e.g, ps -ef | grep a.out | more –e.g, –OS needs to provide mechanisms for IPC client server

IPC related system calls: –the most general IPC mechanism in UNIX is socket (send and receive paradigm with read/write interface, works also for processes on different machines). –What we will discuss: pipes: allow transfer of data between processes in a first-in-first-out manner. signal: send a flag to another process

Pipes: –two types of pipes, named pipes and unnamed pipes –name pipes: like a file (create a named pipe (mknod), open, read/write) can be shared by any number of processes will not be discussed in detail. –Unnamed pipes: an unnamed pipe does not associated with any file can only be shared by related processes (descendants of a process that creates the unnamed pipe). Created using system call pipe().

Pipes and files –Both can be used to share information among processes –Both share the file API –Performance difference: A pipe is usually realized in memory, a file is not. Pipe operations are memory operations, file operations are I/O operations. –Semantic difference: A pipe is a fifo queue: –The content in a fifo queue can only be read once (the information is gone after the read operation). A storage in a file is persistent –The content can be used many times.

The pipe system call –open unnamed pipes –syntax int pipe(int fds[2]) –semantic create a pipe and returns two file descriptors fds[0] and fds[1] a read from fds[0] accesses the data written to fds[1] on a fifo basis. the pipe has a limited size (64K in most systems) -- cannot write to the pipe infinitely.

#include main() { char *s, buf[1024]; int fds[2]; s = “hello world\n”; pipe(fds); write(fds[1], s, strlen(s)); read(fds[0], buf, strlen(s)); printf(“fds[0] = %d, fds[1] = %d\n”, fds[0], fds[1]); write(1, buf, strlen(s)); } /* example1.c */

#include main() { char *s, buf[1024]; int fds[2]; s = “hello world\n”; pipe(fds); if (fork() == 0) { printf(“child process: \n”); write(fds[1], s, 12); exit(0); } read(fds[0], buf, 6); write(1, buf, 6); } /* example2.c : using pipe with fork*/ IPC can be used to enforce the order of the execution of processes.

main() { char *s, buf[1024]; int fds[2]; s = “hello world\n”; pipe(fds); if (fork() == 0) { printf(“11111 \n”); /* how to make before */ read(fds[0], s, 6); printf(“22222\n”); } else { printf(“33333\n”); write(fds[1], buf, 6); printf(“44444\n”) } } /* example3.c */

Anyone writes any programs that take advantage of multi-core in a CPU? –A multiple process solution for computing PI? See pi1.c and pi2.c?

Implementing Pipe in shell. E.g. /usr/bin/ps -ef | /usr/bin/more How shell realizes this command? –Create a process to run ps -ef –Create a process to run more –Create a pipe from ps -ef to more the standard output of the process to run ps -ef is redirected to a pipe streaming to the process to run more the standard input of the process to run more is redirected to be the pipe from the process running ps -ef

Implement “/bin/ps -ef | /bin/more” – first try main() { int fds[2]; char *argv[3]; pipe(fds); // create pipe if (fork() == 0) { close(0); dup(fds[0]); // redirect standard input to fds[0] argv[0] = “/bin/more”; argv[1] = 0; if (execv(argv[0], argv) == -1) exit(0); } if (fork() == 0) { close(1); dup(fds[1]); // redirect standard output to fds[1]; argv[0] = “/bin/ps”; argv[1] = “-ef”; argv[2] = 0; if (execv(argv[0], argv) == -1) exit(0); } wait(); wait(); } /* example4a.c */

Example4a.c not the same as “/bin/ps –ef | /bin/more”, what is missing? –When can ‘more’ be done? When the end of the file is reached. –What is “the end of file” of a pipe? No data in pipe? No data in pipe and no potential writer to the pipe!!!! –In example4a.c, the more program will not finish since there are potential writers. How to fix this program?

Implement “/bin/ps -ef | /bin/more” main() { int fds[2]; char *argv[3]; pipe(fds); // create pipe if (fork() == 0) { close(0); dup(fds[1]); // redirect standard input to fds[1] close(fds[0]); close(fds[1]); // close unused files argv[0] = “/bin/more”; argv[1] = 0; execv(argv[0], argv); exit(0); } if (fork() == 0) { close(1); dup(fds[0]); // redirect standard output to fds[0]; close(fds[0]); close(fds[1]); // close unused files argv[0] = “/bin/ps”; argv[1] = “-ef”; argv[2] = 0; execv(argv[0], argv); exit(0); } close(fds[0]); close(fds[1]); wait(); wait(); } /* example4.c */