ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication Risto Järvinen October 14, 2019 Lecture contents This lecture: Different ways processes can affect each other Classic IPC Stevens: ch10 (again), ch15 (except 15.7-15.10), ch16-17 (except internet sockets) Kerrisk: ch22, ch44, ch49-50, ch56-57,60-61 (ch58-59 = internet sockets) Internet sockets are covered in Network Programming course, so they are informative and useful to study, but not required on this course. Next lecture: System V IPC POSIX IPC ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 2/25 Risto Järvinen 14.10.2019 How can one process affect another? Use of same resources (CPU time, memory, I/O bandwidth,..) Fork() and wait()ing for children. Signals. Using same filesystem, shared files and directories. File locks. Memory mapped files (mmap()) Terminals/pseudoterminals. Pipes and sockets. Dedicated IPC mechanisms: Message queues, Shared memory, Semaphores. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 3/25 Risto Järvinen 14.10.2019 Classic IPC Filesystem and Filelocks. (lect5) Pipes, named and unnamed. (today) Sockets, UNIX-domain and network. (today) Terminals. (lect3) Shared memory via mmap(). (lect3, briefly today) Signals, real-time signals. (lect4, RTS today) Some topics already partially covered. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 4/25 Risto Järvinen 14.10.2019 Signals Covered before. Basic idea is the have asynchronous notification to the process. Implemented as a function (signal handler) that can be called at any time. Limited usability (few signals, no ordering, may lose signals, etc..) ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 5/25 Risto Järvinen 14.10.2019 POSIX Real-Time Signals (RTS) Extension to signals: More signals. Queueing model, with priorities. Sender is identified. Can deliver data. Doesn’t lose multiple signals, though there is limit on how many times a signal can arrive. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 6/25 Risto Järvinen 14.10.2019 Real-Time Signals limitations Amount of signals is implementation specific. Standard doesn’t specify whether RTS operation may be used with normal signals. Delivered data is only an integer or a pointer. And a pointer usually has no meaning across processes. Limits available via sysconf() and signal.h: SIGRTMIN to SIGRTMAX, min _POSIX_RTSIG_MAX. _POSIX_SIGQUEUE_MAX, _SC_SIGQUEUE_MAX, RLIMIT_SIGPENDING. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 7/25 Risto Järvinen 14.10.2019 Real-Time Signals API 1/2 #include <signal.h> int sigqueue(pid_t pid , int sig , const union sigval value); int sigaction( int signum , const struct sigaction ∗act , struct sigaction ∗oldact ); void sa_sigaction( int signum, siginfo_t ∗info , void ∗context ); ”value” is union of integer (int) and pointer (void *). RTS mode handler is defined by setting SA_SIGINFO flag in struct sigaction. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 8/25 Risto Järvinen 14.10.2019 Real-Time Signals API 2/2 #include <signal .h> i n t sigwaitinfo( const sigset_t ∗set, siginfo_t ∗ info ); i n t sigtimedwait( const sigset_t ∗set, siginfo_t ∗ info , const struct timespec ∗ timeout ); Synchronous reception of signals, no need to write asynchronous handler. Blocks until a signal is received (or timeout occurs). If signal arrives between calls to sigwaitinfo(), it’s handled by the given disposition. To avoid surprises, block signals with sigprocmask(). ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 9/25 Risto Järvinen 14.10.2019 Pipes Mentioned briefly before. Data in from one end, out from the other end. Two types: Unnamed and Named The most common use: stdin/stdout/stderr pipes between processes ”ls | less” type command chaining in shell ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 10/25 Risto Järvinen 14.10.2019 Unnamed pipes API #include <unistd .h> int pipe( int pipefd [2]); FILE ∗popen( const char ∗command , const char ∗ type ); int pclose(FILE ∗stream ); Creates two special file descriptors: fd[0] can be read, fd[1] can be written. Pipe is unidirectional. Two-way communications need two pipes. No name or presense besides the FDs. Popen() wraps pipe creation, forking and exec()ing command. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 11/25 Risto Järvinen 14.10.2019 Unnamed pipes semantics For communication between processes within same process group: Parent process creates a pipe and gives the other end to the children. Within single process: f.ex. signal handler hack, or between threads. Pipe can hold at least PIPE_BUF bytes. (defined 4kiB on Linux, but actual default maxsize is 1MiB and it’s adjustable.) Read operation: If data in buffer, returns data. If no data, blocks. If other end has been closed, returns remaining data and then returns EOF (0). Write operation: If space in buffer, writes atomically. If not enough, blocks until there is. If other end has closed, returns errno=EPIPE and process receives SIGPIPE. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 12/25 Risto Järvinen 14.10.2019 Named pipes (FIFO) API int mkfifo( const char ∗pathname, mode_t mode); Pipe that is represented by a special type of file in the filesystem. Named pipe is used like a file; it has access rights, it’s opened with open(), deleted with unlink(). Two-way communication still needs two pipes.. Also command-line utility of the same name. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 13/25 Risto Järvinen 14.10.2019 Named pipe semantics Usable between unrelated processes. Create with mkfifo(), delete with unlink() Open operation: Open with O_WRONLY blocks until other end has opened it the pipe for reading. Open with O_RDONLY blocks until other end has opened the pipe for writing. Read operation: If data in buffer, returns data. If no data, blocks. If other end has been closed, returns remaining data and then returns EOF (0). Write operation: If space in buffer, writes atomically. If not enough, blocks until there is. If other end has closed, returns errno=EPIPE and process receives SIGPIPE. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 14/25 Risto Järvinen 14.10.2019 Sockets BSD-derived communication method, basis of Internet communication. We’ll stick to particular type of sockets: UNIX Domain sockets. Actual Internet communication will be covered in S-38.3610. Basic idea: socket is a file descriptor, with a bunch of additional calls. List of ”domains” that Linux supporst on sockets: UNIX Domain, IPv4, IPv6, IPX, Netlink (kernel), X.25/AX.25, Appletalk and mechanisms to access raw frames of ATM and IP. ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 15/25 Risto Järvinen 14.10.2019 Socket semantics 1/2 Sockets also have different types: SOCK_DGRAM is message-based, unreliable, connectionless. ("UDP") SOCK_STREAM is stream-based, reliable, connected. ("TCP") SOCK_RAW provides access to raw protocol frames. SOCK_RDM is message-based, reliable, connectionless. SOCK_SEQPACKET is message-based, reliable, connected. ... ... SOCK_DGRAM and SOCK_STREAM are only commonly available. Others are for specialized protocols, like TIPC (Transparent Inter-Process Communication). ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 16/25 Risto Järvinen 14.10.2019 Socket semantics 2/2 Connectionless server: socket(), bind(), recvfrom()*N, close() Connectionless client: socket(), (bind(),) sendto()*N, close() Connected server: socket(), bind(), listen(), accept(), read/recv/write/send*N, close() Connected client: socket(), (bind(),) connect(), read/recv/write/send*N, close() ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 17/25 Risto Järvinen 14.10.2019 Socket API 1/2 #include <sys/socket .h> i n t socket( i n t domain , i n t type , i n t protocol ); i n t bind( i n t sockfd , const struct sockaddr ∗addr , socklen_t addrlen); i n t listen( i n t sockfd , i n t backlog ); i n t accept ( i n t sockfd , struct sockaddr ∗addr , socklen_t ∗ addrlen); i n t connect( i n t sockfd , const struct sockaddr ∗addr , socklen_t addrlen); ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 18/25 Risto Järvinen 14.10.2019 Socket API 2/2 ssize_t recv( int sockfd , void ∗buf, size_t len, int flags ); ssize_t recvfrom( int sockfd , void ∗buf, size_t len, int flags , struct sockaddr ∗src_addr , socklen_t ∗addrlen ); ssize_t recvmsg( int sockfd , struct msghdr ∗msg, int flags ); ssize_t send( int sockfd , const void ∗buf, size_t len, int flags ); ssize_t sendto( int sockfd , const void ∗buf, size_t len, int flags , const struct sockaddr ∗dest_addr , socklen_t addrlen ); ssize_t sendmsg( int sockfd , const struct msghdr ∗msg , int flags ); sendto(fd,buf,len,flags,NULL,0) equals send(fd,buf,len,flags). send(fd,buf,len,0) equals write(fd,buf,len). ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 19/25 Risto Järvinen 14.10.2019 UNIX Domain Sockets Socket that communicates inside a UNIX system. Can be datagram or stream based. (Or even reliable datagram.) Address is a path, with a file (type = socket). If a regular file of the same name exists, EADDRINUSE. Can do both SOCK_DGRAM and SOCK_STREAM. struct sockaddr_un { unsigned short sun_family; /∗ Address family ∗/ char sun_path[108]; /∗ Path name ∗/ } ; ELEC-C7310 Sovellusohjelmointi Lecture 6: Inter-process Communication 20/25 Risto Järvinen