Interprocess
communication
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Communicating Processes
• Processes can be started using the exec system calls
• Processes can operate in parallel using the fork system call followed by exec
• Processes sometimes wish to cooperate and exchange information during execution
Communicating Processes
• Why multiple processes? – Synchronous activities
– Asynchronous events
• Files – select
• Shared memory (tutorial)
File Descriptors
• low level I/O is performed on file descriptors that are small integers indicating an open file
• when process is started file descriptor 0 is standard input, 1 is standard output, 2 is standard error output
• low level system call functions operate on file descriptors
I/O system calls
• low level I/O functions include: – creat
– open, close – read, write – ioctl
• eg read 100 characters from standard input into array “buffer”
read(0,buffer,100)
int pipe(int filedes[2]);
• filedesisatwoelementarrayofintegersthatisfilled in with two file descriptors
• filedes[0]isforreading
• filedes[1]isforwriting
• datawrittenintofiledes[1]canbesubsequentlyread from filedes[0]
• thepipefunctionreturns0onsuccess,-1onfailure
Using pipes
• a parent process can communicate with a child by creating a pipe before the fork
• the parent can then write data to fildes[1] and the child can read filedes[0]
• the system has a small amount of buffering
• if the buffer is filled, the writer is suspended until the reader has read some data
Using files
• Process A talks to Process B via a named file.
• A: Open file, write data, close file
• B: Open file, read data, close file
• How can they synchronise read/write?
Using files
• wait for signal
• int fd = open(“shared.txt”, O_WRONLY); • write some data
• close(fd)
• send signal
Using files
• Busy loop for flag, triggered by signal
• int fd = open(“shared.txt”, O_RDONLY); • read some data
• close(fd)
• Send signal back
Using files
• Requires setup signal
• What is the event?
– File has been opened/closed – File has been read/written
• Use select() ! – man 2 select
Using files with select()
• select() monitors multiple file descriptors
• The file descriptors are specified by the file descriptor set.
• select() wait for an I/O event for any of the file descriptors in the set
– There is data available to read from the file – There is space available to write to the file
Using files with select()
• Macros are used to define the set of file descriptors
– FD_ZERO(), FD_SET(), FD_CLR(), FD_ISSET()
• There are three descriptor sets
int select( int nfds,
fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
struct timeval *timeout);
Blocking I/O
Each file descriptor access can cause the processes to block
• pipe/filewritewillblockuntilspaceavailable • pipe/filereadwillblockuntildataavailable
• Processescannotdootherusefulwork.Maymiss events, deadlines
– Realtime systems may not afford to wait for I/O
Blocking I/O
Synchronous sharing of data of
• They each are waiting for signal interrupt, cannot do useful work
• E.g. wait for 100 bytes, but only received 46
• It is possible to open a file in NON BLOCKING MODE
Blocking I/O
• open(”bigfile.bin”, O_NONBLOCK | O_RDONLY);
• Change mode of existing fd (stdin, stdout, stderr)
fcntl(fd, F_SETFL, fcntl(fd, F_GETFL) | O_NONBLOCK);
Use existing flags
• For simple communications between processes a pipe can be used
• For communications where a process needs to be interrupted, signals can be used
• For large amounts of shared data:
– files are helpful
• select() and epoll() • non-blocking
– shared memory
• picture acknowledgement: http://www.pipes.org/Ephemeris
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