Lecture 13
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Announce
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This Week
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Today
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- Finish up syscalls
- UNIX C/C++ Devel

InterProcess Communication (IPC)
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- processes often times need to communicate with eachother

- use IPC methods

	- network connections

	- through files

	- through shared memory

	- through pipes

- simplest are pipes

- pipe() syscall creates a communication channel in OS

- takes a 2 element array, fills with two new file descriptors

	- first is for reading

	- second is for writing

- by itself is useless in a single process

- parent creates pipe, then forks

	- pipe FDs are copied to new process

	- parent closes end of pipe it's not using
	- child closes end of pipe it's not using
	
	- now the two processes can communicate with read/write

- | character in shells uses dup2 and pipe to perform | task

- named pipes for non parent/child processes (mkfifo())


Signals
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- UNIX defines a set of software signals that can be sent and recieved
to and from processes

- for of very simple IPC

- by default, most signals are ignored by the process

- we can change this default behaviour through the signal() syscall

- void *signal(int signum, void *handler);

- signum is the signal number we're setting a handler for

	- signal.h sets up #defines for the signals, gives them names

		SIGINT (ctrl-c)
		SIGTSTP
		SIGKILL
		SIGSTOP
		etc
	- man 7 signal

- we write a function that is to be called when the process recieves a
signal.  

- OS remembers where this function is and if process recvs a sig,
  stops current execution of process, runs handler, returns to normal
  execution

- 'handler' is a function pointer

- one more use of pointers...function pointers!!

- functions can be defined to accept a pointer to a function as an argument

- this way, the program can switch which functions are used in a generic way

void sigfunc(int in) {

	printf("I got a signal!\n");

	signal(SIGTSTP, sigfunc);
}

main() {

	signal(SIGTSTP, sigfunc);

	while(1) {

	}	

}

- currently, under linux, must reset the sighandler


C IO
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- we've talked about UNIX system calls for handling IO

- C defines it's own way to do IO which is very similar

- FILE * instead of file descriptor

- fopen, fclose, fread, fwrite, fprintf, fscanf, fseek