Shared memory for processes

Can't assume all variables are initally zero. OS decide which process maps who which physical address

memory mapping mmap is a family of system calls:

allow two processes to share some region of their memory space.
achieved by creating a memory region that can be shared by other processes

shared memory by two processes mapping to the same file or using fork() after mmap()

random_fd = open("/home/hzy/test/zero", O_RDWR);
ptr = mmap(NULL, 10240, PROT_READ|PROT_WRITE,
	MAP_SHARED | MAP_FILE, random_fd, 0);

private mapping is often used to set up new memory sections.

fd = open("mmap_test_file", O_RDWR); p
tr = mmap(NULL, 10240, PROT_READ|PROT_WRITE, 
	MAP_PRIVATE|MAP_FILE, fd, 0);

Threads

product consumer problem

a producer process writes data to a buffer of fied size
a consumer reads data from the buffer these two processes are being scheduled independently by the CPU

producer must wait if buffer is full consumer must wait if buffer is empty

threads

allows more convenient data sharing
threads within a single process share everything except they have their own program counter, registers and stack space

why

responsiveness - parrallelism - dont have to wait
resource sharing - automaticall share everything in same process
economy - lightweight, creating and switching between threads is faster than creating heavyweight processes
scalability - each to work on multiprocessor architectures

pthread

pthread_create(): create a thread
pthread_exit(): terminate a thread
pthread_join(): wait for a thread to terminate
pthread_detach(): detach a thread so that it won’t be waited
pthread_self(): get the thread id of the current thread
pthread_equal(): compare if two threads are the same
pthread_cancel(): send a cancellation request to a thread

void *myThreadFun(void *vargp) { 
	sleep(1);
	printf("Hello from Thread \n");
	return NULL;
} 
	
int main() { 
	pthread_t thread_id;
	printf("Before Thread\n");
	pthread_create(&thread_id, NULL, myThreadFun, NULL);
	pthread_join(thread_id, NULL);
	printf("After Thread\n");
	exit(0);
}

web server thread

a web server is a process that provides data over the web to requesting clients
a thread is created for each request

Lightweight process - shares everything except the stack

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