10 Thread

A thread of execution is the smallest sequence of programmed instructions that can be managed independently by a scheduler, which is typically a part of the operating system.

Using pthreads library in C

• In your C program you need to #include <pthread.h>.
• There are many functions related to pthreads.
• On a UNIX-based system, a list of these functions can typically be obtained with man -k pthread.
• To know about a specific function see the man page.
• When linking, you must link to the pthread library using compilation flag -lpthread: gcc -lpthread file.c

Spawning a thread using pthread_create()

• A thread is created using pthread_create()

int pthread_create(
	pthread_t *thread_id, // ID number for thread
	const pthread_attr_t *attr, // controls thread attributes
	void *(*function)(void *), // function to be executed
	void *arg // argument of function
);

• Returns 0 if the thread creation is successful.
• Otherwise returns a non-zero value to indicate an error.
• We will use default attributes set by the system, so *attr gets replaced by NULL, the easier syntax is :

int pthread_create(
	pthread_t *thread_id, // ID number for thread
	NULL, // we set it to default thread attributes
	void *(*function)(void *), // function to be executed
	void *arg // argument of function
);

Consider the function with multiple arguments

T *foo (T *, T*)

where T represents any data type. Solution : Pack arguments into a struct and create a wrapper, which takes the compound argument and unpacks before passing the unpacked arguments to foo()

typedef struct Compound {
	T *a, *b;
} Compound_t;
T * foo_wrapper(Compound_t *c) {
	// This can be passed to pthread_create
	T *d;
	d = foo(c->a, c->b);
}