Purpose of while(1); statement in C

Question

What purpose does while(1); serve ? I am aware while(1) (no semicolon) loops infinitely and is similar to a spinlock situation. However I do not

Answer 1

I assume that the while(1); is not associated with a do loop...

The only semi-useful implementation of while(1); I have seen is a do-nothing loop waiting for an interrupt; such as a parent process waiting for a SIGCHLD, indicating a child process has terminated. The parent's SIGCHLD handler, after all child processes have terminated, can terminate the parent thread.

It does the trick, but wastes a lot of CPU-time. Such a usage should perhaps perform some sort of sleep to relinquish the processor periodically.

Answer 2

An usage on embedded software is to implement a software reset using the watchdog:

while (1);

or equivalent but safer as it makes the intent more clear:

do { /* nothing, let's the dog bite */ } while (1);

If the watchdog is enabled and is not acknowledged after x milliseconds we know it will reset the processor so use this to implement a software reset.

Answer 3

If you dig down to assembly, (this is easier to grasp from an embedded systems point of view, or if you tried to program a bootloader)

you will realize that a while loop is just a jmp instruction ... ie

(pseudo code: starting loop address)
add ax, bx
add ax, cx
cmp ax, dx
jz  (pseudo code: another address location)
jmp (pseudo code: starting loop address)

Lets explain how this works, the processor will keep executing instructions sequentially ... no matter what. So the moment it enters this loop it will add register bx to ax and store in ax, add register cx to ax and store to ax, cmp ax, dx (this means subtract dx from ax) the jz instruction means jump to (another address location) if the zero flag is set (which is a bit in the flag register that will be set if the result of the above subtraction is zero), then jmp to starting loop address (pretty straight forward) and redo the whole thing.

The reason I bothered you with all this assembly is to show you that this would translate in C to

int A,B,C,D;
// initialize to what ever;

while(true)
{
A = A + B;
A = A + C;

if((A-D)==0)
{break;}

}

// if((X-Y)==0){break;} is the 
// cmp ax, dx
// jz  (pseudo code: another address location)

So imagine the senario in assembly if you just had a very long list of instructions that didn't end with a jmp (the while loop) to repeat some section or load a new program or do something ... Eventually the processor will reach the last instruction and then load the following instruction to find nothing (it will then freeze or triple fault or something).

That is exactly why, when you want the program to do nothing until an event is triggered, you have to use a while(1) loop, so that the processor keeps jumping in its place and not reach that empty instruction address. When the event is triggered, it jumps to the event handler instructions address, executes it, clears the interrupt and goes back to your while(1) loop just jumping in its place awaiting further interrupts. Btw the while(1) is called a superloop if you want to read more about it ... Just for whoever that is insanely itching to argue and comment negatively at this point, this is not an assembly tutorial or a lecture or anything. It's just plain English explanation that is as simple as possible, overlooking a lot of underlying details like pointers and stacks and whatnot and at some instance over simplifying things to get a point across. No one is looking for documentation accuracy over here and I know this C code won't compile like this, but this is only for Demo !!

Answer 4

One place that I have seen a while(1); is in embedded programming.

The architecture used a main thread to monitor events and worker threads to handle them. There was a hardware watchdog timer (explanation here) that would perform a soft reset of the module after a period of time. Within the main thread polling loop, it would reset this timer. If the main thread detected an unrecoverable error, a while(1); would be used to tie up the main thread, thus triggering the watchdog reset. I believe that assert failure was implemented with a while(1); as well.

Answer 5

while(1);

is actually very useful. Especially when it's a program that has some sort of passcode or so and you want to disable the use of the program for the user because, for an example, he entered the wrong passcode for 3 times. Using a while(1); would stop the program's progress and nothing would happen until the program is rebooted, mostly for security reasons.

Answer 6

This may be used to wait for Interrupt. Basically you initialize all things you need and start waiting for some thing to occur. After that some specific function is called and executed, after that it goes back to waiting state.

That thing could be button pressed, mouse click/move, data received and etc.

What is more I would say, similar stuff is really often used by UI frameworks. While it waits for signals about user actions.