Is there an alternative for sleep() in C?

Alternatives depend in what you are trying to do and what OS you are on.

If you just want to waste time, then these might help:

On most unix-type systems you'll find a 'usleep' function, which is more or less like sleep with greater resolution. Be careful with that one because it usually can not sleep for just one microsecond.

On some unix-type systems, the select system call can be used with all file descriptor sets zero in order to get a fairly accurate sub-second wait.

On windows systems, you have Sleep, which is pretty much the same, but taking a number of milliseconds.

In a multi-tasking operating system, a sleep function can sometimes be given 0 as a parameter. This generally causes the function to give up it's timeslice, but be re-scheduled immediately if no other task is ready to run.

You would not use the code you published to sleep on an embedded system. A decent compiler would entirely remove it, and even if your compiler does not remove it is suboptimal, since running the processor in a tight loop will burn power, which is an issue for embedded system. Even systems not running on battery care about power usage, since lower power usage means cheaper power supplies and cooling.

The way you normally do this is your CPU will implement some sort of IDLE or SLEEP instructions, that will cause it to temporarily stop processing commands. An external interrupt line connected to a timer circuit will wake the processor back up at regular intervals, and which point the CPU checks to see if it has been asleep for long enough, and if not it goes back to sleep.

//Pseudo code
int start = getTime();
int end = start + sleepTime;

while (getTime() < end) {
       asm("SLEEP");
}

The exact details vary from processor to processor. If you are running as a process on an OS the sleep call generally just tells the scheduler to suspend your process, and then the kernel decides whether to schedule another process or to sleep the CPU. Also, the above code will not be adequete for real time systems, which want deadline guarantees, etc. In those cases you will need to get the time in the loop, know the duration of the time interrupt so ou know if you can resleep without blowing the deadline, and potentially reprogram the timer hardware or busy wait.

One common mechanism is to use a select() that is guaranteed to time out, and specify the sleep time as the timeout:

// Sleep for 1.5 sec
struct timeval tv;
tv.tv_sec = 1;
tv.tv_usec = 500000;
select(0, NULL, NULL, NULL, &tv);

The select() is typically used to check a set of file descriptors and wait until at least one is ready to perform I/O. If none is ready (or, in this case, if no fds are specified), it will time out.

The advantage of select() over a busy loop is that it consumes very little resources while sleeping, while a busy loop monopolizes the processor as much as permitted by its priority level.

The kind of loop you describe is called a "busy wait". In real operating systems, sleeping does not cause a busy wait; it tells the operating system to not schedule the process in until the sleep period is over.