What kind of loop is for (;;)?

Question

Found in torvalds/linux-2.6.git -> kernel/mutex.c line 171

I have tried to find it on Google and such to no avail.

What does for (;;) instruct?<

Answer 1

It is an infinite loop which has no initial condition, no increment condition and no end condition. So it will iterate forever equivalent to while(1).

Answer 2

It is functionally equivilent to while(true) { }.

The reason why the for(;;) syntax is sometimes preferred comes from an older age where for(;;) actually compiled to a slightly faster machine code than while(TRUE) {}. This is because for(;;) { foo(); } will translate in the first pass of the compiler to:

lbl_while_condition:
   mov $t1, 1
   cmp $t1, 0
   jnz _exit_while
lbl_block:
   call _foo
   jmp lbl_while_condition

whereas the for(;;) would compile in the first pass to:

lbl_for_init:
   ; do nothing
lbl_for_condition:
   ; always
lbl_for_block:
   call foo;
lbl_for_iterate:
   ; no iterate
   jmp lbl_for_condition

i.e.

 lbl_for_ever:
    call foo
    jmp lbl_for_ever

Hence saving 3 instructions on every pass of the loop.

In practice however, both statements have long since been not only functionally equivalent, but also actually equivalent, since optimisations in the compiler for all builds other than debug builds will ensure that the mov, cmp and jnz are optimised away in the while(1) case, resulting in optimal code for both for(;;) and while(1).

Answer 3

It literally means "do nothing, until nothing happens and at each step, do nothing to prepare for the next". Basically, it's an infinite loop that you'll have to break somehow from within using a break, return or goto statement.

Answer 4

It loops forever (until the code inside the loop calls break or return, of course. while(1) is equivalent, I personally find it more logical to use that.

Answer 5

for(;;)

is an infinite loop just like while(1). Here no condition is given that will terminate the loop. If you are not breaking it using break statement this loop will never come to an end.

Answer 6

It is same as writing infinite loop using " for " statement but u have to use break or some other statement that can get out of this loop.