CODEWITHSUNDEEP
cfor-loopkernelmutex
user242294
user242294

Reputation: 318

What kind of loop is for (;;)?

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?

Upvotes: 14

Views: 1268

Answers (12)

Pawan Sharma
Pawan Sharma

Reputation: 3344

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.

Upvotes: 1

SecurityMatt
SecurityMatt

Reputation: 6743

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).

Upvotes: 1

deddihp
deddihp

Reputation: 663

That was obviously an infinite loop condition.

Upvotes: 0

Anshul garg
Anshul garg

Reputation: 233

It's an infinite loop that you'll have to break somehow from within using a break, return or goto statement. or either some interrupt happens otherwise this loop will run infinitely and executes ;(null statement) every time

Upvotes: 0

JCasso
JCasso

Reputation: 5523

I means:

#define EVER ;;

for(EVER)
{
     // do something
}

Warning: Using this in your code is highly discouraged.

Upvotes: 1

wheaties
wheaties

Reputation: 35980

It's equivalent to while( true )

Edit: Since there's been some debate sparked by my answer (good debate, mind you) it should be clarified that this is not entirely accurate for C programs not written to C99 and beyond wherein stdbool.h has set the value of true = 1.

Upvotes: 4

Abhijit K Rao
Abhijit K Rao

Reputation: 1097

The for(;;) is an infinite loop condition, similar to while(1) as most have already mentioned. You would more often see this, in kernel mutex codes, or mutex eg problem such as dining philosophers. Until the mutex variable is set to a particular value, such that a second process gets access to the resource, the second process keeps on looping, also known as busy wait. Access to a resource can be disk access, for which 2 process are competing to gain access using a mutex such that at a time only one process has the access to the resource.

Upvotes: 20

Swapnil
Swapnil

Reputation: 181

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.

Upvotes: 2

M. Atif Riaz
M. Atif Riaz

Reputation: 502

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).

Upvotes: 9

GuruKulki
GuruKulki

Reputation: 26428

it is an infinite for loop.

Upvotes: 2

Wim
Wim

Reputation: 11252

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.

Upvotes: 5

Blindy
Blindy

Reputation: 67447

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.

Upvotes: 46

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