for(;1;) 在 C 语言中是什么意思？

Question

What does for(;1;) mean in C?

Is it the same as for(;;) in the sense of an infinite loop?

I know that the position where 1 is typed is the condition part of the variable, but in this for loop, I'm not convinced that it is indeed an infinite loop.

Answer 1

According to the C Standard (6.8.5.3 The for statement)

2 Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.

So in fact you yourself instead of the compiler replaced the omitted expression-2 by nonzero constant 1.

So these two statements

for(;1;) { /*...*/ }

and

for(;;) { /*...*/ }

are fully equivalent.

In my opinion it is much better to write

while ( 1 ) { /*...*/ }

instead of the shown above for statements.

Answer 2

Be convinced. For loop works until the condition-part of the expression becomes false. And in C 1 means " true ", which - obviously - will never become false.

Answer 3

Basically, Your both expressions for(;1;) and for(;;) will give you infinite loop.

MSDN says

All of the expressions of the for statement are optional; for example, the following statement is used to write an infinite loop:

for (; ; )
{
    // ...
}

So no criteria in condition of loop will give you a infinite loop.

On you other question for(;1;): you are not convinced that it will give you infinite loop.

It has to a infinite loop, Because result of condition statement is not equal to 0.

Following are some examples:

 for(;1;)   //Infinite loop
 for(;0;)       //Loop will not execute at all
 for(;-1;)       //Infinite loop.