For loop Syntax in C [duplicate]

This question already has an answer here:

• What is the full “for” loop syntax in C (and others in case they are compatible)? 7 answers

I have a strange syntax of for loop here :

for(--index ; index>=0 ; --index)     // (--index) instead of (forInit) 
{
    //code
}

I know for syntax is like this :

for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement

How do we have --index instead ForIniti in the loop. I thought that we can only initialise a variable at that place or can leave it blank like this :

for( ; index>=0 ; index--)
    {
      //code
    }

I haven't been introduced to this syntax before. Please explain why this is syntactically correct.