What is this syntax?

Answer 1

This syntax is really useful when you want to have say 2 iteration variables in a for loop

for ( i = 0, j = 0; i < 10 && j < 10; i++ ) {
  ..
}

Answer 2

It's not a "type of assignment". The comma operator binds very loosely, looser than assignment. So you've written the equivalent of:

((((i = 1), 2), 3), 4), 5;

Integer literals in void contexts are useless (except maybe for avoiding warnings in macros that do nothing in certain cases, like assert), so no, there's no use for exactly this syntax - the need for an expression which sets i to 1 and evaluates to 5 is pretty limited, and even if you found a case for that, the 2,3,4 are redundant.

More useful could be i = 1, code_that_actually_does_something;. The most frequent use of the comma operator is to sneak in multiple side-effects in a context where you're not allowed multiple statements, such as in "if" and "while" conditions, or macros that have to evaluate as expressions.

Answer 3

Application: the Obfuscated C Contest!

Answer 4

It's the comma operator, C's lowest precedence operator. According to C's precedence rules, that line parses as this:

(i = 1), (2), (3), (4), (5);

This could be "useful" if you wanted to do something else on that line:

i = 2, j = 3, k++;

Could save you from using brackets for an if() statement (and could also induce headaches later) or allow you to have multiple expressions in a for() loop's control flow (which is actually a pretty legitimate use of the comma operator).

Answer 5

This is the comma operator, which allows you to combine multiple expressions into one.

The compiler parses it as (i = 1), 2, 3, 4, 5, because = has a higher priority than the comma operator.

Except in for loops, it is not generally useful and its use is frowned upon.

Answer 6

In an assignment, I can't think of where this particular example would be useful.

You can, however, do

x = 1, y = 2, z = 3;

And they'll be treated as a single statement, and evaluated from left-to-right (according to C89).

Answer 7

Another common use for the comma operator is in while loop conditions:

while (c = getchar(), c != EOF && c != '\n')
{

Answer 8

As the others already pointed out: This statement assigns 1 to i, then evaluates 2, then 3, then 4 and then 5. The overall value of the statement is then 5 (the last evaluated expression).

Answer 9

This is useful when you want to create a macro that does several things and returns a value like a function:

#define STEAL_MAGIC_HAT(thing1, thing2, cat) \
(smack(thing1), punch(thing2),get_hat(cat))

extern thing_t g_thing1, g_thing2;
extern cat_t g_cat;
...

    hat_t hat = STEAL_MAGIC_HAT(g_thing1, g_thing2, g_cat);
    don(hat);
...