Why a syntax like "a=b--- // a newline, line ends without ;" does not produce syntax error?

Question

What is the output of this program and how?

#include<stdio.h>

int main(){
     int a=0,b=10;
     a=b---
     printf("the value of b=%d",b);
     printf("the value of a=%d",a);
     return 0;
}

Have you tried it? What output did you get? What output did you expect? — Some programmer dude, Dec 07 '16 at 07:05
Also, you might want to read [Why doesn't the compiler report a missing semicolon?](http://stackoverflow.com/q/40135392/440558). As well as learning what [`printf`](http://en.cppreference.com/w/c/io/fprintf) *returns*. — Some programmer dude, Dec 07 '16 at 07:06
Discussing this kind of nonsense programs is rather pointless. — Jabberwocky, Dec 07 '16 at 07:09
Related stack overflow question : http://stackoverflow.com/questions/7485088/what-does-the-operation-c-ab-mean — msc, Dec 07 '16 at 07:24
There is probably never a reason why you would want to do arithmetic with the result of printf though... — Lundin, Dec 07 '16 at 07:27
@MichaelWalz Agreed. The C tag seems to get these types of questions quite often. — byxor, Dec 07 '16 at 09:05

score 5 · Answer 1 · edited May 23 '17 at 11:45

5

In your code, writing

 a=b---
 printf("the value of b=%d",b);

is same as

a = b---printf("the value of b=%d",b);

which is an expression with undefined behavior, as an attempt is made to change and use the value of variable b in two subexpressions without a sequence point in between. Hence, the output of this code cannot be justified.

Without the above problem, in general, the syntax is similar to

x = (y--) -  (<return value of printf() call>)

which is a perfectly valid syntax.

Note:

Why a = b---<something> is treated as a = (b--) - <something> and not a = b - -- <something> is due to the maximal munch rule.

edited May 23 '17 at 11:45

Community

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1

answered Dec 07 '16 at 07:09

Sourav Ghosh

133,132
16
183
261

but in the above example b's value is 10 so how 'a' got -6 – Shah Junaid Dec 07 '16 at 07:25
@ShahJunaid I get _the value of b=10the value of a=-7_ – BLUEPIXY Dec 08 '16 at 01:56

score 1 · Answer 2 · answered Dec 07 '16 at 09:27

Strictly speaking, as others have said since this is undefined behaviour, the result could be anything . And while they're right, it doesn't explain why you get this specific answer in this specific compiler.

In practice b will usually be printed as 9, or 10 depending on whether the compiler does the decrememnt first or the printf first.

printf returns the number of characters printed. Which I think is 16 or 17 in this case.

So this is like writing a = (b--) - 16; b is 10 before the decrement and it's a post decrement so this is the value used.

Why a syntax like "a=b--- // a newline, line ends without ;" does not produce syntax error?

2 Answers2