llvalue required as left operand of assignment

Question

I wan't to open a file in append mode or erase all it's contents based on $flag

static void do_redirect(int filedes, const char *filename, int flag){
int rc;
int fd;

flag == 1 ? fd = open(filename, O_CREAT|O_RDWR|O_TRUNC, 0644) : 
            fd = open(filename, O_CREAT| O_RDWR, 0644);

}

Why do I get llvalue required as left operand of assignment?

In this case, `if..else` would be much clearer than `?:`, and prevent this bug. — ugoren, Mar 15 '13 at 16:57

score 2 · Accepted Answer · answered Mar 15 '13 at 16:18

2

Because of precedence. The expression is parenthesised implicitly

(flag == 1 ? fd = open(filename, O_CREAT|O_RDWR|O_TRUNC, 0644) : fd) = open(filename, O_CREAT| O_RDWR, 0644);

and the value of the conditional expression is not an lvalue.

Write

flag == 1 ? (fd = open(filename, O_CREAT|O_RDWR|O_TRUNC, 0644)) : 
            (fd = open(filename, O_CREAT| O_RDWR, 0644));

or move the conditional expression inside the open call.

answered Mar 15 '13 at 16:18

Daniel Fischer

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also shouldn't fd be FILE* ? – Barath Ravikumar Mar 15 '13 at 16:21
@BarathBushan Shouldn't you be reading the documentation of `open()` and/or `FILE *`? – Mar 15 '13 at 16:23
@BarathBushan In short, you mistook `open` for `fopen`. `fopen` returns a `FILE*`, and `open` a file descriptor. – Daniel Fischer Mar 15 '13 at 16:25

score 2 · Answer 2 · answered Mar 15 '13 at 16:18

2

It's a question of precedence.

I would suggest you rewrite it as such:

fd = open(filename, O_CREATE | O_RDWR | (flag == 1 ? O_TRUNC : 0), 0644);

answered Mar 15 '13 at 16:18

Some programmer dude

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score 1 · Answer 3 · edited May 23 '17 at 12:21

Why you are getting lvalue error?

You conditional expression is something like:

flag == 1 ? fp = 1 : fp = 2;

that is parsed in C like:

(flag == 1 ? fp = 1 : fp) = 2;  

  ^ expression            = value

and you can't assign a value to an expression that that reason for lvalue error.

The correct answer is given by @Daniel Fischer by adding parenthesis you can rectify your conditional expression(actually overriding precedence).

But interesting this you expression is correct in C++ language!
Read Charles Bailey's answer here: Conditional operator differences between C and C++. You will also find an elaborate answer there why you are getting lvalue error.
(my answer is just a reference to there)

Although @Daniel Fischer and @Joachim Pileborg answer you two tricks you can also achieve same like this:

fd = flag == 1 ? -2 : 3 ;

_{Yes but this is possible only in your case because you are using fd in both true/false expression}

llvalue required as left operand of assignment

3 Answers3