Why does open make my file descriptor 0?

I'm working on a program that is using a pipe and forks and need to change the write end to an output file. But when I open a file the file descriptor is 0 which is usually stdin but which I think is the cause of some of my problems. Here is my code

if (outputfd = open("file", O_RDWR | O_CREAT | O_TRUNC) == -开发者_如何学JAVA1) 
{
    // open failed
}

Can someone let me know why it is 0? Or how to fix it?

outputfd in your line of code is not the output file descriptor but rather is equal to FALSE (0). This is because the file descriptor returned by open is not == -1

It should read:

outputfd = open("file", O_RDWR | O_CREAT | O_TRUNC);
if (outputfd < 0)
{
   // error handling code
}

Or it should read:

if ( ( outputfd = open("file", O_RDWR | O_CREAT | O_TRUNC) ) == -1)
{
    // error handling code
}

Note that this required 3 extra parentheses - one right parenthesis and two left.

It's because you're comparing it to -1.

outputfd doesn't get the result of open. It gets the result of the check for -1.

Just illustrating doron's answer:

>> outputfd = open("file", O_RDWR | O_CREAT | O_TRUNC) == -1)

Let's simplify: first remove errors and add extra punctutation to make it look like an actual stement

   outputfd = open("file", O_RDWR | O_CREAT | O_TRUNC) == -1;

Now, replace function parameters with a placeholder

   outputfd = open(<PLACEHOLDER>) == -1;

Add parenthesis

   outputfd = (open(<PLACEHOLDER>) == -1);

When is the result of open() -1? When the operation failed. So let's assume the operation didn't fail and replace the open with a positive number

   outputfd = (<POSITIVENUMBER> == -1);

No positive number can ever be equal to -1 (barring conversion problems) so the equality test is always false ... and false, in C is, by definition, 0

   outputfd = 0;

In C, relational operators have higher precedence than assignment operators.