c - Why does open make my file descriptor 0?

Question

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) == -1) 
{
    // open failed
}

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

Answer 1

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.