Thread: Checking if a file descriptor is valid

You might need to clarify that question a bit...

Functions like open() will return a certain value (-1) for failure. Otherwise, the easiest way is that it's up to you to keep track of file descriptors.

Alternatively, if you're in Linux, you could read your process's dir in /proc, but again, this all depends - what are you wanting to do?

there are two ways to achieve this:

1. check for the return value when returned from open() system call.

Code:

int fdesc;

if(fdesc = open(//arguments) == -1)
{
    perror("open");
}

2. the other way is ioctl() system calls and check the inode file structure.

Hope i answered your query...

To just see if a file descriptor is valid you could use this:

Code:

int is_valid_fd(int fd)
{
    return fcntl(fd, F_GETFL) != -1 || errno != EBADF;
}

fcntl(GETFL) is probably the cheapest and least likely to fail operation you can perform on a file descriptor, so I've chosen it. In particular, the specification suggests that it cannot be interrupted by signals, nor is it affected by any sort of lock held anywhere.