The OS kernel needs to provide user space processes with access to kernel-managed resources such as files, devices, network sockets, processes, threads, etc. In order to do so, there needs to be some way of identifying these individual resources to the kernel. And then the question is, should every type of resource have a distinct type of identifier? Or should we have a single type of identifier which could refer to an instance of any one of those types of resources?
The pure "everything is a file descriptor" ideology (or its Windows NT equivalent, "everything is a handle") says we should have a single type of identifier, the file descriptor (or handle), which can represent resources of any type for which the process can invoke kernel services – processes, threads, files, network sockets, etc.
Standardised POSIX does a poor job of living up to this ideology, since APIs for managing processes (kill, waitpid, fork, etc) take and return PIDs, not file descriptors. Since a process is not a file descriptor, I can't select()/poll() on it. Using PIDs is also prone to race conditions, whereas file descriptors are less prone to this problem (although not completely immune from it.)
The process descriptor functions such as pdfork provided by FreeBSD do a much better job of living up to "everything is a file descriptor" ideology than pure standardised POSIX does.
> I don't believe everything can be abstracted as a file considering that ioctl() breaks that abstraction. For example, what is it to "read from /proc/{pid for Chrome}"?
Why must every file support read() and write()? Some device files or other special files might only support ioctl(), and maybe also select() and poll(), and I see nothing in principle is wrong with that.
The OS kernel needs to provide user space processes with access to kernel-managed resources such as files, devices, network sockets, processes, threads, etc. In order to do so, there needs to be some way of identifying these individual resources to the kernel. And then the question is, should every type of resource have a distinct type of identifier? Or should we have a single type of identifier which could refer to an instance of any one of those types of resources?
The pure "everything is a file descriptor" ideology (or its Windows NT equivalent, "everything is a handle") says we should have a single type of identifier, the file descriptor (or handle), which can represent resources of any type for which the process can invoke kernel services – processes, threads, files, network sockets, etc.
Standardised POSIX does a poor job of living up to this ideology, since APIs for managing processes (kill, waitpid, fork, etc) take and return PIDs, not file descriptors. Since a process is not a file descriptor, I can't select()/poll() on it. Using PIDs is also prone to race conditions, whereas file descriptors are less prone to this problem (although not completely immune from it.)
The process descriptor functions such as pdfork provided by FreeBSD do a much better job of living up to "everything is a file descriptor" ideology than pure standardised POSIX does.
> I don't believe everything can be abstracted as a file considering that ioctl() breaks that abstraction. For example, what is it to "read from /proc/{pid for Chrome}"?
Why must every file support read() and write()? Some device files or other special files might only support ioctl(), and maybe also select() and poll(), and I see nothing in principle is wrong with that.