Why is there a type for process ids (pid_t), but not for file descriptors (fd)

System calls per se are a concept. They represent actions that processes can ask the kernel to perform.

Those system calls are implemented in the kernel of the UNIX-like system. This implementation (written in C, and in asm for small parts) actually performs the action in the system.

Then, processes use an interface to ask the system for the execution of the system calls. This interface is specified by POSIX. This is a set of functions of the C standard library. They are actually wrappers, they may perform some checks and then call a system-specific function in the kernel that tell it to do the actions required by the system call. And the trick is that those functions which are the interface are named the same as the system calls themselves and are often referred directly as "the system calls".

You could call the function in the kernel that perform the system call directly through the system-specific mechanism. The problem is that it makes your code absolutely not portable.

So, a system call is:

• a concept, a sequence of action performed by the kernel to offer a service to a user process
• the function of the C standard library you should use in your code to get this service from the kernel.

When you use process substitution with <(...) or >(...), bash will open a pipe to the other program on an arbitrary high file descriptor (I think it used to count up from 10, but now it counts down from 63) and pass the name as /dev/fd/N on the command line of the first program. This isn't POSIX, but other shells also support it (it's a ksh88 feature).

That's not exactly a feature of the program you're running though, it just sees /dev/fd/N and tries to open it like a regular file.

The Autoconf manual mentions some historic notes:

A few ancient systems reserved some file descriptors. By convention, file descriptor 3 was opened to /dev/tty when you logged into Eighth Edition (1985) through Tenth Edition Unix (1989). File descriptor 4 had a special use on the Stardent/Kubota Titan (circa 1990), though we don't now remember what it was. Both these systems are obsolete, so it's now safe to treat file descriptors 3 and 4 like any other file descriptors.

Also while I did a google search for this I found a program called runit that uses file descriptors 4 and 5 for some purpose related to log rotation.