How to call a user-defined syscall as a function

Question

Lets say I have a user-defined syscall: foo (with code number 500).

To call it, I simply write in a C file:

syscall(SYS_code, args);

How can I call it using just foo(args)?

Answer 1

You cannot. Not unless you first convince the kernel developers that your syscall is worth being added, then it gets added, and then userspace libraries such as the standard C library (glibc on most Linux distributions) decide to implement a wrapper for it like they do for most of the syscalls.

In other words: since the above is impossible, all you can do is define the wrapper function yourself in your own program.

#define SYS_foo 500

long foo(int a, char *b) {
    return syscall(SYS_foo, a, b);
}