> I've often made the argument that compilers should add builtins for Linux system calls. Just emit code in the right calling convention and the system call instruction, and return the result. Even high level dynamic languages could have their JIT compilers generate this code.

You can only do that, when you compile for a specific machine. In general you are compiling for some abstract notion of an OS. JITs always compile for the machine they are running on, so they don't have that problem. There is code, that is compiled directly to your syscalls specific to your machine, so that abstract code can use this. It's called libc for the C language.

> One of the most annoying things in the Linux manuals is they conflate the glibc wrappers with the actual system calls in Linux. The C library does a lot more than just wrap these things, they dynamically choose the best variants and even implement cancellation/interruption mechanisms. Separating the Linux behavior from libc behavior can be difficult, and in my experience requires reading kernel source code.

In my experience there are often detailed explanation in the notes section. From readv(2):

  NOTES
       POSIX.1  allows  an  implementation  to  place a limit on the number of
       items that can be passed in iov.  An implementation can  advertise  its
       limit  by  defining IOV_MAX in <limits.h> or at run time via the return
       value from sysconf(_SC_IOV_MAX).  On modern Linux systems, the limit is
       1024.  Back in Linux 2.0 days, this limit was 16.

   C library/kernel differences
       The  raw  preadv() and pwritev() system calls have call signatures that
       differ slightly from that of the corresponding GNU  C  library  wrapper
       functions  shown  in  the SYNOPSIS.  The final argument, offset, is un‐
       packed by the wrapper functions into two arguments in the system calls:

           unsigned long pos_l, unsigned long pos

       These arguments contain, respectively, the low order and high order  32
       bits of offset.

   Historical C library/kernel differences
       To  deal  with  the  fact  that IOV_MAX was so low on early versions of
       Linux, the glibc wrapper functions for readv() and  writev()  did  some
       extra  work  if  they  detected  that the underlying kernel system call
       failed because this limit was exceeded.  In the case  of  readv(),  the
       wrapper  function  allocated a temporary buffer large enough for all of
       the items specified by iov, passed that buffer in a  call  to  read(2),
       copied  data from the buffer to the locations specified by the iov_base
       fields of the elements of iov, and then freed the buffer.  The  wrapper
       function  for  writev()  performed the analogous task using a temporary
       buffer and a call to write(2).

       The need for this extra effort in the glibc wrapper functions went away
       with Linux 2.2 and later.  However, glibc continued to provide this be‐
       havior until version 2.10.  Starting with glibc version 2.9, the  wrap‐
       per  functions  provide  this behavior only if the library detects that
       the system is running a Linux kernel older than version 2.6.18 (an  ar‐
       bitrarily  selected  kernel  version).  And since glibc 2.20 (which re‐
       quires a minimum Linux kernel version of  2.6.32),  the  glibc  wrapper
       functions always just directly invoke the system calls.