Difference between slow system calls and fast system calls

Summary: you're correct that receiving a signal is not transparent, neither in case i (interrupted without having read anything) nor in case ii (interrupted after a partial read). To do otherwise in case i would require making fundamental changes both to the architecture of the operating system and the architecture of applications.

The OS implementation view

Consider what happens if a system call is interrupted by a signal. The signal handler will execute user-mode code. But the syscall handler is kernel code and does not trust any user-mode code. So let's explore the choices for the syscall handler:

• Terminate the system call; report how much was done to the user code. It's up to the application code to restart the system call in some way, if desired. That's how unix works.
• Save the state of the system call, and allow the user code to resume the call. This is problematic for several reasons:
  • While the user code is running, something could happen to invalidate the saved state. For example, if reading from a file, the file might be truncated. So the kernel code would need a lot of logic to handle these cases.
  • The saved state can't be allowed to keep any lock, because there's no guarantee that the user code will ever resume the syscall, and then the lock would be held forever.
  • The kernel must expose new interfaces to resume or cancel ongoing syscalls, in addition to the normal interface to start a syscall. This is a lot of complication for a rare case.
  • The saved state would need to use resources (memory, at least); those resources would need to be allocated and held by the kernel but be counted against the process's allotment. This isn't insurmountable, but it is a complication.
    • Note that the signal handler might make system calls that themselves get interrupted; so you can't just have a static resource allotment that covers all possible syscalls.
    • And what if the resources cannot be allocated? Then the syscall would have to fail anyway. Which means the application would need to have code to handle this case, so this design would not simplify the application code.
• Remain in progress (but suspended), create a new thread for the signal handler. This, again, is problematic:
  • Early unix implementations had a single thread per process.
  • The signal handler would risk overstepping on the syscall's shoes. This is an issue anyway, but in the current unix design, it's contained.
  • Resources would need to be allocated for the new thread; see above.

The main difference with an interrupt is that the interrupt code is trusted, and highly constrained. It's usually not allowed to allocate resources, or run forever, or take locks and not release them, or do any other kind of nasty things; since the interrupt handler is written by the OS implementer himself, he knows that it won't do anything bad. On the other hand, application code can do anything.

The application design view

When an application is interrupted in the middle of a system call, should the syscall continue to completion? Not always. For example, consider a program like a shell that's reading a line from the terminal, and the user presses Ctrl+C, triggering SIGINT. The read must not complete, that's what the signal is all about. Note that this example shows that the read syscall must be interruptible even if no byte has been read yet.

So there must be a way for the application to tell the kernel to cancel the system call. Under the unix design, that happens automatically: the signal makes the syscall return. Other designs would require a way for the application to resume or cancel the syscall at its leasure.

The read system call is the way it is because it's the primitive that makes sense, given the general design of the operating system. What it means is, roughly, “read as much as you can, up to a limit (the buffer size), but stop if something else happens”. To actually read a full buffer involves running read in a loop until as many bytes as possible have been read; this is a higher-level function, fread(3). Unlike read(2) which is a system call, fread is a library function, implemented in user space on top of read. It's suitable for an application that reads for a file or dies trying; it's not suitable for a command line interpreter or for a networked program that must throttle connections cleanly, nor for a networked program that has concurrent connections and doesn't use threads.

The example of read in a loop is provided in Robert Love's Linux System Programming:

ssize_t ret;
while (len != 0 && (ret = read (fd, buf, len)) != 0) {
  if (ret == -1) {
    if (errno == EINTR)
      continue;
    perror ("read");
    break;
  }
  len -= ret;
  buf += ret;
}

It takes care of case i and case ii and few more.

Conceptually, a library function is part of your process.

At run-time, your executable code and the code of any libraries (such as libc.so) it depends on, get linked into a single process. So, when you call a function in such a library, it executes as part of your process, with the same resources and privileges. It's the same idea as calling a function you wrote yourself (with possible exceptions like PLT and/or trampoline functions, which you can look up if you care).

Conceptually, a system call is a special interface used to make a call from your code (which is generally unprivileged) to the kernel (which has the right to escalate privileges as necessary).

For example, see the Linux man brk. When a C program calls malloc to allocate memory, it is calling a library function in glibc.

If there is already enough space for the allocation inside the process, it can do any necessary heap management and return the memory to the caller.

If not, glibc needs to request more memory from the kernel: it (probably) calls the brk glibc function, which in turn calls the brk syscall. Only once control has passed to the kernel, via the syscall, can the global virtual memory state be modified to reserve more memory, and map it into your process' address space.