The functions of the OS is to manage system resources. What are system resources? These are the hardware components of the computer that support the execution of a programmer or organization of information. Typically, we describe the set of system resources coordinated by the OS as:

• Device Management : Hardware devices, such as keyboard, monitors, printers, hard drives, etc., are resources managed by the computer. When a programmer wishes to interact with these devices, a unified interface provided by the OS is used.
• Process Management : The invocation and execution of a program, a process, is managed by the OS, including managing its current state, running or stopped, as well as the loading of code.
• Memory Management : The access to physical and virtual memory is controlled by the OS, and a programs memory layout and current allocations is carefully managed.
• File System Management : The OS is also responsible for ensuring that programs can read and write from the filesystem, but also that programs don't corrupt the file sysystem or access files/directory that they do not have permission to.

So far in this class, we've written programs (either in Bash or C) that have required access to those resources. For example, we've read user-input through keyboard (device management); we've invoked and executed programs through bash (process management); we've allocated and deallocated memory in C using malloc() and calloc() (memory management); and, we've read, written, and created files in both C and Bash (file system management).

In each of those cases, while it is nice to think that we, as the programmer, have done these things, in fact, the operating system has performed these actions on our behalf in a supervisory role. This is mostly for our protection and convenience. Would you really want to have to read directly from the keyboard driver in order to get input from the user? Would you want to write to the display driver to print information back? Maybe you do, if you're nuts about computing, but most of us don't. And further, if you do want to perform these low level actions yourself, it's really easy to mess it up, at which point, your computer may be broken forever. For example, if you had to manipulate the filesystem directly, and you made a mistake —oops, you just lost all your files!

The kernel of the OS is a program that is trusted to perform all the protected system resource actions. The kernel is trusted software and executes in supervisory mode, and all the basic OS functionality is implemented from with the kernel software. We describe the domain of the kernel as kernel-space. Actions that can be performed without privilege, that is, do not require the kernel, are described as part of the user domain, or user-space.

The distinction between these two domains is important. For example, adding two numbers together, a process completed by an add instruction on the processor, is unprivileged and is performed in user-space. Similarly, the action of iterating through an array and reading and writing data already allocated in memory is also unprivileged and performed in user-space. But, the allocation of new memory, by adjusting the break point, for example, is a privileged process, and must be completed by the kernel.

The invocation of the kernel to perform a context switch occurs through a trap. A trap is a special instruction to the processor that an operation is needed from the kernel. The processor interrupts the current execution of the program, saves the state, and invokes the kernel with the trap information. In the running example, this will be a trap for the kernel function sys_sbrk() which was invoked via the system call sbrk().

The kernel will then fulfill the request via the kernel function. Once that function returns, the kernel is context switched out, the user process is context switched in, and execution continues as before.

So far in this class we haven't been using the system call interface directly, but rather we have used the C standard library to interface for us. This is going to change as we explore the Unix system, and it is important that you can identify the differences between library functions and system calls.

The easiest way to do this is via the manuals. The man pages are divided into sections to better organize the plethora of manuals available. There is a total of 8 sections, and below are the relevant ones.

• Section 1: General commands, such as those found in the bash environment
• Section 2: System calls, such as sbrk()
• Section 3: Library functions, such as malloc()
• Section 8: System Administration, … get to that later

For example, if we type, man malloc, and inspect the header of the manual, we can learn a lot of information:

MALLOC(3)           Linux Programmer's Manual                    MALLOC(3)

NAME
       malloc, free, calloc, realloc - Allocate and free dynamic memory

SYNOPSIS
       #include <stdlib.h>

       void *malloc(size_t size);
       void free(void *ptr);

First we can see that malloc() is in section 3 of the manual via MALLOC(3) header. Also, from the synopsis, we see that it is a part of the C standard library via the #include <stdlib.h>. As a comparison, let's look at the manual for sbrk().

BRK(2)              Linux Programmer's Manual                        BRK(2)

NAME
       brk, sbrk - change data segment size

SYNOPSIS
       #include <unistd.h>

       int brk(void *addr);

       void *sbrk(intptr_t increment);

The sbrk() command, with brk(), is in section 2 of the manual via the BRK(2) header, and it also in the unix standard library, which we know from #include <unistd.h>.

One problem you may encounter is that there are manuals that have the same name. For example, there is the read command for bash, which is a general command in section 1 of the manual, and there is also the system call read(), which is in section 2 of the manual. The preference for the man command is to always retrieve lower numbered manuals. For example,

man 2 read

will display the bash read command and not the system call read(). To access the system call manual for read(), use

#> man 2 read
READ(2)                  Linux Programmer's Manual                     READ(2)

NAME
       read - read from a file descriptor

SYNOPSIS
       #include <unistd.h>

       ssize_t read(int fd, void *buf, size_t count);