Muchen He

Week 9

Updated 2022-03-15

Original slides and lecture by Mieszko Lis.

Files

We have used them. They’re everywhere. But what are they?

Definition

On a very simple defiition: it’s a collection of data that we can open, view, write, and modify. However for unix, the defition is a lot more abstract, that:

This abstraction boils down to: a file is a sequence of bytes and that we the only operation we have on the file is reading the next character (if there is one).

So what isn’t a file (in unix)? Files might be on disk, but it could live on RAM or nowhere at all. Files might have a path and names, but they might not.

For example, when we “pipe” the output of one program to another (e.g. grep), both programs treat the input/output as a file, but this “file” doesn’t exist anywhere.

cat /etc/passwd | grep hello

At an OS level – outside of the file abstraction – files are structured and formatted data since they could take form of different type of files (e.g. indicated by extensions mp4, pdf, etc.). They’re outside of the file abstraction and the OS simply just treats it differently by application. Key is: only the OS-level application (Finder, Explorer, etc.) knows the difference between files.

Filename extensions has no effect on the files themselves, but is just a convention for human or OS to recognize the files.

At a low level, the file abstraction is also encoding-agnostic. Only the program reading such file should care if the file is encoded in binary, ASCII, or Unicode, etc.

Example

cat 1337.txt
ph33r m3 n00bz

Example: directories are special files

ls /tmp

Example: processes and their executables in the memory space (and only exist in memory) can be examined as a file

xxd -p /proc/self/exe

Example: use a hardware random number generator and read it like a file

xxd -p -l 10 /dev/random

Example: we can write / dump something to a “file” that act as void

echo "Something important" > /dev/null

Example: the command tty tells us what terminal we are currently looing at. When we write to a terminal, it will appear on the screen.

For example, suppose we found our “device file” for our terminal:

tty
/dev/pts/3

Then writing to that “file” prints to the screen

echo "hello" > /dev/pts/3
hello

Example: Disks and partitions are special files. We can read and write to it (but it will obviously brick your computer lol).

df /dev/sda1

The above command shows free space on partition sda1.

Example: we can also talk to the kernel; here we can read about list of CPU information

cat /proc/cpuinfo

The reverse is also true: we can write to the kernel.

echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all

This command sends a ICMP packet and ignore it.

Example: PIPE (as seen above)

cat /etc/passwd | grep hello

File Operations & Interfaces

syscall Interface

The syscall interface is an OS-level specifical. This means it’s not portable since it depends on which OS you’re on. There exists standards like POSIX that enable interchangeability.

Files have file descriptors – a set of small bytes that identify open files.

Possible operations are:

The OS only cares about bytes and not even semantically – it doesn’t know about line feeds, escape characters, etc. This makes it safe for syscall handlers.

Every file interaction has to interact with kernel. If we have a higher-level interface (e.g. a web browser that loads a file), under the hood it would still have to ask the kernel.

libc Interface

This is a step higher than syscall interface, and is implemented on top of the syscall interface. This is nice because we can write code using libc and it’s portable across systems.

Instead of having file descriptors, instead of have file pointers and streams to identify open files.

As it implements on top of syscall, we have functions that facilitate similar tasks:

FILE *fopen(const char *filename, const char *mode)
int fclose(FILE *fp);
int fflush(FILE *fp);
/* more ... */

It’s generally unsafe for syscall handlers. And the changes are buffered in a separate buffer structure. The buffer is only flushed to the actual file when the buffer is full (or when flush is called).

Q: why do we buffer things? See File Buffering

libcsyscall interaction

insert diagram

Suppose our code uses the libc library, then when we use fgetc() function to read from file. The function actually performs a syscall to the kernel to read the file into the buffer.

File Buffering

Analogy: suppose an delivery airplane can hold 10,000 packages, it takes an hour each to prepare the plane for loading and deloading. But once it is prepared, it only takes 1 second to load/unload each package.

So back to our file example, having a buffer allows us to amortize the latency to write to a file. We can fill a buffer and write once (in a flush call), instead of having our disk (e.g. spin up/down for a hard drive – which takes a long time) for every write call.

Buffer is not exclusive to files, it’s actually used everywhere – in the kernel, on the disks.

Buffers are typically transparent in theory, as in, the user/applications doesn’t know that buffers exist.

Problem: It gets complicated if we want guarantee about when I/O actually happens. What if we try to write data to disk, but suddenly the disk loses power out while data is still in the buffer? Furthermore if the disk fails, the write’s error is delayed while data is sitting in the buffer.

Reading A File

If we use the abstraction where file is a sequence of bytes and we can only read the next bytes. Then we must track the position of where we are currently reading/writing:

insert diagram

The position act as an index for us to access bytes from a file, but doesn’t allow random access (it’s more like a cassette tape where we can only seek back and forward).

Note: not all devices are seekable (for example, terminal devices or peripherals like keyboard cannot be seeked).

Example:

Consider the following code that uses syscall in C:

uint8_t buf[10];
int fd = open("/dev/random", O_RDONLY);
ssize_t n = read(fd, buf, 10);
close(fd);

Where:

Writing To File

Note: draft from this point on… to be finished…

Example:

/* todo: insert code */

Where

Error Reporting

We saw that read/write operations return number of bytes actually read/written.

How do we know what kind of error when an error occurs? We can check errno global variable and compare it against a set of predefined values: e.g. ENOENT, EACCES, EINTR, etc.