# UNIX Abstraction: Everything is a File #

A file is a named collection of data in a file system. In the POSIX (Portable Operating System Interface for UNIX) standard, every file includes data (a sequence of bytes) and metadata (such as size, modification time, owner, security, and access control).

Files are organized into directories, which are folders containing files and other directories. Directories employ hierarchical naming (/path/to/file.txt) to ensure that ever file has a unique identifier.

Every process has a current working directory (CWD), which can be set using int chdir(const char *path).

UNIX I/O has several key ideas:

• Uniformity: all IO operations use the same syscalls (open, close, read, write).
• Open before use: A device or channel must be opened and checked for permissions before any IO can commence.
• Byte Oriented: Everything can be represented as a series of bytes in an array.
• Kernel-buffered reads and writes: All data, whether incoming or outgoing, is stored in a kernel buffer and returned on request or when devices become available. This frees the processor to perform other tasks while IO is occurring but not ready.
• Kernel buffering is typically done in blocks to enable efficient caching; as a consequence, int fflush(FILE *stream) must be called to push all buffered contents out of the file and complete the file operation.
• Explicit Close: Applications which are done accessing a file or device must call close to signal to the OS that the file is available.

# High level IO: Streams #

High-level C libraries operate on streams (unformatted sequences of bytes with a position). When using high-level IO operations, the libraries automatically provide caching and write buffering to improve performance.

To read or write using the high-level file API, we must first call FILE* fopen(const char *filename, const char *mode) to open the file.

The mode can be set to read (r), write(w), append(a), readwrite existing (r+), readwrite existing or create non-existing (w+), readwrite as append (a+), or binary versions of all stated (such as rb or ab+).

After opening the file, size_t fread(void *restrict ptr, size_t size, size_t nmemb, FILE *restrict stream) can be called to read contents from the file into the buffer at *ptr. This function reads nmemb items of data, each size bytes long, from stream.

The fwrite function can also be used (it has the same signature as fread) to write items from the buffer at ptr into stream.

Here’s an example function that copies the file contents from the path *src into the file at path *dest:

void copy(const char *src, const char *dest) {
char buffer [100];
FILE* write_file = fopen(dest, "w");
fwrite(buffer, 1, buf_size, write_file);
fclose(write_file);
}


### Stream Operations #

Streams can be accessed in the same manner as files. Some API Standard Streams include stdin (input stream), stdout (output stream), and stderr (error stream).

The C libraries include several operations to work with streams:

• int fputc(int c, FILE *stream) puts a character c at the current position in *stream.
• fputs writes a string at the current position.
• fgetc and fgets reads the next character/string at the current position.

# Low Level IO: File Descriptors #

Rather than working with streams, low level IO functions interface with the syscalls directly, and handle file identification using file descriptors. Every opened file is assigned an integer index in the kernel’s file descriptor table, which can then be used to read and write to and from that file.

Unlike high level IO, where everything will be written at once from a buffer, operations on low-level file descriptors are visible immediately. As such, mixing high and low level IO can be problematic because it’s very difficult to predict which sections of the file will be accessed at any particular point in time.

By default, every program starts with the three standard streams stdin, stdout, and stderr, which are assigned the file descriptors 0, 1, and 2 respectively. Every future file opened will be assigned some number greater than 2. These numbers can be accessed with the constants STDIN_FILENO, STDOUT_FILENO, and STDERR_FILENO which require an #include <unistd.h>.

To open a file, use int open(const char* filename, int flags, mode_t mode).

• Flags handle access modes (Rd, Wr, etc), as well as operating modes (Create, append, etc.).
• The mode is a bit vector of permission bits, bitwise OR’d (|) together.
• For a full list of flags and modes, see the man page ( https://man7.org/linux/man-pages/man2/openat.2.html).
• There is also the int creat(const char* filename, mode_t mode) function, which is equivalent to calling open with the flags O_CREAT|O_WRONLY|O_TRUNC (create the file, write only, replace existing file completely).

To read and write, use ssize_t read(int filedes, void *buffer, size_t size) and int write(int filedes, void *buffer, size_t size) respectively. These functions return the number of bytes that were read or written.

To change the current location within a file, use lseek(int filedes, off_t offset, int whence).

• Several options for whence include:
• SEEK_SET, which interprets the offset from the beginning of the file (position 0),
• SEEK_END, which subtracts the offset from the end of the file, and
• SEEK_CUR, which adds the offset to the current position in the file.

To duplicate descriptors (so that multiple processes can operate on the same file), there are two options:

1. Use int dup(int oldfd), which creates and returns a new file descriptor which acts as an alias for oldfd.
2. Use dup2(int old, int new), using new instead of assigning a new file descriptor. This is useful for redirecting output (for example, specifying new == STDOUT_FILENO would print the file to standard output).

An implementation of the copy example from the high-level IO section using low-level IO might look something like this:

void copy(const char *src, const char *dest) {
char buffer [100];
int buf_size = 0;
}
int bytes_written = 0;
int write_fd = open(dest, O_WRONLY);
while (bytes_written < buf_size) {
bytes_written += write(write_fd, &buffer[bytes_written], buf_size - bytes_written);
}
close(write_fd);
}


For an example of using dup2 in the context of forks and pipes to execute processes, see http://www.cs.loyola.edu/~jglenn/702/S2005/Examples/dup2.html.

### High vs Low Level IO #

High level streams are buffered in user memory:

## Be careful when mixing high and low level IO #

High level IO operations interface with the user-level buffer in chunks. However, low-level IO operations interface with the file directly. It is very hard to predict which portions of the file will be edited by which at what time.

# Pipes #

UNIX pipes provide a way to communicate information both within and between processes. Using pipes is very similar to reading and writing from a file, with some additional checks involved.

To create a pipe, use int pipe(int fileds[2]), where fileds[2] is a two-integer array. When pipe is called, it automatically assigns two new file descriptors to the array.

To read and write to and from a pipe, the write and read low-level calls can be made on fileds[1] and fileds[0], respectively.

In the kernel, pipes are implemented as a fix-sized queue. This causes some issues when the allocated size is overflowed:

• If the buffer is full and Process A tries to write, then the pipe blocks (process gets put to sleep).
• If the buffer is empty and Process B tries to read, then the pipe also blocks.

### Closing Pipes #

Just like files, pipes should be closed after use with close(fileds[0]) for the read stream and close(fileds[1]) for the write stream.

If a pipe is accessed after it is closed, several things could happen:

• If the write descriptor is closed, then the pipe will continually return EOF’s if read from.
• If the read descriptor is closed, then attempting to write to the pipe will generate a SIGPIPE signal.

Here’s an example of how to use pipes to transfer a string:

int main() {
int fds[2];
pipe(fds);
char *str = "hello world";
size_t bytes_written = 0;
size_t total = 0;
while (bytes_written = write(fds[1], &str[total], strlen(&str[total]) + 1)) {
total += bytes_written;
if (str[total - 1] == ’\0’) break;
}
close(fds[1]);
char *read_str = malloc(strlen(str) + 1);
total = 0;
}
return 0;
}


# Signals #

A signal is a software interrupt which provides a method of communicating state information about processes, the OS, or hardware. Different signals can have different handlers, or be ignored entirely, depending on the program.

Here is a list of standard Linux signals:

Custom signal handlers can be specified using the int signal(int signum, void (*handler)(int)) function. For example, signal(SIGINT, printMsg) will call the printMsg() function when the user quits out of the program using Ctrl+C.

Instead of specifying a custom handler, you also have the option of passing in SIG_IGN, which ignores the signal, or SIG_DFL, which runs the default behavior for that signal.

# Sockets #

#include <sys/socket.h>

/* Creates an endpoint for communication and returns a
file descriptor corresponding to that endpoint. */
int socket(int domain, int type, int protocol);


https://man7.org/linux/man-pages/man2/socket.2.html

#include <netdb.h>

int ai_flags;
int ai_family;
int ai_socktype;
int ai_protocol;
char * ai_canonname;
};

#define BUF_SIZE 1024
// Provides the preferred connection properties.
//If this is NULL, any connection is acceptible
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
int rv = getaddrinfo(NULL, port, &hints, &server); // Connects to loopback (node is NULL)
if (rv != 0) {
return NULL;
}
return server;
}
void *serve_client(void *client_socket_arg) {
int client_socket = (int)client_socket_arg;
char buf[BUF_SIZE];
ssize_t n;
while ((n = read(client_socket, buf, BUF_SIZE)) > 0) {
buf[n] = ’\0’;
printf("Client Sent: %s\n", buf);
if (write(client_socket, buf, n) == -1) {
close(client_socket);
}
}
close(client_socket);
}

int main(int argc, char **argv) {
if (argc < 2) {
printf("Usage: %s <port>\n", argv[0]);
return 1;
}

if (server == NULL) return 1;
int server_socket = socket(server->ai_family, server->ai_socktype, server->ai_protocol);
if (server_socket == -1) return 1;
if (listen(server_socket, 1) == -1) return 1;
while (1) {
int connection_socket = accept(server_socket, NULL, NULL);
if (connection_socket == -1) {
perror("accept");
}
serve_client, (void *)connection_socket);
if (err != 0) {
}
}