Systems Programming Exam 1

Systems Programming - Aubrey Knight

What is systems programming?

The knowledge and skillset required to write/modify an operating system OR write programs using an OS's API.

Which definition of systems programming is emphasized in this course?

Writing programs using a UNIX-like OS API.

Why use a UNIX-like OS for systems programming?

Simplicity, exposure, and availability.

What does POSIX stand for?

Portable Operating Systems Interface.

What standard defines common UNIX-like OS features?

POSIX (part of SUSv4)

What is the kernel?

The central software that manages and allocates computer resources.

What is the shell?

A command interpreter that reads user commands and executes programs.

Examples of shells?

sh, csh, ksh, bash.

What is a user?

An individual system account identified by a username and user ID.

What is a group?

A category of users identified by a group name and group ID.

What is meant by a single directory hierarchy?

All files and directories are accessed from one root directory.

What is the root directory in UNIX-like systems?

/

What file types exist in UNIX-like systems?

Regular, device, FIFO, socket, directory, symbolic link.

What is universality of I/O?

All file types use the same basic I/O system calls.

Four basic I/O system calls?

open, read, write, close.

What is a process?

An instance of an executing program.

What information does the kernel track about a process?

User, PID, termination status, parent process.

Major regions of a process's memory?

Text, globals/statics, heap, stack.

What is memory mapping?

Mapping a file or memory region into a process's virtual address space.

What is file mapping?

Mapping a file into virtual memory.

What is anonymous mapping?

Memory mapping without an associated file, initialized to zero.

What is IPC?

Interprocess Communication.

Examples of IPC mechanisms?

Signals, pipes, sockets, file locking, message queues, semaphores, shared memory.

What are signals?

Kernel-generated notifications of events or exceptional conditions.

Why are signals called software interrupts?

They interrupt normal program execution.

Examples of signal-generating events?

Ctrl-C, child termination, timer expiration, divide by zero, invalid memory access.

What is a thread?

A lightweight execution unit within a process.

What do threads share?

Code, global data, heap, virtual memory.

What do threads not share?

Their execution stacks.

What is a system call?

A controlled entry point into the kernel.

Why are system calls necessary?

Some actions must be performed by the kernel.

Examples of kernel services?

Process creation, IPC creation, I/O processing, file systems.

What is the universal I/O model?

The same basic I/O system calls work on all file types.

What is a file descriptor?

A small non-negative integer representing an open file.

What does file descriptor 0 represent?

Standard input.

What does file descriptor 1 represent?

Standard output.

What does file descriptor 2 represent?

Standard error.

What does open() do?

Opens a file and returns a file descriptor.

What does read() do?

Reads bytes from a file into a buffer.

What does write() do?

Writes bytes from a buffer to a file.

What does close() do?

Closes a file descriptor and releases resources.

Prototype of open()?

int open(const char *path, int flags, mode_t mode)

Success return value of open()?

File descriptor >= 0.

Failure return value of open()?

-1.

What does O_RDONLY mean?

Open file for reading only.

What does O_WRONLY mean?

Open file for writing only.

What does O_RDWR mean?

Open file for reading and writing.

What does O_CREAT do?

Create file if it doesn't exist.

What does O_TRUNC do?

Truncate existing file.

What does O_APPEND do?

Append writes to end of file.

What is mode used for in open()?

File permissions when creating a file.

What does S_IRUSR mean?

User read permission.

What does S_IWUSR mean?

User write permission.

What does S_IXUSR mean?

User execute permission.

What does S_IRGRP mean?

Group read permission.

What does S_IWGRP mean?

Group write permission.

What does S_IXGRP mean?

Group execute permission.

What does S_IROTH mean?

Other read permission.

What does S_IWOTH mean?

Other write permission.

What does S_IXOTH mean?

Other execute permission.

What does errno store?

Error information for failed system calls.

ENOENT?

File does not exist.

EACCES?

Permission denied.

EMFILE?

Process has no more file descriptors available.

ENFILE?

System-wide open file limit reached.

EROFS?

Write attempted on read-only filesystem.

What is blocking I/O?

The process waits until the resource becomes available.

What is non-blocking I/O?

The process continues immediately if resource is unavailable.

Prototype of read()?

ssize_t read(int fd, void *buffer, size_t count)

What does read return on EOF?

0.

What does read return on error?

-1.

Prototype of write()?

ssize_t write(int fd, const void *buffer, size_t count)

What does write return?

Number of bytes written or -1 on error.

Prototype of close()?

int close(int fd)

What does close return on success?

0.

What is a file system?

An organized collection of files and directories.

What is a disk partition?

A non-overlapping region of a disk treated as a device.

What are device special files?

Files that represent hardware devices.

Types of device special files?

Character devices and block devices.

Examples of file systems?

ext2, ext3, ext4, XFS, FAT32, NTFS, HFS.

What is a logical block?

The basic unit for allocating space in a file system.

What is the boot block?

The first filesystem block containing boot information.

What is the superblock?

Filesystem metadata including size and layout.

What is an inode?

A structure storing metadata about a file.

What metadata does an inode store?

Type, owner, permissions, timestamps, size, links, block pointers.

What is stat()?

Retrieves file metadata.

Difference between stat() and fstat()?

fstat() uses a file descriptor.

What structure stores file metadata?

struct stat.

What member stores file type and permissions?

st_mode.

What member stores file size?

st_size.

What member stores inode number?

st_ino.

What member stores owner UID?

st_uid.

What member stores owner GID?

st_gid.

What are the three UNIX permission categories?

User, Group, Other.

What are the three permission bits?

Read, Write, Execute.

What does permission 644 mean?

User rw-, Group r--, Other r--.

What does S_IRWXU mask?

All user permission bits.

What does S_IRWXG mask?

All group permission bits.

What does S_IRWXO mask?

All other permission bits.

How is file type extracted from st_mode?

st_mode & S_IFMT