Signals and File I/O in Unix Programming

Signal

Software interrupts for handling asynchronous events.

SIG

Prefix for all signal names in Unix.

<signal.h>

Header defining signal constants in Unix.

Signal number

Positive integer constant representing a signal.

SIGKILL

Signal that cannot be ignored or caught.

SIGSTOP

Signal that cannot be ignored or caught.

Terminal-generated signals

Signals from user terminal key presses.

Hardware exceptions

Signals generated by hardware errors.

kill() function

Sends signals to other processes.

Signal disposition

Action taken when a signal is received.

SIG IGN

Constant to ignore a signal.

SIG DFL

Constant for default signal action.

Signal handler

Function called when a signal occurs.

fork

System call creating a child process.

Unreliable signals

Signals that may be lost in earlier Unix versions.

EINTR

Error code for interrupted system calls.

Slow system calls

Calls that can block indefinitely.

wait function

Always interrupted when a signal is caught.

waitpid function

Similar to wait; interrupted by signals.

Reentrant functions

Functions that can be interrupted safely.

Signal handler arguments

First: signo; second: function pointer.

<sys/signal.h>

User-level header for signal handling in Mac OSX.

Signal Handler

Function that responds to signals in UNIX.

Async-Signal Safe

Functions safe to call within signal handlers.

Reentrant Function

Can be interrupted and safely called again.

errno

Global variable for error number in C.

Nonreentrant Function

Cannot be safely called from signal handlers.

File Descriptor

Non-negative integer referencing a file.

Standard Input

File descriptor 0, for input operations.

Standard Output

File descriptor 1, for output operations.

Standard Error

File descriptor 2, for error messages.

open Function

Opens a file and returns its descriptor.

openat Function

Opens a file relative to a directory.

TOCTTOU Error

Vulnerability from non-atomic file operations.

creat Function

Creates a file for write-only access.

close Function

Closes an open file descriptor.

lseek Function

Sets the file offset for reading/writing.

SEEK_SET

Set offset from beginning of file.

SEEK_CUR

Set offset from current position.

SEEK_END

Set offset from end of file.

Unbuffered I/O

Each read/write invokes a system call.

openat Use Case 1

Absolute pathname, fd is ignored.

openat Use Case 2

Relative pathname, fd specifies starting location.

openat Use Case 3

Relative pathname, fd is AT_FDCWD.

File Creation Modes

Options for file creation in open.

lseek

Records current file offset in kernel only.

File offset

Position in file for read/write operations.

File extension

Occurs when offset exceeds current file size.

Hole in file

Unbacked space in file, no storage required.

read function

Reads data from file, returns bytes read.

read return values

Returns bytes read, 0 for EOF, -1 for error.

Partial read

Less data read than requested due to various reasons.

write function

Writes data to file, returns bytes written.

write return values

Returns bytes written, -1 on error.

write error causes

Disk full or file size limit exceeded.

O_APPEND option

Sets file offset to end before writing.

I/O efficiency

System reads more data on sequential reads.

File sharing

Multiple processes can share open files.

Process table

Contains entries for each process's open files.

File descriptor flags

Flags associated with each file descriptor.

File table

Kernel maintains a table for all open files.

v-node structure

Contains file type info and operation pointers.

Atomic operations

Operations requiring multiple calls cannot be atomic.

pread function

Reads data without updating current file offset.

pwrite function

Writes data without updating current file offset.

Signal interruption

Can cause partial reads during read operations.

Atomic Operation

Operation performed entirely or not at all.

dup Function

Duplicates a file descriptor, returning new descriptor.

dup2 Function

Duplicates a file descriptor to specified value.

File Descriptor

Non-negative integer representing an open file.

sync Function

Flushes file system buffers to disk.

fsync Function

Flushes buffers for a specific file descriptor.

fdatasync Function

Flushes only data portions of a file.

Delayed Write

Data queued for later writing to disk.

Process ID (PID)

Unique identifier for each process.

Process ID 0

Scheduler process, also known as swapper.

Process ID 1

Init process, invoked at bootstrap completion.

getpid Function

Returns calling process's unique process ID.

getppid Function

Returns parent process's ID of calling process.

getuid Function

Returns real user ID of calling process.

geteuid Function

Returns effective user ID of calling process.

getgid Function

Returns real group ID of calling process.

getegid Function

Returns effective group ID of calling process.

fork Function

Creates a new process, returning twice.

Child Process

Process created by the fork function.

Process Creation

Existing process creates a new one via fork.

Scheduling Algorithm

Determines execution order of parent and child.

Process Termination

Releases process ID for future reuse.

Fork

Creates a child process duplicating the parent.

File Descriptor Duplication

Child inherits all open file descriptors.

Shared File Offset

Parent and child share the same file offset.

Intermixed Output

Uncoordinated writes from parent and child mix.

Inherited Properties

Child inherits various attributes from parent.

Process IDs

Parent and child have different process IDs.

Return Value of Fork

Fork returns different values for parent and child.

Pending Alarms

Cleared for the child process upon fork.

Zombie Process

Terminated process not yet waited on by parent.

vfork Function

Creates child without copying parent's address space.

Spawn

Fork followed by exec to run a new program.

Exit Function

Terminates a process in various defined ways.