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system calls

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API for Process Management​

Concepts​

  • OS provides API of functions for user programs
  • API consists of system calls executing in privileged kernel mode
  • Privileged operations (e.g., hardware access) require higher privilege level
  • System calls can be blocking (e.g., read(), causing context switch) or non-blocking (e.g., getpid(), returning immediately)

Portability Across OS​

Concepts​

  • POSIX API standardizes system calls for code portability across compliant OSes
  • Recompilation may still be needed for different hardware architectures
  • Language libraries (e.g., C's printf) often wrap system calls (e.g., write)
  • Application Binary Interface (ABI) defines interface between machine code and hardware

Concepts​

  • fork(): Creates new child process - all processes are forked from parent
  • exec(): Replaces current process image with new executable
  • exit(): Terminates current process
  • wait(): Blocks parent process until child terminates
  • Language libraries provide variants of these calls

Fork() System Call​

Concepts​

  • Parent's call to fork() creates new child process with new PID
  • Child receives copy of parent's memory image

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Flow​

  • After fork()
    • Parent and child resume execution from fork() call
    • fork() returns 0 to child and child's PID to parent
    • Parent and child run independently with separate memory - changes in one do not affect other
  • Execution order is non-deterministic without synchronization
  • Nested fork() calls create multiple processes (e.g., fork(); fork(); creates 4 total processes)

Exit() System Call​

Concepts​

  • Process calls exit() to terminate when execution finishes
  • OS switches process out and never runs it again
  • In C, exit() is called automatically at end of main()
  • Exiting process cannot free its own memory
  • Terminated process becomes zombie

Wait() System Call​

Concepts​

  • Parent calls wait() to reap (clean up) zombie child
  • wait() cleans up one terminated child's resources
  • If child is running, wait() blocks parent until child exits
  • If child is zombie, wait() reaps it and returns immediately
  • If no children exist, wait() returns immediately
  • waitpid() reaps specific child by PID
  • Parent should eventually call wait() for each fork()
  • "Orphan" process (parent exited first) is adopted and reaped by init process
  • Failing to call wait() creates zombie processes - can exhaust system resources
  • Using wait() enforces deterministic execution order (parent waits for child)

Exec() System Call​

Concepts​

  • exec() replaces current process code and memory with new program
  • Takes executable file as argument
  • Process memory image is completely replaced by new executable's image
  • Successful exec() does not return - new program starts executing
  • Unsuccessful exec() returns error - original program continues

Shell and Terminal​

Concepts​

  • Init process is first process on boot and spawns shell (e.g., bash)
  • All other processes are forked from existing ones
  • Shell primary loop: read command, fork() child, exec() command in child, wait() for child
  • Commands like ls are executables that shell exec()s
  • Some commands (e.g., cd) are built-in and executed directly by shell process - necessary to change shell's own working directory, not a temporary child's

Foreground and Background Execution​

Concepts​

  • Default user command runs in foreground - shell cannot accept next command until previous finishes
  • For background commands (&), shell forks but does not wait(), returning to prompt immediately
  • Shell reaps background processes later, sometimes using non-blocking wait() calls
  • Multiple commands can run serially or in parallel

I/O Redirection​

Concepts​

  • Every process has default file descriptors: STDIN, STDOUT, and STDERR
  • Shell can manipulate child file descriptors before exec() to redirect I/O

Examples​

  • ls > foo.txt: Shell closes child's STDOUT and opens foo.txt, taking the same file descriptor number

Shell Commands With Pipes​

Concepts​

  • Shell can pipe output of one command to input of another
  • Connects one child's STDOUT to another's STDIN using kernel pipe

Commands​

cat foo.c | grep factorial
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