標準的控制流程是一種可預測的演進：程式計數器根據順序邏輯或明確的跳轉，從位址 $a_k$ 移動至 $a_{k+1}$。然而， 例外控制流程（ECF） 代表了發生在正常流程之外的「突兀」轉移。
1. 數學模型
處理器執行是一個序列 $a_0, a_1, \dots, a_{n-1}$，其中每個 $a_k$ 對應一個指令 $I_k$。當處理器狀態發生變動——一個 事件——會觸發跳轉至一個應用程式即時碼路徑中不存在的專用處理常式。
2. 實作層級
ECF 橋接了硬體與軟體之間的差距。其範圍涵蓋從硬體層級的 例外 （錯誤、中斷）到作業系統層級的 上下文切換 以及 訊號。
3. 「突兀」的現實
無論是使用者按下 Ctrl+C 或是系統呼叫請求磁碟存取，例外控制流程都會迫使中央處理器跳轉至另一個「世界」——核心——以確保系統能對動態狀態變化保持回應。
main.py
TERMINALbash — 80x24
> Ready. Click "Run" to execute.
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QUESTION 1
Practice Problem 8.1: Given Process A(1-3), B(2-5), and C(4-6), which pair is NOT concurrent?
Processes A and B
Processes A and C
Processes B and C
All processes are concurrent.
✅ Correct!
Processes A(1-3) and C(4-6) do not overlap in time; A finishes before C starts.❌ Incorrect
Concurrency requires a time overlap. A(1-3) and B(2-5) overlap at time 2. B(2-5) and C(4-6) overlap at time 4.QUESTION 2
Which function is called once but returns only if there is an error?
fork()
exit()
execve()
waitpid()
✅ Correct!
execve() overwrites the current process image; if successful, it never returns to the caller.❌ Incorrect
fork() returns twice. exit() never returns. waitpid() returns when a child changes state.QUESTION 3
In the mathematical model of control flow, what triggers an 'exceptional' transition?
A standard sequential instruction Iₖ₊₁
An explicit loop or function call
A change in processor state (Event)
The end of the program counter
✅ Correct!
ECF is defined by transitions triggered by events external or internal to the instruction stream, not normal logic.❌ Incorrect
Standard transitions (loops, calls) are part of normal control flow, not exceptional.QUESTION 4
Which function behavior matches: 'Called once, returns either a pointer or NULL'?
getenv()
fork()
exit()
longjmp()
✅ Correct!
getenv() searches for an environment variable and returns its pointer or NULL if not found.❌ Incorrect
fork() returns a PID/0. exit() returns nothing. longjmp() returns to setjmp().QUESTION 5
If a process receives a signal of type k while another signal of type k is pending, what happens?
The signal is queued.
The signal is discarded.
The kernel crashes.
The handler is interrupted by the new signal.
✅ Correct!
Signals are non-queuing; if a signal is already pending, subsequent instances of that same signal are dropped.❌ Incorrect
Signals of the same type do not queue in standard Unix systems.Case Study: Analyzing Execution Flow in global-waitprob1.c
Synchronization and ECF Transitions
Consider a program where a parent process calls fork() to create a child, and the parent immediately calls waitpid() to reap it. Analyze the resulting control flow divergence and convergence.
Q
How many output lines does this program generate and what is the ordering principle?
Solution:
The program generates 2 output lines. The ordering is strictly deterministic: the child's output must appear before the parent's final output because waitpid() acts as a synchronization barrier, forcing the parent's sequential flow to pause until the child's exceptional exit state is processed.
The program generates 2 output lines. The ordering is strictly deterministic: the child's output must appear before the parent's final output because waitpid() acts as a synchronization barrier, forcing the parent's sequential flow to pause until the child's exceptional exit state is processed.
Q
Required Output Task: Analyze the global-waitprob1.c execution flow (approx. 200 words).
Solution:
The execution of global-waitprob1.c serves as a textbook illustration of Exceptional Control Flow through process management. Upon reaching the fork() call at address a_k, the system state undergoes a profound transition. The operating system creates a duplicate logical control flow—the child—which begins its own sequence starting from a duplicate set of registers and address space. This creates a divergence where two distinct sequences (a_0...a_n) coexist. The parent process immediately encounters the waitpid() system call, which is a 'Trap' class exception. This call effectively places the parent's sequential instruction stream in a suspended state, waiting for a hardware-triggered event: the child's termination. In this moment, the child process executes its unique path independently. When the child calls exit(), an exceptional event occurs that notifies the kernel. The kernel then context-switches back to the parent, resolving the waitpid() block. This synchronization barrier ensures that the child's logical flow is 'reaped' before the parent proceeds. Ultimately, the two flows, though briefly concurrent and independent, are reconciled by the OS to ensure predictable output, proving that ECF is not chaotic but a highly structured collaboration between the hardware, kernel, and application logic.
The execution of global-waitprob1.c serves as a textbook illustration of Exceptional Control Flow through process management. Upon reaching the fork() call at address a_k, the system state undergoes a profound transition. The operating system creates a duplicate logical control flow—the child—which begins its own sequence starting from a duplicate set of registers and address space. This creates a divergence where two distinct sequences (a_0...a_n) coexist. The parent process immediately encounters the waitpid() system call, which is a 'Trap' class exception. This call effectively places the parent's sequential instruction stream in a suspended state, waiting for a hardware-triggered event: the child's termination. In this moment, the child process executes its unique path independently. When the child calls exit(), an exceptional event occurs that notifies the kernel. The kernel then context-switches back to the parent, resolving the waitpid() block. This synchronization barrier ensures that the child's logical flow is 'reaped' before the parent proceeds. Ultimately, the two flows, though briefly concurrent and independent, are reconciled by the OS to ensure predictable output, proving that ECF is not chaotic but a highly structured collaboration between the hardware, kernel, and application logic.