Understanding the exit status Bits

🔢 Understanding the status Bits

This status is a 16-bit integer returned by the kernel to describe how a child process changed state. It can represent:

1. Normal termination (via exit() or return)
2. Killed by a signal
3. Stopped by a signal
4. Continued (after being stopped)

🧱 Bit Layout for Each Case

Let's read this from least significant bit (bit 0) to most (bit 15):

✅ 1. Normal Termination

Bits 0–7:    Always zero
Bits 8–15:   Exit status (0–255)

So the value looks like:

0x00SS  (where SS is the exit status byte)

• Detected by WIFEXITED(status) macro.
• WIFSIGNALED(status) → false

☠️ 2. Killed by Signal

Bits 0–6:   Signal number (non-zero)
Bit 7:      Core dump flag
Bits 8–15:  Unused (0)

Example: if the process was killed by SIGSEGV (11) and dumped core:

Signal = 11  → bits 0-6 = 0001011
Core   = 1   → bit 7 = 1

status = 00000000 10001011  → 0x008B

• WIFSIGNALED(status) checks that:

  • Bits 0–6 ≠ 0 (so signal present)
  • Bits 8–15 = 0 (not a stopped or continued signal)

• If so → returns true

Then:

• WTERMSIG(status) → reads bits 0–6 → 11
• WCOREDUMP(status) → reads bit 7

✋ 3. Stopped by Signal

Bits 0–7:   0x7F (decimal 127)
Bits 8–15: Stop signal

• Looks like: 0xSS7F where SS = signal
• Detected by WIFSTOPPED(status)
• WIFSIGNALED(status) → false

▶️ 4. Continued

status = 0xFFFF

• Used with waitpid(..., WCONTINUED)
• Detected by WIFCONTINUED(status)
• WIFSIGNALED(status) → false

✅ Summary