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Zobair Najdaoui
Zobair Najdaoui

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Understanding the exit status Bits

#programming #c #cpp #computerscience

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🔢 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)
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So the value looks like:

0x00SS  (where SS is the exit status byte)
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  • 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)
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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
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  • 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
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  • Looks like: 0xSS7F where SS = signal
  • Detected by WIFSTOPPED(status)
  • WIFSIGNALED(status) → false

▶️ 4. Continued 

status = 0xFFFF
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  • Used with waitpid(..., WCONTINUED)
  • Detected by WIFCONTINUED(status)
  • WIFSIGNALED(status) → false

✅ Summary

Status Type Bit Pattern Summary Detected by Related Macros
Normal exit bits 0–7 = 0, bits 8–15 = exit code WIFEXITED(status) WEXITSTATUS(status)
Killed by signal bits 0–6 = signal, bit 7 = core dump flag WIFSIGNALED(status) WTERMSIG(status), WCOREDUMP(status)
Stopped bits 0–7 = 0x7F, bits 8–15 = stop signal WIFSTOPPED(status) WSTOPSIG(status)
Continued status == 0xFFFF WIFCONTINUED(status)

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