Break down WIFSIGNALED

🧠 Context: What is status?

• status is an int passed by reference to wait() / waitpid().
• When a child process terminates, the kernel writes information about how it terminated into status.
• You then use macros like WIFSIGNALED(status) to interpret that information.

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🔍 1. WIFSIGNALED(status)

"Was the child process terminated by an uncaught signal?"

• This macro checks if the child died because of a signal, not because it called exit() or returned from main().

• Returns non-zero (true) if:

  • The child process ended due to a signal, not normal exit.
  • The signal was not caught or ignored, meaning it went unhandled.

💡 Example:

#include <sys/wait.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    pid_t pid = fork();
    if (pid == 0) {
        // Child process: cause segmentation fault
        int *p = NULL;
        *p = 1;  // SIGSEGV
    } else {
        int status;
        waitpid(pid, &status, 0);

        if (WIFSIGNALED(status)) {
            printf("Child was killed by signal %d\n", WTERMSIG(status));

            if (WCOREDUMP(status))
                printf("Core dump was generated\n");
        }
    }
}

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🛑 2. WTERMSIG(status)

"Which signal caused the termination?"

• If WIFSIGNALED(status) is true, you can use WTERMSIG(status) to get the signal number.

• For example:

  • SIGSEGV = 11 (Segmentation fault)
  • SIGKILL = 9
  • SIGINT = 2 (CTRL+C)

• These signal numbers are defined in <signal.h>.

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💾 3. WCOREDUMP(status)

"Did the process create a core dump?"

• Some systems generate a core dump file when a process crashes due to a signal.
• WCOREDUMP(status) checks whether that happened.
• It’s not part of the official POSIX (SUSv3) standard, but it’s available on many systems like Linux, BSD, etc.

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🧩 How does this work under the hood?

Internally, the status integer returned by wait() contains bit fields that encode:

• Whether the process exited normally or by signal
• What signal or exit code
• Whether a core dump occurred

Macros like WIFSIGNALED just check specific bits in that integer.

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🧪 Summary Table

Macro	What it tells you	Only valid when
WIFSIGNALED(s)	Process was killed by an uncaught signal	Always
WTERMSIG(s)	The signal number that killed it	When WIFSIGNALED(s) true
WCOREDUMP(s)	If a core dump was produced	When WIFSIGNALED(s) true (and supported)