Why is signal() discouraged in favor of sigaction?

A strong answer

A strong candidate frames Why is signal() discouraged in favor of sigaction? around the exact memory layout, compiler semantics, and hardware execution model covered in Signal Architecture and Delivery with sigaction. Explain signal() unreliable semantics across systems, and sigaction's precise control over masks, flags, and restart behavior.

By default, Unix processes terminate abruptly when common external events occur:

• A user presses Ctrl+C in the terminal: the kernel generates SIGINT (Interrupt).
• The controlling terminal window is closed: the kernel delivers SIGHUP (Hangup).
• A process writes to a pipe whose reading end is closed: the kernel delivers SIGPIPE (Broken Pipe).

When an unhandled terminating signal arrives, the process dies immediately. Any open database transactions remain uncommitted, temporary scratch files on disk are left behind, and active network connections terminate without clean shutdown handshakes.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Configure reliable signal handlers using the POSIX sigaction interface.
• Mastery of core invariant: Trace kernel signal frame injection, user-stack redirection, and sigreturn restoration.
• Mastery of core invariant: Prevent unintended EINTR errors during blocking I/O using the SA_RESTART flag.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.