What makes a function async-signal-safe?

A strong answer

A strong candidate frames What makes a function async-signal-safe? around the exact memory layout, compiler semantics, and hardware execution model covered in Signal Masks, Pending Signals, and Async-Signal Safety. Explain that async-signal-safe functions can be called from a signal handler, and list examples like write but not printf/malloc.

Signal handlers interrupt execution asynchronously at arbitrary instruction boundaries. This asynchronous nature introduces severe concurrency hazards within a single-threaded program.

Now trace what occurs when a child process terminates at the worst possible moment:

1. The main thread executes add_job() and finishes node->next = job_list.
2. Right before job_list = node executes, the child process exits.
3. The CPU interrupts the main thread mid-instruction and runs sigchld_handler().
4. remove_job() traverses job_list, which is currently in a half-updated, inconsistent state. The pointers corrupt, causing a segmentation fault or a memory leak.

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: Manipulate signal sets using sigemptyset, sigfillset, sigaddset, and sigdelset.
• Mastery of core invariant: Protect critical sections against interruption using sigprocmask.
• Mastery of core invariant: Differentiate non-queued standard POSIX signals from real-time queued signals.
• 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.