C errno & Syscall Return Handling

When to use

• C crossing the OS boundary: read/write/accept/connect, open/close, and libc functions that report errors via errno (strtol, strtod, fread, fwrite, malloc, fopen).
• Flaky I/O: lost or duplicated data, false errors, hangs, failures only under load or after signals.
• Retry/aggregation: read a buffer "until done", write "until all bytes are out".

When not to use

• Pure computation with no libc/OS boundary — see c-undefined-behavior.
• Win32 FALSE/GetLastError/HRESULT conventions — different model; this skill covers CRT _read/_write and POSIX, not CreateFile/ReadFile.
• C++ exceptions/RAII — cpp-object-lifecycle.
• Inside signal handlers — c-signal-handler-safety (requires this skill).

What the agent often gets wrong