Read Exactly N Bytes

POSIX. Implement ssize_t read_exact(int fd, void *buf, size_t n) — loop recv until exactly n bytes are accumulated; return n, or -1 on EOF-before-complete (peer closed mid-message), error, or NULL/zero-argument misuse (n==0 returns 0 immediately). Partial data stays in buf on failure — the tests only check return values.