Direct native standard streams
The Linux x86-64 runtime implements print, println, and eprint under the following stream contract. Target availability and platform qualification are specified by each compiler release.
kelft-native-streams/1 uses the unchanged kelft-native/1 four-pointer calling convention and 64-byte context. Each intrinsic takes one borrowed packed UTF-8 string and returns void without writing a result buffer, allocating, retaining or releasing the input. The argument is evaluated once before output. print and println select descriptor 1; eprint selects descriptor 2. println writes exactly one LF after its payload, including an empty payload. NULs, CRLFs, BOMs and Unicode normalization forms are preserved byte for byte. Output is synchronous and unbuffered in the language runtime; these calls complete before a subsequent explicit process exit. Calls are not transactional or guaranteed to be atomic across processes or between stdout and stderr.
Kelft private-IR builders emit the checked write loop. A narrow Linux mechanism emits write directly, returning an i32 count or negative kernel errno through the result pointer. Nonempty output retries EINTR with the same range, advances by positive partial counts and checks each count before pointer arithmetic. Zero progress, an oversized count, invalid private lengths and hard errors (including EAGAIN) trap with KRT_IO. Empty print/eprint calls make no syscall, even if their descriptor is closed. Diagnostics use the existing best-effort stderr trap path; if stderr is closed, exit 1 still occurs without a diagnostic. These error rules do not introduce a recoverable stream API or change the existing JavaScript, Wasm and C drivers.
The process inherits its SIGPIPE disposition. With the default disposition, a broken pipe terminates it with SIGPIPE. With SIGPIPE ignored, write returns EPIPE and the checked helper traps with KRT_IO. The runtime does not install signal handlers or alter the signal mask. Other inherited-descriptor properties, such as blocking mode, are also retained.
Core cannot select, open or redirect descriptors. There is no stdin reader, arbitrary descriptor API, file access, subprocess execution or signal interface. The existing empty filesystem/execute maximum remains enforced by the supported operation set. A caller may redirect inherited stdout/stderr; this does not grant the program filesystem paths or compiler/linker authority. The private raw write mechanism and byte-range helper are trusted runtime interfaces, not source FFI.
Verification requires deterministic object bytes and statically linked execution with exact output bytes. They include binary controls, Unicode, CRLF, empty output, evaluation order, a 294912-byte payload, and 20000-iteration cleanup under a 4096-byte heap and 128 KiB stack. A separate C ABI probe verifies borrowed inputs, void/i32 output widths, zero allocation, real pipe routing and preservation of stdin. A test-only weak mechanism allows an independent C stub to inject partial writes, EINTR, zero progress, hard errors and oversized counts into the emitted loop. Production objects remain unchanged. All original code, FDE bytes and FDE relocations remain unchanged in the mock. ELF writers bind FDEs to a local text-section symbol and the emitted body offset; replacing a global function symbol cannot redirect its unwind record. The injected mechanism calls an independent native unwinder, which must find both the emitted stream-write frame and its print/println/eprint caller. Fourteen required native checks cover those failures, invalid lengths, closed descriptors, /dev/full and both SIGPIPE dispositions; language host checks execute the actual linker-only program. This fault injection supplements real execution and does not provide cross-target release evidence.
Verification reports must bind source, toolchain, fixture, original/mock object, and ABI executable identities. Fault injection supplements real execution and cannot establish qualification for a different target.
Source: public Kelft compiler, commit f567c21684362db37e0aa0a68e71e7614502c45a. Compiler 0.1.0-alpha.2; Core 0.1; Intent 0.2.