sx/issues/0137-jit-run-no-main-segfault.md

0137 — sx run on a program with no main segfaults (JIT entry lookup unguarded)

RESOLVED. A pre-JIT entry-point check in main.zig now emits a clean error: no 'main' function found … diagnostic and exits non-zero before any codegen/JIT, so a no-main program never reaches the garbage-pointer call. A defensive main_addr == 0 guard in target.zig's runJITFromObject (ORC reports lookup success but leaves the address degenerate) remains as a backstop. Regression test: examples/1188-diagnostics-run-no-main.sx.

Symptom

sx run <file> on a program that defines no main function crashes (SIGSEGV/abort, "Segmentation fault at address 0x60") instead of emitting a clean diagnostic like error: no 'main' function found.

• Observed: process crash, exit 134 (abort) / 139 (SIGSEGV); no diagnostic.
• Expected: a normal compile-style error ("no main entry point") and a clean non-zero exit, the same way any other missing-entry condition reports.

Independent of inline assembly — surfaced while writing an ASM-stream probe that omitted main, but reproduces with an ordinary, asm-free program (see below).

Reproduction

A file with only an (uncalled) function and no main:

foo :: (n: u64) -> u64 { return n + 1; }

sx run that.sx
# => "Segmentation fault at address 0x60", exit 134
# expected: "error: no 'main' function found" (or similar), clean non-zero exit

Root cause (suspected)

src/target.zig JIT-run path, ~lines 256–273. After the ORC lookup:

var main_addr: c.LLVMOrcExecutorAddress = 0;
err = c.LLVMOrcLLJITLookup(jit, &main_addr, "main");
if (err != null) { /* prints "JIT lookup error" and returns error.CompileError */ }

// no guard for main_addr == 0 here:
const main_fn: *const fn () callconv(.c) i32 = @ptrFromInt(main_addr);
const result = main_fn();          // <- calls a null/garbage pointer when no main

When the module has no main symbol, the lookup leaves main_addr at 0 (or ORC returns a degenerate success), so @ptrFromInt(main_addr) + main_fn() calls into null → the crash. There is no main_addr == 0 check.

Investigation prompt (paste into a fresh session)

sx run on a program with no main segfaults instead of diagnosing. The JIT run path in src/target.zig (~lines 256–273) looks up "main" via LLVMOrcLLJITLookup, then unconditionally casts main_addr to a function pointer and calls it. When the program defines no main, main_addr is 0 (or the lookup degenerately "succeeds"), so the call dereferences null and crashes.

Fix: after the lookup's err check, add if (main_addr == 0) { … } that emits a clean user-facing error ("no main function found" / "program has no entry point") and returns error.CompileError (matching the existing JIT lookup error style), BEFORE the @ptrFromInt + call. Consider whether a pre-JIT check (the module/program already knows whether a main decl exists — e.g. emit_llvm.zig:631 already null-checks LLVMGetNamedFunction(.., "main")) is the better choke point so the diagnostic carries a source span rather than a bare message. Either is acceptable; the hard requirement is no crash.

Verification: printf 'foo :: (n: u64) -> u64 { return n + 1; }\n' > /tmp/x.sx && sx run /tmp/x.sx — expect a clean error message + non-zero exit, NOT a segfault. Add a pinned repro under issues/ (or an examples/11xx-diagnostics-* once the message is settled) asserting the diagnostic on stderr + the exit code.