Compiler Notes for fortsh

TL;DR

• Linux x86_64: Use gfortran (works great)
• Linux aarch64: Use gfortran (auto-enables C stat helpers for struct layout differences)
• macOS ARM64 (M1/M2/M3/M4): Use LLVM Flang (flang-new) — gfortran has serious bugs
• macOS x86_64: Use gfortran with -frecursive

The Makefile auto-detects your platform and selects the right compiler. Just run make.

macOS ARM64: Why flang-new?

gfortran on Apple Silicon has at least 8 confirmed bugs that make it unusable:

1. Stack corruption — Large stack arrays (600KB+) corrupt memory
2. Deferred-length allocatable bug — character(len=:), allocatable loses length descriptor
3. Intent(out) crashes — Subroutine return epilogue segfaults
4. Allocatable string assignment corruption — Assigning to allocatable strings in types corrupts heap
5. Automatic finalization crashes — Crashes during automatic cleanup
6. Substring slice crashes — buffer(:length) operations segfault
7. Empty string assignment corruption — buffer = '' corrupts heap
8. flush() in loops corruption — Frequent stderr flush in tight loops corrupts heap

Install flang-new:

brew install flang

flang-new String Buffer Issue (Resolved)

flang-new has a known issue where Fortran string operations (substring slicing, direct assignment) on buffers larger than 128 bytes can cause heap corruption.

This limitation has been fully worked around via the C string library (src/c_interop/fortsh_strings.c), which routes all critical string operations through C code instead of flang-new's Fortran runtime. The C string library is auto-enabled for all flang-new builds (USE_C_STRINGS).

Additionally, a safe_assign_alloc_str routine performs char-by-char copies for allocatable strings >16 bytes, and the expansion pipeline uses C-backed growing buffers (buffer_grow, buffer_append_chars) for all variable and parameter expansion.

The workaround is transparent — no command length limits, no feature restrictions. macOS ARM64 passes the full test suite (3,600+ POSIX tests, 850+ builtin tests, 200+ stress tests) identically to Linux.

flang-new Fortran I/O Caveat

flang-new's write(output_unit, ...) and write(error_unit, ...) cache file descriptors at process startup and don't follow dup2 redirections. This means builtin output written via Fortran I/O bypasses shell redirections like > /dev/null.

Fix: Key builtins use write_stdout/write_stderr from io_helpers.f90, which call C write() directly to fd 1/2. This respects all dup2 redirections. Files affected: builtins.f90, aliases.f90, shell_options.f90, better_errors.f90, grammar_parser.f90, fd_redirection.f90, variables.f90, ast_executor.f90.

When adding new builtin output that users might redirect, use write_stdout/write_stderr instead of write(output_unit/error_unit, ...).

Linux aarch64: struct stat Layout

glibc on aarch64 uses a different struct stat layout than x86_64:

• st_mode and st_nlink are swapped (mode at offset 16 on aarch64, offset 24 on x86_64)
• st_nlink is 4 bytes (unsigned int) on aarch64, 8 bytes (unsigned long) on x86_64
• st_blksize is 4 bytes on aarch64, 8 bytes on x86_64
• Total struct size: 128 bytes (aarch64) vs 144 bytes (x86_64)

Fix: C stat helper functions in fd_wrapper.c (fortsh_stat_mode, fortsh_stat_size, etc.) use system headers for the correct layout. Enabled via -DUSE_C_STAT (auto-set by Makefile when uname -m is aarch64). x86_64 uses the Fortran stat_t struct directly.

Compiler Selection

Force a specific compiler:

make FC=gfortran clean all    # Force gfortran
make FC=flang-new clean all   # Force LLVM Flang

Build flags:

make NO_C_STRINGS=1     # Disable C string library (will crash on flang-new)
make NO_MEMPOOL=1       # Disable memory pooling
make MEMPOOL_DEBUG=1    # Enable memory pool debug output