macOS Apple Silicon

fortsh on Apple Silicon (M1/M2/M3) has a significant limitation due to Fortran compiler issues.

Key limitation: Commands are limited to 127 characters.

The Problem

gfortran Issues

gfortran on ARM64 macOS has multiple severe bugs:

1. Stack corruption with programs using more than ~600KB stack
2. Deferred-length string length loss during argument passing
3. Segfaults on intent(out) string arguments
4. Heap corruption on string assignment

These bugs make gfortran unusable for fortsh on Apple Silicon.

flang-new Workaround

fortsh uses flang-new (LLVM's Fortran compiler) on Apple Silicon. However, flang-new has its own limitation:

128-byte string operation limit: Substring operations on strings longer than 128 bytes cause heap corruption.

This limits command lines to 127 characters (128 minus null terminator).

Impact

• Very long commands must be split or aliased
• Long paths may need to be cd'd into rather than typed
• Complex one-liners may need to be saved as scripts
• Normal interactive use is generally unaffected

Workaround Implementation

fortsh includes a C string library (src/c_interop/fortsh_strings.c) that implements string operations safely on Apple Silicon:

// C functions that bypass the flang-new bug
void c_string_copy(char* dest, const char* src, int max_len);
void c_string_concat(char* dest, const char* src, int max_len);
int c_string_length(const char* str);

This workaround is automatically enabled when building on macOS ARM64.

Installation

Via Homebrew

brew install FortranGoingOnForty/tap/fortsh

From Source

# Install LLVM (provides flang-new)
brew install llvm

# Add to PATH
export PATH="/opt/homebrew/opt/llvm/bin:$PATH"

# Build
git clone https://github.com/fortrangoingonforty/fortsh.git
cd fortsh
make
sudo make install

The Makefile automatically detects ARM64 and uses flang-new.

Checking Your Setup

# Verify the limit
fortsh -c 'echo "${#0}"'  # Shows max line length

# Check compiler used
fortsh -c 'echo $FORTSH_COMPILER'  # Shows "flang-new" on ARM64

Configuration Differences

Future

This limitation may be resolved when:

• flang-new fixes the 128-byte substring bug, OR
• gfortran fixes its ARM64 bugs, OR
• fortsh implements a full C string backend

Until then, the 127-character limit remains on Apple Silicon.

Workarounds

Use Aliases

# Instead of typing long commands
alias deploy='cd ~/projects/myapp && ./scripts/deploy.sh --env=prod'

Use Variables

# Store long paths
export APP="$HOME/long/path/to/application"
cd "$APP"

Use Scripts

For complex one-liners, save as a script instead:

# Instead of a 200-character one-liner:
cat > script.sh << 'EOF'
# Your complex commands here
EOF
fortsh script.sh

System Information

Fortsh detects the platform at compile time:

#if defined(__APPLE__) && defined(__arm64__)
    ! Use C string library
    USE_C_STRINGS = .true.
    MAX_LINE_LEN = 128
#endif

From src/common/constants.f90 and build-time detection in Makefile.