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saltwater: the part of the sea causing lots of rust

A C compiler written in Rust, with a focus on good error messages.

This project is no longer maintained.


swcc reads from standard in by default, so you can type in code directly. It's not interactive though, you have to hit Ctrl+D to indicate end of file (Ctrl+Z on Windows).

Use swcc --help for all options (or see below).

Running on Windows

You need to have cc on your PATH. You can either install mingw + gcc or MSVC. Other than that, it should work exactly the same as on Linux.


brew install saltwater

Unimplemented features

  • Defining functions taking variadic arguments. Note that calling variadic functions (like printf) is already supported.
  • Variable-length arrays (int a[n])
  • Multiple translation units (files)
  • Bitfields
  • Compiling on non-x86 platforms
  • Cross-compilation


$ cat tests/runner-tests/readme.c
// output: j is 6
int printf(const char *, ...);

typedef struct s *sp;

int i = 1;
int a[3] = {1, 2, 3};
float f = 2.5;

struct s {
  int outer;
} my_struct;

int g(int);

int main(void) {
  sp my_struct_pointer = &my_struct;
  const int c = my_struct_pointer->outer = 4;
  // should return 6
  int j = i + f*a[2] - c/g(1);
  printf("j is %d\n", j);
  return j;

int g(int i) {
  if (i < 0 || i >= 3) {
    return 0;
  return a[i];
$ swcc tests/runner-tests/readme.c
$️ ./a.out
j is 6

Debug output

$ cat tests/runner-tests/cpp/if/defined.c
// code: 2

#define a
#define b

#if defined(a)
int i = 2;

#ifndef b
syntax error

# if defined b && defined(a)
    int main() { return i; }
$ swcc -E tests/runner-tests/cpp/if/defined.c
int i = 2 ; int main ( ) { return i ; }
$ echo 'int i = 1 + 2 ^ 3 % 5 / 2 & 1; int main(){}' | swcc --debug-ast
ast: int i = ((1) + (2)) ^ ((((3) % (5)) / (2)) & (1));
ast: int main(){
$ cat tests/runner-tests/hello_world.c
int main() {
    puts("Hello, world!");
$ swcc --debug-ir tests/runner-tests/hello_world.c
function u0:0() -> i32 system_v {
    gv0 = symbol colocated u1:3
    sig0 = (i64) -> i32 system_v
    fn0 = u0:26 sig0

    v0 = global_value.i64 gv0
    v1 = call fn0(v0)
    v2 = iconst.i32 0
    return v2
$ ./a.out
Hello, world!

All options

$ swcc --help
swcc 0.9.0
Joshua Nelson <>
A C compiler written in Rust, with a focus on good error messages.

usage: swcc [FLAGS] [OPTIONS] [<file>]

        --debug-ast        If set, print the parsed abstract syntax tree (AST) in addition to compiling.
                            The AST does no type checking or validation, it only parses.
        --debug-hir        If set, print the high intermediate representation (HIR) in addition to compiling.
                            This does type checking and validation and also desugars various expressions.
        --debug-ir         If set, print the intermediate representation (IR) of the program in addition to compiling.
        --debug-lex        If set, print all tokens found by the lexer in addition to compiling.
        --jit              If set, will use JIT compilation for C code and instantly run compiled code (No files produced).
                            NOTE: this option only works if swcc was compiled with the `jit` feature.
    -h, --help             Prints help information
    -c, --no-link          If set, compile and assemble but do not link. Object file is machine-dependent.
    -E, --preprocess-only  If set, preprocess only, but do not do anything else.
                            Note that preprocessing discards whitespace and comments.
                            There is not currently a way to disable this behavior.
    -V, --version          Prints version information

        --color <when>       When to use color. May be "never", "auto", or "always". [default: auto]
    -o, --output <output>    The output file to use. [default: a.out]
        --max-errors <max>   The maximum number of errors to allow before giving up.
                             Use 0 to allow unlimited errors. [default: 10]
    -I, --include <dir>      Add a directory to the local include path (`#include "file.h"`).
                              Can be specified multiple times to add multiple directories.
    -D, --define <id[=val]>  Define an object-like macro.
                              Can be specified multiple times to add multiple macros.
                              `val` defaults to `1`.

    <file>    The file to read C source from. "-" means stdin (use ./- to read a file called '-').
              Only one file at a time is currently accepted. [default: -]


cargo test
# optionally, you can fuzz the compiler
# it may be more helpful to just `grep -R unimplemented src`, though

# libFuzzer/AFL

# Honggfuzz:
# Running Honggfuzz locally requires some parameters to use it at its full potential,
# so it is probably a good idea to have a look here:
# and here:
# we suggest the following:
HFUZZ_RUN_ARGS="--tmout_sigvtalrm --exit_upon_crash" tests/



Implementation Defined Behavior



See This also includes reporting bugs.