What causes "Expected 3 arguments, got %d\n" in bazelbuild/bazel?

Invoke with exactly three paths: launcher_maker .

Expected 3 arguments, got %d\n

Error message

Expected 3 arguments, got %d\n

What it means

launcher_maker's main requires exactly three positional arguments (launcher binary, launch-info params file, output executable); argc must be at least 4 (program name + 3). If fewer are passed it prints the count it actually received and returns exit code 1. Bazel generates these actions internally, so hitting it manually or via a broken rule means the action's argv is malformed.

Source

Thrown at src/tools/launcher/launcher_maker.cc:64

  if (!blaze_util::AsAbsoluteWindowsPath(path, &wpath, &error)) {
    fprintf(stderr, "Failed to make absolute path for %s: %s\n", path,
            error.c_str());
    exit(1);
  }
  return wpath;
}

#else  // _WIN32

#define STRING_TYPE std::string
#define STRING_FORMAT "%s"
std::string convert_path(char* path) { return path; }

#endif  // _WIN32

int main(int argc, char** argv) {
  if (argc < 4) {
    fprintf(stderr, "Expected 3 arguments, got %d\n", argc);
    return 1;
  }

  STRING_TYPE launcher_path = convert_path(argv[1]);
  STRING_TYPE info_params = convert_path(argv[2]);
  STRING_TYPE output_path = convert_path(argv[3]);

  std::ifstream src(launcher_path.c_str(), std::ios::binary);
  if (!src.good()) {
    fprintf(stderr, "Failed to open " STRING_FORMAT ": %s\n",
            launcher_path.c_str(), strerror(errno));
    return 1;
  }
  std::ofstream dst(output_path.c_str(), std::ios::binary);
  if (!dst.good()) {
    fprintf(stderr, "Failed to create " STRING_FORMAT ": %s\n",
            output_path.c_str(), strerror(errno));
    return 1;

View on GitHub (pinned to e6e199d060)

Solutions

Invoke with exactly three paths: launcher_maker <launcher> <info-file> <output>.
Inspect the failing action with --subcommands to see the actual argv Bazel produced.
Fix custom action definitions so all three ctx.actions.run() args are always supplied.
Quote paths in wrapper scripts so spaces do not collapse argument counts.

Example fix

# before: launcher_maker.exe launcher.exe
# after: launcher_maker.exe launcher.exe launch-info.txt my-launcher.exe

Defensive patterns

Strategy: validation

Validate before calling

if (argc != 4) {
  fprintf(stderr, "usage: %s <launcher> <info-file> <output>\n", argv[0]);
  return 1;
}

Prevention

Always supply all three positional paths in order.
Quote paths with spaces so argv counts stay correct.
In Starlark actions, build args from a fixed list so none can be dropped.

When it happens

Trigger: Running launcher_maker with 0-2 arguments; a custom Starlark action accidentally dropping the output argument; shell scripts mangling quoted paths into a single argv entry.

Common situations: Hand-testing the tool; custom rules that build the args list conditionally and skip a parameter.

Related errors

AI-assisted analysis of bazelbuild/bazel@e6e199d060 (2026-08-14). Data as JSON: /api/errors/6f45b5f8c5347a32. Report an issue: GitHub.