How the binding works

swmm.* — the binding, function by function

Playground extras (not part of the C API)

Property codes (swmm5.h, verbatim)

What swmm.set() actually accepts

Read straight from swmm_setValue() in swmm5.c (5.2.4) — anything else is silently ignored:

Three lifecycle rules this playground enforces

Which approach do you need?

Approach 1 — batch mode with plain Lua

-- run the official CLI and read the report back (any Lua 5.x)
local inp, rpt, out = "model.inp", "model.rpt", "model.out"

-- Windows:  runswmm.exe (ships with the EPA installer)
-- Linux:    runswmm from your own build (see the build card below)
local cmd = ('runswmm "%s" "%s" "%s"'):format(inp, rpt, out)
local ok = os.execute(cmd)
assert(ok, "SWMM run failed")

-- pull both continuity errors out of the .rpt
local f = assert(io.open(rpt, "r"))
local text = f:read("*a"); f:close()
for v in text:gmatch("Continuity Error %(%%%)[%s%.]*(-?%d+%.%d+)") do
  print("continuity:", v .. " %")
end

Approach 2 — LuaJIT FFI on the official shared library

local ffi = require("ffi")   -- LuaJIT

ffi.cdef[[
  int    swmm_run(const char *f1, const char *f2, const char *f3);
  int    swmm_open(const char *f1, const char *f2, const char *f3);
  int    swmm_start(int saveFlag);
  int    swmm_step(double *elapsedTime);
  int    swmm_stride(int strideStep, double *elapsedTime);
  int    swmm_end(void);
  int    swmm_report(void);
  int    swmm_close(void);
  int    swmm_getMassBalErr(float *runoffErr, float *flowErr, float *qualErr);
  int    swmm_getVersion(void);
  int    swmm_getError(char *errMsg, int msgLen);
  int    swmm_getWarnings(void);
  int    swmm_getCount(int objType);
  void   swmm_getName(int objType, int index, char *name, int size);
  int    swmm_getIndex(int objType, const char *name);
  double swmm_getValue(int property, int index);
  void   swmm_setValue(int property, int index, double value);
  double swmm_getSavedValue(int property, int index, int period);
  void   swmm_writeLine(const char *line);
  void   swmm_decodeDate(double date, int *y, int *mo, int *d,
                         int *h, int *mi, int *s, int *dow);
]]

local swmm = ffi.load("swmm5")        -- swmm5.dll / libswmm5.so on the path

-- property codes (swmm5.h): NODE=2, LINK=3,
-- NODE_DEPTH=303, LINK_FLOW=410, LINK_SETTING=407
local NODE, LINK = 2, 3
local NODE_DEPTH, LINK_FLOW, LINK_SETTING = 303, 410, 407

assert(swmm.swmm_open("model.inp", "model.rpt", "model.out") == 0)
assert(swmm.swmm_start(1) == 0)

local pond = swmm.swmm_getIndex(NODE, "POND")
local gate = swmm.swmm_getIndex(LINK, "OR1")
local elapsed = ffi.new("double[1]")

while true do
  if swmm.swmm_step(elapsed) ~= 0 or elapsed[0] <= 0 then break end
  local d = swmm.swmm_getValue(NODE_DEPTH, pond)
  -- the same rule sample 03 runs in your browser:
  swmm.swmm_setValue(LINK_SETTING, gate, d > 6.5 and 1.0 or 0.15)
end

swmm.swmm_end(); swmm.swmm_report(); swmm.swmm_close()

Building the shared library

# Linux / macOS — one command, from the EPA source tree (tag v5.2.4):
gcc -O2 -fPIC -shared -fopenmp \
    src/solver/*.c -Isrc/solver -Isrc/solver/include -lm \
    -o libswmm5.so

# Windows — the EPA installer already ships swmm5.dll (5.2.x exports the
# full swmm_getValue/setValue API); or build with CMake from the same repo:
#   cmake -B build ; cmake --build build --config Release

Verified on this build

Corrections to the source note

This app grew out of an AI-written note on using Lua with SWMM5. The note's architecture (batch → FFI → hybrid) is sound; three details needed fixing:

Provenance

Honest limits