Stata C Plugins

Stata C/C++ Plugin Development

Build high-performance C/C++ plugins for Stata. This skill covers the full lifecycle from SDK setup through cross-platform distribution, based on real experience building production Stata plugins for statistical imputation, random forests, string matching, and causal inference.

This skill assumes macOS (Apple Silicon or Intel) as the development platform. Build commands, cross-compilation workflows, and Docker instructions are all Mac-oriented. The plugins themselves target all four platforms (macOS ARM64, macOS x86_64, Linux x86_64, Windows x86_64), but the development environment is macOS. If you need to develop on Linux or Windows natively, adapt the compilation and Docker sections accordingly.

How to Approach Every Task

Before writing any code, enter plan mode. A good plan covers:

1. Complete inventory — every feature, option, and component to build (for translation: exhaustive catalog of the source package's API)
2. Architecture decisions — wrap C++ backend vs. write C from scratch vs. pure Stata
3. Relevant reference files — identify up front which of this skill's reference files contain info you'll need, and cite them explicitly in the plan steps so they get loaded at the right time:
   • references/translation_workflow.md — full translation workflow, test repurposing, fidelity audit
   • references/testing_strategy.md — test layers, reference data generation, Layer 0 (repurpose original tests)
   • references/performance_patterns.md — pthreads, XorShift RNG, quickselect, pre-sorted indices
   • references/packaging_and_help.md — .toc/.pkg/.sthlp templates, build scripts
   • references/cpp_plugins.md — C++ wrapping, extern "C", exception safety, compilation
4. Phase-by-phase steps with dependencies between them
5. For each step: what gets built, what tests get written, and that the review loop runs before proceeding
6. For translation projects: a final fidelity audit as the last step (see translation_workflow.md)

Implement sequentially across components, in parallel within each component. Once an interface is defined, dispatch independent sub-tasks as parallel subagents (e.g., C plugin implementation, .ado wrapper, and test suite can run simultaneously). Merge their work, run the full test suite, then proceed to the review loop before moving to the next component.

Run the review loop after every component:

• Default: dispatch 2-3 review agents in parallel, ideally from different models (e.g., Claude + GPT + Gemini) for diversity of perspective. Use whatever multi-model tools are available in your environment.
• If only one model is available: dispatch 2-3 agents with different review focuses (correctness, completeness, architecture). Different prompts approximate the diversity of different models.
• Each agent reviews the diff, test results, and requirements — instruction: "List any gaps, bugs, or issues. Say LGTM if everything looks correct."
• Fix all issues raised, re-dispatch, loop until all agents say LGTM. Then proceed.

Wrap First, Write From Scratch Second

When translating a package, always check for an existing C/C++ backend before writing any algorithm code. Many R packages have C++ in src/. Many Python packages have Cython or vendored C/C++ libraries. Standalone C++ libraries exist for string matching, linear algebra, tree algorithms, and more.

If a C++ implementation exists, wrap it. Do not reimplement the algorithm in C. Wrapping gives you identical output (same code path), production-grade performance, and a fraction of the code. The plugin is just a thin extern "C" glue layer between Stata's SDK and the library's API. Binary size is irrelevant — statically link everything (-static-libstdc++ -static-libgcc) and ship whatever size the binary turns out to be, even 10-15 MB on Windows. Users don't care about plugin file size; they care about correct results.

See references/cpp_plugins.md for the full pattern and references/translation_workflow.md for the workflow. Working examples of this approach (wrapping C++ backends, multi-plugin dispatching, save/load for scoring on new data) can be found in the repos listed in the project CLAUDE.md under "Example Applications."

For translation projects, also: repurpose the original package's test suite and data (see references/testing_strategy.md Layer 0), write additional Stata-specific tests, and end the plan with a multi-agent fidelity audit. See references/translation_workflow.md for the complete workflow.

The Plugin SDK

Download stplugin.h and stplugin.c from: https://www.stata.com/plugins/

These two files define the interface between your C code and Stata:

Indexing is 1-based. Both variable indices and observation indices start at 1, not 0. Off-by-one errors here are silent and catastrophic — you read the wrong variable's data with no warning.

Memory Safety

A crash in your plugin kills the entire Stata session. No save prompt, no recovery. The user loses all unsaved work. This is the single most important thing to internalize.

• Check every malloc()/calloc() return for NULL
• Validate argc before accessing argv[]
• Build with -fsanitize=address during development
• Test on small data first, scale up gradually
• Pre-allocate all memory upfront in stata_call(), free at the end

The stata_call() Entry Point

Every plugin implements one function. Plugins can also be written in C++ — the entry point just needs extern "C" linkage so Stata can find it; everything else can be full C++. The obvious case for C++ is when existing C++ code is available to wrap (e.g., an R package's src/ directory). C++ also helps when you need complex data structures or threading via std::thread. For practical C++ guidance — the extern "C" pattern, exception safety, compilation commands, wrapping libraries — see references/cpp_plugins.md. The rest of this file focuses on C because it's the simpler default.

#include "stplugin.h"

// For C++ plugins, wrap the entry point with extern "C":
//   extern "C" {
//     STDLL stata_call(int argc, char *argv[]) { ... }
//   }

STDLL stata_call(int argc, char *argv[]) {
    // 0. Validate arguments BEFORE accessing argv[]
    if (argc < 3) {
        SF_error("myplugin requires 3 arguments: n_train n_test seed\n");
        return 198;  // Stata's "syntax error" code
    }

    // 1. Parse arguments (all strings — use atoi/atof)
    int n_train = atoi(argv[0]);
    int n_test  = atoi(argv[1]);
    int seed    = atoi(argv[2]);

    // 2. Get dimensions
    ST_int nobs  = SF_nobs();
    // CAUTION: SF_nvar() returns ALL variables in the dataset, not just
    // the ones passed to `plugin call`. If the .ado creates tempvars
    // (touse, merge_id, etc.) the count will be higher than expected.
    // Pass the variable count via argv instead of relying on SF_nvar().
    int p = atoi(argv[3]);  // safer: pass feature count explicitly

    // 3. Allocate memory
    double *X    = calloc(nobs * p, sizeof(double));
    double *y    = calloc(nobs, sizeof(double));
    double *pred = calloc(nobs, sizeof(double));
    if (!X || !y || !pred) {
        SF_error("myplugin: out of memory\n");
        if (X) free(X); if (y) free(y); if (pred) free(pred);
        return 909;
    }

    // 4. Read data from Stata (1-indexed!)
    ST_double val;
    for (ST_int obs = 1; obs <= nobs; obs++) {
        SF_vdata(1, obs, &val);      // var 1 = depvar
        y[obs-1] = val;
        for (int j = 0; j < p; j++) {
            SF_vdata(j + 2, obs, &val);  // vars 2..nvars-1 = features
            X[(obs-1) * p + j] = val;
        }
    }

    // 5. Run your algorithm
    int rc = my_algorithm(X, y, pred, n_train, n_test, p, seed);
    if (rc != 0) {
        SF_error("myplugin: algorithm failed\n");
        free(X); free(y); free(pred);
        return 909;
    }

    // 6. Write results back to Stata
    for (ST_int obs = 1; obs <= nobs; obs++) {
        SF_vstore(nvars, obs, pred[obs-1]);  // last var = output
    }

    free(X); free(y); free(pred);
    return 0;  // 0 = success
}

Return Codes

• 0 — success
• 198 — syntax error (bad arguments)
• 909 — insufficient memory
• 601 — file not found
• Any non-zero triggers a Stata error

The .ado Wrapper Pattern

Users never call plugin call directly. An .ado file provides the Stata-native interface.

The Preserve/Merge Pattern

This is the core pattern for plugins that operate on a subset of data:

program define mycommand, rclass
    syntax varlist(min=2) [if] [in], GENerate(name) [SEED(integer 12345) REPlace]

    gettoken depvar indepvars : varlist

    if "`replace'" != "" {
        capture drop `generate'
    }
    confirm new variable `generate'

    // Mark sample: novarlist ALLOWS missing depvar (critical for imputation)
    marksample touse, novarlist
    markout `touse' `indepvars'   // but DO exclude missing predictors

    // Stable merge key — create BEFORE any sorting or subsetting
    tempvar merge_id
    quietly gen long `merge_id' = _n

    // Count subsets
    quietly count if `touse' & !missing(`depvar')
    local n_train = r(N)
    quietly count if `touse' & missing(`depvar')
    local n_test = r(N)

    // Create output variable (all missing initially)
    quietly gen double `generate' = .

    // Preserve, subset, call plugin
    preserve
    quietly keep if `touse'

    // Sort if plugin requires it (donors first, test second)
    tempvar sort_order
    quietly gen `sort_order' = missing(`depvar')
    quietly sort `sort_order'

    // Call plugin
    plugin call myplugin `depvar' `indepvars' `generate', ///
        `n_train' `n_test' `seed'

    // Save results and restore
    tempfile results
    quietly keep `merge_id' `generate'
    quietly save `results'
    restore

    // Merge predictions back (update replaces missing with non-missing)
    quietly merge 1:1 `merge_id' using `results', nogenerate update
end

Why update works: The generate variable is all-missing before preserve. After restore, it's still all-missing. The update option replaces missing values with non-missing ones from the merge file. The replace option is handled earlier via capture drop, so by merge time the variable is always freshly created.

Plugin Sorting Contract

CRITICAL: Some plugins expect data sorted a specific way (training rows first, test rows second). Others handle missing data internally. Sorting mismatches are among the most dangerous bugs — the plugin silently reads the wrong data, producing garbage output with no error message. A mismatched sort order can drop prediction quality dramatically (e.g., correlation going from 0.99 to 0.38) because the plugin treats test observations as training data and vice versa.

• If the plugin checks SF_is_missing() internally: do NOT sort in the .ado wrapper
• If the plugin expects n_train contiguous rows then n_test rows: sort by missing(depvar) before calling

Document which pattern your plugin uses.

Plugin Loading (Cross-Platform)

Use the gtools-style OS detection pattern. This detects the OS via c(os) and constructs a bare filename. The bare filename is resolved via Stata's adopath, which is reliable across all platforms.

/* ---- Load plugin (gtools-style: detect OS, bare filename) ---- */
if ( inlist("`c(os)'", "MacOSX") | strpos("`c(machine_type)'", "Mac") ) local c_os_ macosx
else local c_os_: di lower("`c(os)'")

cap program drop myplugin
program myplugin, plugin using("myplugin_`c_os_'.plugin")

This resolves to myplugin_macosx.plugin, myplugin_windows.plugin, or myplugin_unix.plugin depending on platform.

WARNING — DO NOT use findfile + absolute paths. The following pattern is BROKEN on Windows and must never be used:

* BROKEN — DO NOT USE
capture findfile myplugin.plugin
capture program myplugin, plugin using("`r(fn)'")

findfile returns an absolute path (e.g., C:\ado\plus\m\myplugin.plugin). On Windows, Stata's LoadLibrary call fails when given certain absolute paths via using(). The gtools-style pattern avoids this by passing a bare filename (no path), which Stata resolves via the adopath — exactly how gtools, ftools, and other major packages work.

Similarly, do not use a nested if/else cascade trying each platform-arch suffix. This was the old pattern in several packages and fails for the same reason if findfile is involved, plus it's fragile and verbose.

Plugin file naming: pluginname_os.plugin where os is one of macosx, unix, windows. Examples: qrf_plugin_macosx.plugin, grf_plugin_windows.plugin.

Note: clear all wipes loaded plugin definitions. If a test script starts with clear all, all program ... plugin definitions are gone. Reload them.

Cross-Platform Compilation

Build for three platforms (ARM Macs run x86_64 via Rosetta, so one macOS binary suffices). Install the Windows cross-compiler first: brew install mingw-w64.

All platforms: -O3 -fPIC for release, add -g -fsanitize=address for development.

For C++ plugins: use g++ instead of gcc. Add -std=c++ at the version the library requires (check its docs — C++11, C++14, and C++17 are all common). Header-only C++ libraries can be vendored into c_source/ and included with -I.. Always use -static-libstdc++ -static-libgcc on Windows and Linux.

Naming convention: pluginname_os.plugin (e.g., qrf_plugin_macosx.plugin, grf_plugin_windows.plugin). The os suffix must match what the gtools-style loader produces: macosx, unix, or windows.

macOS note: use -bundle, NOT -shared. This is a common mistake.

Linux from macOS (Docker Required)

There is no native Linux cross-compiler on macOS. Use Docker via Colima (brew install colima docker, then colima start). Build with a one-liner:

docker run --rm --platform linux/amd64 -v "$(pwd):/build" -w /build ubuntu:18.04 \
    bash -c "apt-get update -qq && apt-get install -y -qq g++ gcc make > /dev/null 2>&1 && make linux"

glibc compatibility: Build on Ubuntu 18.04 for maximum compatibility (requires only GLIBC 2.14, works on any Linux from ~2012+). Building on Ubuntu 22.04+ requires GLIBC 2.34, which excludes RHEL 8, Ubuntu 20.04, and many HPC environments.

Performance Optimization

See references/performance_patterns.md for detailed code examples of:

1. Pre-sorted feature indices — Sort feature values once, scan linearly at each tree node. O(n) per split instead of O(n log n).
2. Precomputed distance norms — Exploit ||a-b||^2 = ||a||^2 + ||b||^2 - 2*a'b for KNN.
3. Quickselect — O(n) partial sort for finding k-th nearest neighbor.
4. Parallel ensemble training (pthreads) — Train multiple models concurrently. Each thread gets its own data copy and RNG state. Never call Stata SDK functions (SF_vdata, SF_vstore, SF_display) from worker threads — read all data on the main thread first, dispatch computation to workers, write results back on the main thread after joining.
5. XorShift RNG — C plugins cannot access Stata's internal RNG (runiform()). XorShift128+ is fast, statistically sound, and thread-safe (each thread gets its own state). Seed from argv[] for reproducibility.
6. Dense arrays for trees — Flat node arrays instead of linked lists for cache locality.

Debugging

Debugging is hard because you can't attach a debugger to Stata's plugin host.

Strategies

1. Printf via SF_display():

   char buf[256];
   snprintf(buf, sizeof(buf), "Debug: n=%d, p=%d\n", n, p);
   SF_display(buf);
   

2. Write diagnostic files:

   FILE *f = fopen("plugin_debug.log", "w");
   fprintf(f, "value at [%d][%d] = %f\n", i, j, val);
   fclose(f);
   

3. Test standalone first. Write a main() that reads CSV and calls your algorithm. Debug with normal tools (gdb, valgrind, sanitizers). Then adapt for the plugin interface.

4. Build with sanitizers during development: -g -fsanitize=address

5. Check SF_vdata() return values. It returns RC (0=success). Non-zero means invalid obs/var index.

Common Failure Modes

Packaging and Distribution

Use platform-specific .pkg files so users only download the binary for their OS. Stata's net install has no conditional logic, so the way to avoid shipping all 4 binaries to every user is to offer separate packages per platform. All packages install the same .ado and .sthlp files — only the .plugin binary differs.

mypackage/
├── stata.toc                          # lists all package variants
├── mypackage.pkg                      # all platforms (for users who don't care)
├── mypackage_mac.pkg                  # macOS only
├── mypackage_linux.pkg                # Linux only
├── mypackage_win.pkg                  # Windows only
├── mycommand.sthlp                    # overview help file (short name!)
├── mycommand.ado                      # user-facing command
├── myplugin_macosx.plugin
├── myplugin_unix.plugin
├── myplugin_windows.plugin
└── c_source/                          # NOT distributed, for building
    ├── build.py
    ├── stplugin.c
    ├── stplugin.h
    └── algorithm.c

Users install their platform's package:

* macOS
net install mypackage_mac, from("https://raw.githubusercontent.com/user/repo/main") replace
* Linux
net install mypackage_linux, from("https://raw.githubusercontent.com/user/repo/main") replace
* Windows
net install mypackage_win, from("https://raw.githubusercontent.com/user/repo/main") replace

All platform binaries ship via the all-platform .pkg, or users can install platform-specific packages. Stata loads only the matching plugin at runtime via gtools-style OS detection. Windows C++ binaries can be 10-15MB due to static linking, which is normal.

See references/packaging_and_help.md for .toc, .pkg, .sthlp templates and SMCL formatting.

Common Pitfalls

1. Sorting destroys merge keys. If you sort inside preserve/restore, the merge_id linkage breaks. Always create merge_id BEFORE preserve.

2. 1-indexed everything. SF_vdata(var, obs, &val) — both var and obs start at 1. Off-by-one errors are silent.

3. marksample excludes missing by default. For imputation (where missing depvar IS the point), use marksample touse, novarlist.

4. macOS c(os) returns "MacOSX". Use the gtools pattern: inlist("c(os)'", "MacOSX") | strpos("c(machine_type)'", "Mac") to detect Mac. For other platforms, lower(c(os)) gives "windows" or "unix".

5. argv[] has no bounds checking. Accessing argv[3] when argc == 2 is a segfault. Always check argc first.

6. clear all wipes plugins. Reload plugin definitions after clear all in test scripts.

7. Only the first program define in a .ado file is auto-discovered. Subprograms need their own .ado files or explicit run to load.

8. Normalize inputs when the algorithm requires it (neural networks, gradient-based methods, distance-based methods like KNN). Scale to mean=0, sd=1 in the .ado wrapper, denormalize predictions after. The plugin should receive clean, normalized data — let the .ado handle the scaling.

9. pthreads on Windows needs -lwinpthread. Use conditional linker flags.

10. Memory errors crash Stata with no recovery. Pre-allocate everything, check every allocation, build with sanitizers during development.

11. glibc version mismatch. Building Linux plugins on a modern distro produces binaries that won't load on older systems. Use Ubuntu 18.04 in Docker for maximum compatibility.

12. SF_nvar() returns total dataset variables. It counts ALL variables in the dataset, not just the ones in the plugin call varlist. If the .ado creates tempvars (touse, merge_id, sort keys), the count will be higher than expected. Never use SF_nvar() to validate argument counts — pass the expected count via argv instead.

13. findfile + absolute paths breaks on Windows. findfile returns an absolute path that Stata's LoadLibrary can't resolve on Windows. Use the gtools-style OS detection pattern instead (see Plugin Loading section above) — it constructs a bare filename that Stata resolves via the adopath.

Naming Conventions

• Use method() not model() for method selection options
• Use generate() (abbreviation gen()) for output variable naming
• Use replace as a flag option, not replace()
• Plugin files: algorithm_plugin_os.plugin where os is macosx, unix, or windows
• .ado files: lowercase, underscores for multi-word
• Stata option convention: options lowercase, abbreviations capitalized (GENerate, MAXDepth)
• Target Stata 14.0+ (version 14.0) for plugin support
• Help files use the short command name, not the repo name. If the repo is called mypackage_stata, the overview help file should still be mypackage.sthlp (so help mypackage works). Don't append "stata" to help file or command names — the user is already in Stata.