@@ -173,6 +173,106 @@ logEachMap( 'x: %0.4f, \t s: %0.4f, \t f(x,s): %0.4f', x, s, gammainc );
 
 <!-- /.examples -->
 
+<!-- C interface documentation. -->
+
+* * *
+
+<section class="c">
+
+## C APIs
+
+<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->
+
+<section class="intro">
+
+</section>
+
+<!-- /.intro -->
+
+<!-- C usage documentation. -->
+
+<section class="usage">
+
+### Usage
+
+```c
+#include "stdlib/math/base/special/gammainc.h"
+```
+
+#### stdlib_base_gammainc( x, a, regularized, upper )
+
+Evaluates the [incomplete gamma function][incomplete-gamma-function] for inputs `x` and `a`. The third and fourth parameters of the function can be used to specify whether instead to evaluate the non-regularized and/or upper incomplete gamma functions, respectively.
+
+```c
+double out = stdlib_base_gammainc( 0.0, 1.0, true, false );
+// returns 0.0
+
+out = stdlib_base_gammainc( 6.0, 2.0, true, false );
+// returns ~0.9826
+```
+
+The function accepts the following arguments:
+
+- **x**: `[in] double` input value.
+- **a**: `[in] double` input value.
+- **regularized**: `[in] bool` input value.
+- **upper**: `[in] bool` input value.
+
+```c
+double stdlib_base_gammainc( const double x, const double a, const bool regularized, const bool upper );
+```
+
+</section>
+
+<!-- /.usage -->
+
+<!-- C API usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->
+
+<section class="notes">
+
+</section>
+
+<!-- /.notes -->
+
+<!-- C API usage examples. -->
+
+<section class="examples">
+
+### Examples
+
+```c
+#include "stdlib/math/base/special/gammainc.h"
+#include <stdlib.h>
+#include <stdio.h>
+
+static double random_uniform( const double min, const double max ) {
+double v = (double)rand() / ( (double)RAND_MAX + 1.0 );
+return min + ( v*(max-min) );
+}
+
+int main( void ) {
+double x;
+double a;
+double y;
+int i;
+
+for ( i = 0; i < 10; i++ ) {
+x = random_uniform( 0.0, 1000.0 );
+a = random_uniform( 1.0, 1000.0 );
+y = stdlib_base_gammainc( x, a, true, false );
+printf( "x: %lf, s: %lf, f(x,s): %lf\n", x, a, y );
+}
+}
+```
+
+</section>
+
+<!-- /.examples -->
+
+</section>
+
+<!-- /.c -->
+
 <!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->
 
 <section class="related">

@@ -0,0 +1,135 @@
+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2025 The Stdlib Authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+'use strict';
+
+// MODULES //
+
+var resolve = require( 'path' ).resolve;
+var bench = require( '@stdlib/bench' );
+var uniform = require( '@stdlib/random/array/uniform' );
+var isnan = require( '@stdlib/math/base/assert/is-nan' );
+var tryRequire = require( '@stdlib/utils/try-require' );
+var pkg = require( './../package.json' ).name;
+
+
+// VARIABLES //
+
+var gammainc = tryRequire( resolve( __dirname, './../lib/native.js' ) );
+var opts = {
+'skip': ( gammainc instanceof Error )
+};
+
+
+// MAIN //
+
+bench( pkg+'::native:regularized=true,upper=true', opts, function benchmark( b ) {
+var x;
+var y;
+var z;
+var i;
+
+x = uniform( 100, 0.0, 100.0 );
+y = uniform( 100, 1.0, 100.0 );
+
+b.tic();
+for ( i = 0; i < b.iterations; i++ ) {
+z = gammainc( x[ i%x.length ], y[ i%y.length ], true, true );
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+}
+b.toc();
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+b.pass( 'benchmark finished' );
+b.end();
+});
+
+bench( pkg+'::native:regularized=true,upper=false', opts, function benchmark( b ) {
+var x;
+var y;
+var z;
+var i;
+
+x = uniform( 100, 0.0, 100.0 );
+y = uniform( 100, 1.0, 100.0 );
+
+b.tic();
+for ( i = 0; i < b.iterations; i++ ) {
+z = gammainc( x[ i%x.length ], y[ i%y.length ], true, false );
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+}
+b.toc();
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+b.pass( 'benchmark finished' );
+b.end();
+});
+
+bench( pkg+'::native:regularized=false,upper=true', opts, function benchmark( b ) {
+var x;
+var y;
+var z;
+var i;
+
+x = uniform( 100, 0.0, 100.0 );
+y = uniform( 100, 1.0, 100.0 );
+
+b.tic();
+for ( i = 0; i < b.iterations; i++ ) {
+z = gammainc( x[ i%x.length ], y[ i%y.length ], false, true );
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+}
+b.toc();
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+b.pass( 'benchmark finished' );
+b.end();
+});
+
+bench( pkg+'::native:regularized=false,upper=false', opts, function benchmark( b ) {
+var x;
+var y;
+var z;
+var i;
+
+x = uniform( 100, 0.0, 100.0 );
+y = uniform( 100, 1.0, 100.0 );
+
+b.tic();
+for ( i = 0; i < b.iterations; i++ ) {
+z = gammainc( x[ i%x.length ], y[ i%y.length ], false, false );
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+}
+b.toc();
+if ( isnan( z ) ) {
+b.fail( 'should not return NaN' );
+}
+b.pass( 'benchmark finished' );
+b.end();
+});

@@ -0,0 +1,146 @@
+#/
+# @license Apache-2.0
+#
+# Copyright (c) 2025 The Stdlib Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#/
+
+# VARIABLES #
+
+ifndef VERBOSE
+QUIET := @
+else
+QUIET :=
+endif
+
+# Determine the OS ([1][1], [2][2]).
+#
+# [1]: https://en.wikipedia.org/wiki/Uname#Examples
+# [2]: http://stackoverflow.com/a/27776822/2225624
+OS ?= $(shell uname)
+ifneq (, $(findstring MINGW,$(OS)))
+OS := WINNT
+else
+ifneq (, $(findstring MSYS,$(OS)))
+OS := WINNT
+else
+ifneq (, $(findstring CYGWIN,$(OS)))
+OS := WINNT
+else
+ifneq (, $(findstring Windows_NT,$(OS)))
+OS := WINNT
+endif
+endif
+endif
+endif
+
+# Define the program used for compiling C source files:
+ifdef C_COMPILER
+CC := $(C_COMPILER)
+else
+CC := gcc
+endif
+
+# Define the command-line options when compiling C files:
+CFLAGS ?= \
+-std=c99 \
+-O3 \
+-Wall \
+-pedantic
+
+# Determine whether to generate position independent code ([1][1], [2][2]).
+#
+# [1]: https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code-Gen-Options
+# [2]: http://stackoverflow.com/questions/5311515/gcc-fpic-option
+ifeq ($(OS), WINNT)
+fPIC ?=
+else
+fPIC ?= -fPIC
+endif
+
+# List of includes (e.g., `-I /foo/bar -I /beep/boop/include`):
+INCLUDE ?=
+
+# List of source files:
+SOURCE_FILES ?=
+
+# List of libraries (e.g., `-lopenblas -lpthread`):
+LIBRARIES ?=
+
+# List of library paths (e.g., `-L /foo/bar -L /beep/boop`):
+LIBPATH ?=
+
+# List of C targets:
+c_targets := benchmark.out
+
+
+# RULES #
+
+#/
+# Compiles source files.
+#
+# @param {string} [C_COMPILER] - C compiler (e.g., `gcc`)
+# @param {string} [CFLAGS] - C compiler options
+# @param {(string|void)} [fPIC] - compiler flag determining whether to generate position independent code (e.g., `-fPIC`)
+# @param {string} [INCLUDE] - list of includes (e.g., `-I /foo/bar -I /beep/boop/include`)
+# @param {string} [SOURCE_FILES] - list of source files
+# @param {string} [LIBPATH] - list of library paths (e.g., `-L /foo/bar -L /beep/boop`)
+# @param {string} [LIBRARIES] - list of libraries (e.g., `-lopenblas -lpthread`)
+#
+# @example
+# make
+#
+# @example
+# make all
+#/
+all: $(c_targets)
+
+.PHONY: all
+
+#/
+# Compiles C source files.
+#
+# @private
+# @param {string} CC - C compiler (e.g., `gcc`)
+# @param {string} CFLAGS - C compiler options
+# @param {(string|void)} fPIC - compiler flag determining whether to generate position independent code (e.g., `-fPIC`)
+# @param {string} INCLUDE - list of includes (e.g., `-I /foo/bar`)
+# @param {string} SOURCE_FILES - list of source files
+# @param {string} LIBPATH - list of library paths (e.g., `-L /foo/bar`)
+# @param {string} LIBRARIES - list of libraries (e.g., `-lopenblas`)
+#/
+$(c_targets): %.out: %.c
+$(QUIET) $(CC) $(CFLAGS) $(fPIC) $(INCLUDE) -o $@ $(SOURCE_FILES) $< $(LIBPATH) -lm $(LIBRARIES)
+
+#/
+# Runs compiled benchmarks.
+#
+# @example
+# make run
+#/
+run: $(c_targets)
+$(QUIET) ./$<
+
+.PHONY: run
+
+#/
+# Removes generated files.
+#
+# @example
+# make clean
+#/
+clean:
+$(QUIET) -rm -f *.o *.out
+
+.PHONY: clean