diff --git a/base/assert/is-nonpositive-integer/README.md b/base/assert/is-nonpositive-integer/README.md
index 9d7e43b27..69467691c 100644
--- a/base/assert/is-nonpositive-integer/README.md
+++ b/base/assert/is-nonpositive-integer/README.md
@@ -110,6 +110,97 @@ bool = isNonPositiveInteger( NaN );
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+* * *
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+## C APIs
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+### Usage
+
+```c
+#include "stdlib/math/base/assert/is_nonpositive_integer.h"
+```
+
+#### stdlib_base_is_nonpositive_integer( x )
+
+Tests if a finite double-precision floating-point number is a nonpositive integer.
+
+```c
+#include
+
+bool out = stdlib_base_is_nonpositive_integer( -5.0 );
+// returns true
+
+out = stdlib_base_is_nonpositive_integer( 0.0 );
+// returns true
+```
+
+The function accepts the following arguments:
+
+- **x**: `[in] double` input value.
+
+```c
+bool stdlib_base_is_nonpositive_integer( const double x );
+```
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+### Examples
+
+```c
+#include "stdlib/math/base/assert/is_nonpositive_integer.h"
+#include
+#include
+
+int main( void ) {
+ const double x[] = { 5.0, -5.0, 3.14, -3.14, 0.0, 0.0/0.0 };
+
+ bool b;
+ int i;
+ for ( i = 0; i < 6; i++ ) {
+ b = stdlib_base_is_nonpositive_integer( x[ i ] );
+ printf( "x = %lf, is_nonpositive_integer(x) = %s\n", x[ i ], ( b ) ? "True" : "False" );
+ }
+}
+```
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