diff --git a/apps/test-fpclassify.c b/apps/test-fpclassify.c
new file mode 100644
index 00000000..f50ca4f5
--- /dev/null
+++ b/apps/test-fpclassify.c
@@ -0,0 +1,24 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdint.h>
+#include <math.h>
+
+int main(int argc, char * argv[]) {
+	if (argc < 2) return 1;
+
+	union {
+		double asFloat;
+		uint64_t asInt;
+	} bits;
+
+	if (!strcmp(argv[1], "inf")) {
+		bits.asFloat = INFINITY;
+	} else if (!strcmp(argv[1], "nan")) {
+		bits.asFloat = NAN;
+	} else {
+		bits.asFloat = strtod(argv[1], NULL);
+	}
+
+	printf("0x%016zx %d\n", bits.asInt, fpclassify(bits.asFloat));
+	return 0;
+}
diff --git a/base/usr/include/math.h b/base/usr/include/math.h
index eb3709ec..d616bde3 100644
--- a/base/usr/include/math.h
+++ b/base/usr/include/math.h
@@ -46,4 +46,26 @@ extern double modf(double x, double *iptr);
 
 extern double hypot(double x, double y);
 
+extern double trunc(double x);
+extern double acosh(double x);
+extern double asinh(double x);
+extern double atanh(double x);
+extern double erf(double x);
+extern double erfc(double x);
+extern double gamma(double x);
+extern double lgamma(double x);
+extern double copysign(double x, double y);
+extern double remainder(double x, double y);
+
+enum {
+    FP_NAN, FP_INFINITE, FP_ZERO, FP_SUBNORMAL, FP_NORMAL
+};
+
+extern int fpclassify(double x);
+
+#define isfinite(x) ((fpclassify(x) != FP_NAN && fpclassify(x) != FP_INFINITE))
+#define isnormal(x) (fpclassify(x) == FP_NORMAL)
+#define isnan(x)    (fpclassify(x) == FP_NAN)
+#define isinf(x)    (fpclassify(x) == FP_INFINITE)
+
 _End_C_Header
diff --git a/libc/math/bad.c b/libc/math/bad.c
index a42b934b..934e6e06 100644
--- a/libc/math/bad.c
+++ b/libc/math/bad.c
@@ -49,3 +49,28 @@ double expm1(double x) {
 	BAD;
 	return exp(x) - 1.0;
 }
+
+double trunc(double x) {
+	BAD;
+	return (double)(long)x;
+}
+
+double acosh(double x) { BAD; return 0.0; }
+double asinh(double x) { BAD; return 0.0; }
+double atanh(double x) { BAD; return 0.0; }
+double erf(double x)   { BAD; return 0.0; }
+double erfc(double x)  { BAD; return 0.0; }
+double gamma(double x) { BAD; return 0.0; }
+double lgamma(double x){ BAD; return 0.0; }
+double remainder(double x, double y) { BAD; return 0.0; }
+
+double copysign(double x, double y) {
+	if (y < 0) {
+		if (x < 0) return x;
+		return -x;
+	} else {
+		if (x < 0) return -x;
+		return x;
+	}
+}
+
diff --git a/libc/math/math.c b/libc/math/math.c
index 44bc406d..bc3e1eff 100644
--- a/libc/math/math.c
+++ b/libc/math/math.c
@@ -469,3 +469,18 @@ double tanh(double x) {
 	return (exp(2.0*x) - 1.0) / (exp(2.0*x) + 1.0);
 }
 
+int fpclassify(double x) {
+	union {
+		double asFloat;
+		uint64_t asInt;
+	} bits = {x};
+	uint64_t exponent = (bits.asInt >> 52) & 0x7FF;
+	uint64_t mantissa = (bits.asInt & 0xFffffFFFFffffULL);
+
+	if (exponent == 0x7FF) {
+		return mantissa ? FP_NAN : FP_INFINITE;
+	} else if (exponent == 0) {
+		return mantissa ? FP_SUBNORMAL : FP_ZERO;
+	}
+	return FP_NORMAL;
+}