diff --git a/exercises/practice/all-your-base/all_your_base.h b/exercises/practice/all-your-base/all_your_base.h
index a48979964..73ed28095 100644
--- a/exercises/practice/all-your-base/all_your_base.h
+++ b/exercises/practice/all-your-base/all_your_base.h
@@ -3,4 +3,7 @@
 
 #define DIGITS_ARRAY_SIZE 64
 
+size_t rebase(int8_t digits[DIGITS_ARRAY_SIZE], int16_t from_base,
+              int16_t to_base, size_t num_digits);
+
 #endif
diff --git a/exercises/practice/allergies/allergies.h b/exercises/practice/allergies/allergies.h
index e34a18cdf..42f93fc0d 100644
--- a/exercises/practice/allergies/allergies.h
+++ b/exercises/practice/allergies/allergies.h
@@ -20,4 +20,7 @@ typedef struct {
    bool allergens[ALLERGEN_COUNT];
 } allergen_list_t;
 
+bool is_allergic_to(allergen_t allergen, unsigned int score);
+allergen_list_t get_allergens(unsigned int score);
+
 #endif
diff --git a/exercises/practice/circular-buffer/circular_buffer.h b/exercises/practice/circular-buffer/circular_buffer.h
index 9f809d42c..ec13e26d0 100644
--- a/exercises/practice/circular-buffer/circular_buffer.h
+++ b/exercises/practice/circular-buffer/circular_buffer.h
@@ -1,4 +1,30 @@
 #ifndef CIRCULAR_BUFFER_H
 #define CIRCULAR_BUFFER_H
 
+#include <stddef.h>
+#include <stdint.h>
+
+typedef int16_t buffer_value_t;
+typedef struct circular_buffer_t circular_buffer_t;
+
+// constructs a new buffer
+circular_buffer_t *new_circular_buffer(size_t capacity);
+
+// read next value from buffer
+int16_t read(circular_buffer_t *buffer, buffer_value_t *value);
+
+// write a value to the buffer
+int16_t write(circular_buffer_t *buffer, buffer_value_t value);
+
+// write a value to the buffer
+// overwrites the oldest value if the buffer is already full
+int16_t overwrite(circular_buffer_t *buffer, buffer_value_t value);
+
+// clear the buffer
+void clear_buffer(circular_buffer_t *buffer);
+
+// destroy the entire buffer
+// buffer will be a dangling pointer after calling this method on it
+void delete_buffer(circular_buffer_t *buffer);
+
 #endif
diff --git a/exercises/practice/darts/darts.h b/exercises/practice/darts/darts.h
index 287cf1f32..194a6df8a 100644
--- a/exercises/practice/darts/darts.h
+++ b/exercises/practice/darts/darts.h
@@ -1,4 +1,12 @@
 #ifndef DARTS_H
 #define DARTS_H
 
+#include <stdint.h>
+
+typedef struct {
+   float x, y;
+} coordinate_t;
+
+uint8_t score(coordinate_t landing_position);
+
 #endif
diff --git a/exercises/practice/perfect-numbers/perfect_numbers.h b/exercises/practice/perfect-numbers/perfect_numbers.h
index 13f32a648..400c126ee 100644
--- a/exercises/practice/perfect-numbers/perfect_numbers.h
+++ b/exercises/practice/perfect-numbers/perfect_numbers.h
@@ -8,4 +8,6 @@ typedef enum {
    ERROR = -1
 } kind;
 
+kind classify_number(int);
+
 #endif
diff --git a/exercises/practice/pythagorean-triplet/pythagorean_triplet.h b/exercises/practice/pythagorean-triplet/pythagorean_triplet.h
index a911dfa55..aca768c0f 100644
--- a/exercises/practice/pythagorean-triplet/pythagorean_triplet.h
+++ b/exercises/practice/pythagorean-triplet/pythagorean_triplet.h
@@ -1,4 +1,20 @@
 #ifndef PYTHAGOREAN_TRIPLET_H
 #define PYTHAGOREAN_TRIPLET_H
 
+#include <stdint.h>
+#include <stdlib.h>
+
+typedef struct {
+   uint16_t a, b, c;
+} triplet_t;
+
+typedef struct {
+   size_t count;
+   triplet_t triplets[];
+} triplets_t;
+
+triplets_t *triplets_with_sum(uint16_t sum);
+
+void free_triplets(triplets_t *triplets);
+
 #endif
diff --git a/exercises/practice/rational-numbers/rational_numbers.h b/exercises/practice/rational-numbers/rational_numbers.h
index f00b697de..0928221dc 100644
--- a/exercises/practice/rational-numbers/rational_numbers.h
+++ b/exercises/practice/rational-numbers/rational_numbers.h
@@ -1,4 +1,26 @@
 #ifndef RATIONAL_NUMBERS_H
 #define RATIONAL_NUMBERS_H
 
+#include <stdint.h>
+
+typedef struct {
+   int16_t numerator, denominator;
+} rational_t;
+
+rational_t add(rational_t r1, rational_t r2);
+
+rational_t subtract(rational_t r1, rational_t r2);
+
+rational_t multiply(rational_t r1, rational_t r2);
+
+rational_t divide(rational_t r1, rational_t r2);
+
+rational_t absolute(rational_t r);
+
+rational_t exp_rational(rational_t r, uint16_t n);
+
+float exp_real(uint16_t n, rational_t r);
+
+rational_t reduce(rational_t r);
+
 #endif
diff --git a/exercises/practice/resistor-color-duo/resistor_color_duo.h b/exercises/practice/resistor-color-duo/resistor_color_duo.h
index d432fc8e3..fe8f3f5d9 100644
--- a/exercises/practice/resistor-color-duo/resistor_color_duo.h
+++ b/exercises/practice/resistor-color-duo/resistor_color_duo.h
@@ -1,4 +1,11 @@
 #ifndef RESISTOR_COLOR_DUO_H
 #define RESISTOR_COLOR_DUO_H
 
+#include <stdint.h>
+
+typedef enum {
+} resistor_band_t;
+
+uint16_t color_code(resistor_band_t colors[]);
+
 #endif
diff --git a/exercises/practice/resistor-color-trio/resistor_color_trio.h b/exercises/practice/resistor-color-trio/resistor_color_trio.h
index 903a06726..f96f6a48c 100644
--- a/exercises/practice/resistor-color-trio/resistor_color_trio.h
+++ b/exercises/practice/resistor-color-trio/resistor_color_trio.h
@@ -1,4 +1,19 @@
 #ifndef RESISTOR_COLOR_TRIO_H
 #define RESISTOR_COLOR_TRIO_H
 
+#include <stdint.h>
+
+typedef enum {
+} resistor_band_t;
+
+typedef enum {
+} resistor_unit_t;
+
+typedef struct {
+   uint16_t value;
+   resistor_unit_t unit;
+} resistor_value_t;
+
+resistor_value_t color_code(resistor_band_t colors[]);
+
 #endif
diff --git a/exercises/practice/resistor-color/resistor_color.h b/exercises/practice/resistor-color/resistor_color.h
index 6adf79773..5a09a9752 100644
--- a/exercises/practice/resistor-color/resistor_color.h
+++ b/exercises/practice/resistor-color/resistor_color.h
@@ -1,8 +1,13 @@
 #ifndef RESISTOR_COLOR_H
 #define RESISTOR_COLOR_H
 
-typedef enum {
+#include <stdint.h>
 
+typedef enum {
 } resistor_band_t;
 
+uint16_t color_code(resistor_band_t color);
+
+const resistor_band_t *colors(void);
+
 #endif
diff --git a/exercises/practice/saddle-points/saddle_points.h b/exercises/practice/saddle-points/saddle_points.h
index ab8d5ea7f..9006f998c 100644
--- a/exercises/practice/saddle-points/saddle_points.h
+++ b/exercises/practice/saddle-points/saddle_points.h
@@ -1,4 +1,21 @@
 #ifndef SADDLE_POINTS_H
 #define SADDLE_POINTS_H
 
+#include <stddef.h>
+#include <stdint.h>
+
+typedef struct {
+   size_t row, column;
+} saddle_point_t;
+
+typedef struct {
+   size_t count;
+   saddle_point_t points[];
+} saddle_points_t;
+
+saddle_points_t *saddle_points(size_t rows, size_t columns,
+                               uint8_t matrix[][columns]);
+
+void free_saddle_points(saddle_points_t *saddle_points);
+
 #endif
diff --git a/exercises/practice/square-root/square_root.h b/exercises/practice/square-root/square_root.h
index 685c00f24..8580295d2 100644
--- a/exercises/practice/square-root/square_root.h
+++ b/exercises/practice/square-root/square_root.h
@@ -1,4 +1,8 @@
 #ifndef SQUARE_ROOT_H
 #define SQUARE_ROOT_H
 
+#include <stdint.h>
+
+uint16_t square_root(uint16_t radicand);
+
 #endif
diff --git a/exercises/practice/triangle/triangle.h b/exercises/practice/triangle/triangle.h
index 9722a9b64..8bedf0e44 100644
--- a/exercises/practice/triangle/triangle.h
+++ b/exercises/practice/triangle/triangle.h
@@ -1,10 +1,16 @@
 #ifndef TRIANGLE_H
 #define TRIANGLE_H
 
+#include <stdbool.h>
+
 typedef struct {
    double a;
    double b;
    double c;
 } triangle_t;
 
+bool is_equilateral(triangle_t sides);
+bool is_isosceles(triangle_t sides);
+bool is_scalene(triangle_t sides);
+
 #endif