diff --git a/src/kwil_def.h b/src/kwil_def.h
new file mode 100644
index 0000000..3502418
--- /dev/null
+++ b/src/kwil_def.h
@@ -0,0 +1,126 @@
+#ifndef _kwil_def_H
+#define _kwil_def_H
+
+#include <stddef.h>
+
+// Type tag for kwil fields
+enum kwil_field_tag_t {
+    KWIL_FIELD_UNKNOWN = 0,         // the field's type is invalid/not known to kwil
+    KWIL_FIELD_FLOAT = 1,           // the field is a float
+    KWIL_FIELD_INT = 2,             // the field is an int
+    KWIL_FIELD_UNSIGNED = 3,        // the field is an unsigned int
+    KWIL_FIELD_CHAR = 4,            // the field is a character
+    KWIL_FIELD_CUSTOM = 6           // the field is a KWIL_STRUCT or KWIL_ENUM
+};
+extern enum kwil_field_tag_t kwil_field_tag_from_string(const char* str);
+
+enum kwil_type_tag_t {
+    KWIL_TYPE_STRUCT,
+    KWIL_TYPE_ENUM
+};
+
+struct kwil_enum_value_t {
+    char name[48];
+};
+
+struct kwil_enum_t {
+    // an array of strings with the possible enum values
+    struct kwil_enum_value_t* enum_values;
+    // the length of the enum_values array
+    size_t enum_values_len;
+    // the capacity of the enum_values array
+    size_t enum_values_cap;
+};
+
+// Instantiate a new kwil enum with a name and no values yet.
+extern void kwil_init_enum(struct kwil_enum_t* self);
+// Register an enum value with an enum
+extern int kwil_enum_add_value(struct kwil_enum_t* self, const char* name);
+// Free all memory related to a kwil enum.
+// DOES NOT FREE self*, if the self* was heap-allocated, make sure to free it as well.
+extern int kwil_free_enum(struct kwil_enum_t* self);
+
+// Data of a marked field of a marked struct.
+struct kwil_field_t {
+    // name of the field
+    char name_str[48];
+    // the type string of the field
+    char type_str[48];
+    // how to serialize the type
+    enum kwil_field_tag_t type_tag;
+    // the length of the static sized array, >0 if type_tag is an array
+    size_t array_length;
+    int array_dynamic;
+
+    // if type_tag is 
+    struct kwil_type_t* type_ptr;
+};
+
+// Instantiate a new kwil field with a name and type.
+extern int kwil_init_field(struct kwil_field_t* self, const char* field_name, const char* type_str);
+
+struct kwil_struct_t {
+    // the list of all fields in the type
+    struct kwil_field_t* fields;
+    // the used length of the fields array
+    size_t fields_len;
+    // the capacity of the fields array
+    size_t fields_cap;
+};
+
+// Instantiate new kwil struct data without fields
+// NOTE: intended to be used in tandem with kwil_type_t to store the struct's name.
+// kwil_init_struct does not store the name of the type.
+extern int kwil_init_struct(struct kwil_struct_t* self);
+// Register a field to a kwil struct
+extern int kwil_struct_add_field(struct kwil_struct_t* self, struct kwil_field_t* new);
+// Free all memory allocated for a kwil struct
+// DOES NOT FREE self*, if the self* was heap allocated, make sure to free it as well.
+extern int kwil_free_struct(struct kwil_struct_t* self);
+
+// A type that can be serialized with kwil
+struct kwil_type_t {
+    // the declared name of the type
+    char type_name[48];
+    // the type name switched to a tag
+    enum kwil_type_tag_t type_tag;
+    union {
+        // if type_tag is KWIL_FIELD_ENUM
+        struct kwil_enum_t enum_type;
+        // if type_tag is KWIL_FIELD_STRUCT
+        struct kwil_struct_t struct_type;
+    };
+};
+
+// Create a new kwil type from an existing kwil struct.
+// Copies the contents of the 'from' pointer.
+// 'from' pointer only needs to be valid for the duration of the function.
+extern int kwil_type_from_struct(struct kwil_type_t* self, struct kwil_struct_t* from, const char* name);
+// Create a new type from an existing enum.
+// Copies the contents of the 'from' pointer.
+// 'from' pointer only needs to be valid for the duration of the function.
+extern int kwil_type_from_enum(struct kwil_type_t* self, struct kwil_enum_t* from, const char* name);
+
+// All kwil-serializable data in a header
+struct kwil_header_t {
+    // name of the header without any other path elements
+    char file_name[48];
+    // array of kwil types defined in this header
+    struct kwil_type_t* types;
+    // length of kwil_types
+    size_t types_len;
+    // capacity of types
+    size_t types_cap;
+};
+
+// Instantiate a new kwil header with a filename
+extern int kwil_init_header(struct kwil_header_t* self, const char* filename);
+// Add an existing kwil type to the kwil header.
+// Copies the kwil_type_t and inserts it into kwil_types.
+// The 'new' pointer only needs to remain valid until the function returns.
+extern int kwil_header_add_type(struct kwil_header_t* self, struct kwil_type_t* new);
+// Free kwil header data and all associated memory.
+// Invalidates all pointers stored in kwil_types.
+extern int kwil_free_header(struct kwil_header_t* self);
+
+#endif // _kwil_def_H