diff --git a/ffi/src/main/kotlin/gay/pizza/pork/ffi/FfiFunctionDefinition.kt b/ffi/src/main/kotlin/gay/pizza/pork/ffi/FfiFunctionDefinition.kt
index 2173f3d..92caadf 100644
--- a/ffi/src/main/kotlin/gay/pizza/pork/ffi/FfiFunctionDefinition.kt
+++ b/ffi/src/main/kotlin/gay/pizza/pork/ffi/FfiFunctionDefinition.kt
@@ -3,7 +3,8 @@ package gay.pizza.pork.ffi
 class FfiFunctionDefinition(
   val library: String,
   val function: String,
-  val returnType: String
+  val returnType: String,
+  val parameters: List<String>
 ) {
   companion object {
     fun parse(def: String): FfiFunctionDefinition {
@@ -15,7 +16,13 @@ class FfiFunctionDefinition(
           "but '${def}' was specified")
       }
       val (library, function, returnType) = parts
-      return FfiFunctionDefinition(library, function, returnType)
+      val parametersString = if (parts.size == 4) parts[3] else ""
+      return FfiFunctionDefinition(
+        library,
+        function,
+        returnType,
+        parametersString.split(",")
+      )
     }
   }
 }
diff --git a/ffi/src/main/kotlin/gay/pizza/pork/ffi/JnaNativeProvider.kt b/ffi/src/main/kotlin/gay/pizza/pork/ffi/JnaNativeProvider.kt
index 97925fd..ed0d5e7 100644
--- a/ffi/src/main/kotlin/gay/pizza/pork/ffi/JnaNativeProvider.kt
+++ b/ffi/src/main/kotlin/gay/pizza/pork/ffi/JnaNativeProvider.kt
@@ -1,6 +1,7 @@
 package gay.pizza.pork.ffi
 
 import com.sun.jna.Function
+import com.sun.jna.Pointer
 import gay.pizza.pork.ast.ArgumentSpec
 import gay.pizza.pork.evaluator.CallableFunction
 import gay.pizza.pork.evaluator.NativeProvider
@@ -12,20 +13,22 @@ class JnaNativeProvider : NativeProvider {
     return CallableFunction { functionArgs ->
       val ffiArgs = mutableListOf<Any?>()
       for ((index, spec) in arguments.withIndex()) {
+        val ffiType = functionDefinition.parameters[index]
         if (spec.multiple) {
           val variableArguments = functionArgs.values
             .subList(index, functionArgs.values.size)
           ffiArgs.addAll(variableArguments)
           break
         } else {
-          ffiArgs.add(functionArgs.values[index])
+          val converted = convert(ffiType, functionArgs.values[index])
+          ffiArgs.add(converted)
         }
       }
       invoke(function, ffiArgs.toTypedArray(), functionDefinition.returnType)
     }
   }
 
-  private fun invoke(function: Function, values: Array<Any?>, type: String): Any = when (type) {
+  private fun invoke(function: Function, values: Array<Any?>, type: String): Any = when (rewriteType(type)) {
     "void*" -> function.invokePointer(values)
     "int" -> function.invokeInt(values)
     "long" -> function.invokeLong(values)
@@ -35,4 +38,48 @@ class JnaNativeProvider : NativeProvider {
     "char*" -> function.invokeString(values, false)
     else -> throw RuntimeException("Unsupported ffi return type: $type")
   }
+
+  private fun rewriteType(type: String): String = when (type) {
+    "size_t" -> "long"
+    else -> type
+  }
+
+  private fun convert(type: String, value: Any?): Any? = when (rewriteType(type)) {
+    "short" -> numberConvert(type, value) { toShort() }
+    "unsigned short" -> numberConvert(type, value) { toShort().toUShort() }
+    "int" -> numberConvert(type, value) { toInt() }
+    "unsigned int" -> numberConvert(type, value) { toInt().toUInt() }
+    "long" -> numberConvert(type, value) { toLong() }
+    "unsigned long" -> numberConvert(type, value) { toLong().toULong() }
+    "double" -> numberConvert(type, value) { toDouble() }
+    "float" -> numberConvert(type, value) { toFloat() }
+    "char*" -> notNullConvert(type, value) { toString() }
+    "void*" -> nullableConvert(type, value) { this as Pointer }
+    else -> throw RuntimeException("Unsupported ffi type: $type")
+  }
+
+  private fun <T> notNullConvert(type: String, value: Any?, into: Any.() -> T): T {
+    if (value == null) {
+      throw RuntimeException("Null values cannot be used for converting to type $type")
+    }
+    return into(value)
+  }
+
+  private fun <T> nullableConvert(type: String, value: Any?, into: Any.() -> T): T? {
+    if (value == null) {
+      return null
+    }
+    return into(value)
+  }
+
+  private fun <T> numberConvert(type: String, value: Any?, into: Number.() -> T): T {
+    if (value == null) {
+      throw RuntimeException("Null values cannot be used for converting to numeric type $type")
+    }
+
+    if (value !is Number) {
+      throw RuntimeException("Cannot convert value '$value' into type $type")
+    }
+    return into(value)
+  }
 }