diff --git a/src/include/stdlib.h b/src/include/stdlib.h
index 9f34e11..4d99f82 100644
--- a/src/include/stdlib.h
+++ b/src/include/stdlib.h
@@ -12,4 +12,6 @@ void free(void *ptr);
 
 _Noreturn void exit(int status);
 
+int abs(int j);
+
 #endif
diff --git a/src/stdlib.c b/src/stdlib.c
index aae99d7..9c4a2ef 100644
--- a/src/stdlib.c
+++ b/src/stdlib.c
@@ -12,3 +12,7 @@ void free(void *ptr) {
 _Noreturn void exit(int status) {
   sys_exit(status);
 }
+
+int abs(int i) {
+  return i > 0 ? i : -i;
+}