diff --git a/c-programming/io/freadln.c b/c-programming/io/freadln.c
index d6b0b6f..86af0f4 100644
--- a/c-programming/io/freadln.c
+++ b/c-programming/io/freadln.c
@@ -86,6 +86,7 @@ int freadln(FILE* f, char** output, size_t* length_out) {
 # include <inttypes.h>
 # include <stdbool.h>
 # include <stdint.h>
+# include <string.h>
 
 # if POSIX
 #  include <unistd.h>