diff --git a/include/sys/string.h b/include/sys/string.h
index c056406..c0463cc 100644
--- a/include/sys/string.h
+++ b/include/sys/string.h
@@ -16,6 +16,8 @@
 #define MAX(x, y) ((x) > (y) ? (x) : (y))
 #define MIN(x, y) ((x) < (y) ? (x) : (y))
 
+int atoi(const char *str);
+
 void *memcpy(void *restrict dst, const void *restrict src, size_t cnt);
 uint64_t *memcpyq(uint64_t *restrict dst, const uint64_t *restrict src, size_t cnt);
 void *memmove(void *dst, const void *src, size_t cnt);
diff --git a/sys/string.c b/sys/string.c
index 1e26af7..778562d 100644
--- a/sys/string.c
+++ b/sys/string.c
@@ -1,4 +1,14 @@
 #include "sys/string.h"
+#include "sys/ctype.h"
+
+int atoi(const char *str) {
+    int res = 0;
+    while (isdigit(*str)) {
+        res *= 10;
+        res += (*str++) - '0';
+    }
+    return res;
+}
 
 char *strcat(char *dst, const char *src) {
     size_t l = strlen(dst);