diff --git a/libc/mlibc.c b/libc/mlibc.c
index 8688d7fd..98bcd9dd 100644
--- a/libc/mlibc.c
+++ b/libc/mlibc.c
@@ -312,6 +312,25 @@ isdigit (int c)
 {
   return (c>='0') && (c<='9');
 }
+
+int
+atoi (char const *s)
+{
+  int i = 0;
+  int sign = 1;
+  if (*s && *s == '-')
+    {
+      sign = -1;
+      s++;
+    }
+  while (isdigit (*s))
+    {
+      i *= 10;
+      i += (*s - '0');
+      s++;
+    }
+  return i * sign;
+}
 #endif
 
 char itoa_buf[10];
diff --git a/module/mes/libc.mes b/module/mes/libc.mes
index 993699fa..31607f8e 100644
--- a/module/mes/libc.mes
+++ b/module/mes/libc.mes
@@ -285,6 +285,32 @@ isdigit (char c)
                 parse-c99)))
     ast))
 
+(define atoi
+  (let* ((ast (with-input-from-string
+                  "
+int
+atoi (char const *s)
+{
+  int i = 0;
+  int sign = 1;
+  if (*s && *s == '-')
+    {
+      sign = -1;
+      s++;
+    }
+  while (isdigit (*s))
+    {
+      i *= 10;
+      i += (*s - '0');
+      s++;
+    }
+  return i * sign;
+}
+"
+;;paredit:"
+                parse-c99)))
+    ast))
+
 (define malloc
   (let* ((ast (with-input-from-string
                   "
@@ -374,6 +400,7 @@ getenv (char const* s)
    strcmp
    itoa
    isdigit
+   atoi
    malloc
    realloc
    strncmp