diff --git a/photon/libc/include/string.h b/photon/libc/include/string.h
index 5d7822e..d0d58f3 100644
--- a/photon/libc/include/string.h
+++ b/photon/libc/include/string.h
@@ -20,6 +20,7 @@ extern char *strcpy(char *dest, const char *src);
 extern char *strncpy(char *dest, const char *src, size_t sz);
 
 extern char *strrchr(const char *s, int c);
+extern size_t strcspn(const char *str1, const char *str2);
 
 extern char *strdup(const char *s);
 
diff --git a/photon/libc/string/strcspn.c b/photon/libc/string/strcspn.c
new file mode 100644
index 0000000..9220d63
--- /dev/null
+++ b/photon/libc/string/strcspn.c
@@ -0,0 +1,15 @@
+#include <stddef.h>
+
+size_t strcspn(const char *str1, const char *str2)
+{
+    size_t i;
+    for (i = 0; str1[i]; i++) {
+        for (size_t ii = 0; str2[ii]; ii++) {
+            if (str1[i] == str2[ii]) {
+                return i;
+            }
+        }
+    }
+
+    return i;
+}