From 28c14d475fbd16d07db88c8d12edddfe9cc226ab Mon Sep 17 00:00:00 2001
From: Michael Jones <michaelrj@google.com>
Date: Thu, 12 Jun 2025 15:57:58 -0700
Subject: [PATCH] [libc] Independent strcat/strncat/stpcpy (#142643)

The previous implementations called other entrypoints. This patch fixes
strcat, strncat, and stpcpy to be properly independent.
---
 libc/src/string/CMakeLists.txt |  3 ---
 libc/src/string/stpcpy.cpp     |  5 ++---
 libc/src/string/strcat.cpp     |  9 +++++----
 libc/src/string/strncat.cpp    | 10 +++++-----
 4 files changed, 12 insertions(+), 15 deletions(-)

diff --git a/libc/src/string/CMakeLists.txt b/libc/src/string/CMakeLists.txt
index c3b414d87285..8784bc3750cb 100644
--- a/libc/src/string/CMakeLists.txt
+++ b/libc/src/string/CMakeLists.txt
@@ -87,7 +87,6 @@ add_entrypoint_object(
   HDRS
     stpcpy.h
   DEPENDS
-    .mempcpy
     .string_utils
 )
 
@@ -108,7 +107,6 @@ add_entrypoint_object(
   HDRS
     strcat.h
   DEPENDS
-    .strcpy
     .string_utils
     libc.include.llvm-libc-types.size_t
 )
@@ -265,7 +263,6 @@ add_entrypoint_object(
   HDRS
     strncat.h
   DEPENDS
-    .strncpy
     .string_utils
     libc.include.llvm-libc-types.size_t
 )
diff --git a/libc/src/string/stpcpy.cpp b/libc/src/string/stpcpy.cpp
index 979edd72c1f1..48c0db950ace 100644
--- a/libc/src/string/stpcpy.cpp
+++ b/libc/src/string/stpcpy.cpp
@@ -8,7 +8,6 @@
 
 #include "src/string/stpcpy.h"
 #include "src/__support/macros/config.h"
-#include "src/string/mempcpy.h"
 #include "src/string/string_utils.h"
 
 #include "src/__support/common.h"
@@ -18,8 +17,8 @@ namespace LIBC_NAMESPACE_DECL {
 LLVM_LIBC_FUNCTION(char *, stpcpy,
                    (char *__restrict dest, const char *__restrict src)) {
   size_t size = internal::string_length(src) + 1;
-  char *result =
-      reinterpret_cast<char *>(LIBC_NAMESPACE::mempcpy(dest, src, size));
+  __builtin_memcpy(dest, src, size);
+  char *result = dest + size;
 
   if (result != nullptr)
     return result - 1;
diff --git a/libc/src/string/strcat.cpp b/libc/src/string/strcat.cpp
index 6a6f068bd475..7dce6d15a65c 100644
--- a/libc/src/string/strcat.cpp
+++ b/libc/src/string/strcat.cpp
@@ -9,7 +9,6 @@
 #include "src/string/strcat.h"
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/null_check.h"
-#include "src/string/strcpy.h"
 #include "src/string/string_utils.h"
 
 #include "src/__support/common.h"
@@ -21,9 +20,11 @@ LLVM_LIBC_FUNCTION(char *, strcat,
   LIBC_CRASH_ON_NULLPTR(dest);
   LIBC_CRASH_ON_NULLPTR(src);
   size_t dest_length = internal::string_length(dest);
-  size_t src_length = internal::string_length(src);
-  LIBC_NAMESPACE::strcpy(dest + dest_length, src);
-  dest[dest_length + src_length] = '\0';
+  size_t i;
+  for (i = 0; src[i] != '\0'; ++i)
+    dest[dest_length + i] = src[i];
+
+  dest[dest_length + i] = '\0';
   return dest;
 }
 
diff --git a/libc/src/string/strncat.cpp b/libc/src/string/strncat.cpp
index 4926b7d244d1..6d8bb6960748 100644
--- a/libc/src/string/strncat.cpp
+++ b/libc/src/string/strncat.cpp
@@ -10,7 +10,6 @@
 #include "src/__support/macros/config.h"
 #include "src/__support/macros/null_check.h"
 #include "src/string/string_utils.h"
-#include "src/string/strncpy.h"
 
 #include "src/__support/common.h"
 
@@ -23,11 +22,12 @@ LLVM_LIBC_FUNCTION(char *, strncat,
     LIBC_CRASH_ON_NULLPTR(dest);
     LIBC_CRASH_ON_NULLPTR(src);
   }
-  size_t src_length = internal::string_length(src);
-  size_t copy_amount = src_length > count ? count : src_length;
   size_t dest_length = internal::string_length(dest);
-  LIBC_NAMESPACE::strncpy(dest + dest_length, src, copy_amount);
-  dest[dest_length + copy_amount] = '\0';
+  size_t i;
+  for (i = 0; i < count && src[i] != '\0'; ++i)
+    dest[dest_length + i] = src[i];
+
+  dest[dest_length + i] = '\0';
   return dest;
 }