[libc] Independent strcat/strncat

The previous implementations called other entrypoints. This patch fixes
strcat and strncat to be properly independent.

Author: michaelrj-google

libc/src/string/CMakeLists.txt

@@ -108,7 +108,6 @@ add_entrypoint_object(
   HDRS
     strcat.h
   DEPENDS
-    .strcpy
     .string_utils
     libc.include.llvm-libc-types.size_t
 )
@@ -265,7 +264,6 @@ add_entrypoint_object(
   HDRS
     strncat.h
   DEPENDS
-    .strncpy
     .string_utils
     libc.include.llvm-libc-types.size_t
 )

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;
 }
 

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;
 }