1  /* Append a null-terminated string to another string, with length checking.
       2     Copyright (C) 2023 Free Software Foundation, Inc.
       3     This file is part of the GNU C Library.
       4  
       5     The GNU C Library is free software; you can redistribute it and/or
       6     modify it under the terms of the GNU Lesser General Public
       7     License as published by the Free Software Foundation; either
       8     version 2.1 of the License, or (at your option) any later version.
       9  
      10     The GNU C Library is distributed in the hope that it will be useful,
      11     but WITHOUT ANY WARRANTY; without even the implied warranty of
      12     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
      13     Lesser General Public License for more details.
      14  
      15     You should have received a copy of the GNU Lesser General Public
      16     License along with the GNU C Library; if not, see
      17     <https://www.gnu.org/licenses/>.  */
      18  
      19  #include <stdint.h>
      20  #include <string.h>
      21  
      22  size_t
      23  __strlcat (char *__restrict dest, const char *__restrict src, size_t size)
      24  {
      25    size_t src_length = strlen (src);
      26  
      27    /* Our implementation strlcat supports dest == NULL if size == 0
      28       (for consistency with snprintf and strlcpy), but strnlen does
      29       not, so we have to cover this case explicitly.  */
      30    if (size == 0)
      31      return src_length;
      32  
      33    size_t dest_length = __strnlen (dest, size);
      34    if (dest_length != size)
      35      {
      36        /* Copy at most the remaining number of characters in the
      37  	 destination buffer.  Leave for the NUL terminator.  */
      38        size_t to_copy = size - dest_length - 1;
      39        /* But not more than what is available in the source string.  */
      40        if (to_copy > src_length)
      41  	to_copy = src_length;
      42  
      43        char *target = dest + dest_length;
      44        memcpy (target, src, to_copy);
      45        target[to_copy] = '\0';
      46      }
      47  
      48    /* If the sum wraps around, we have more than SIZE_MAX + 2 bytes in
      49       the two input strings (including both null terminators).  If each
      50       byte in the address space can be assigned a unique size_t value
      51       (which the static_assert checks), then by the pigeonhole
      52       principle, the two input strings must overlap, which is
      53       undefined.  */
      54    _Static_assert (sizeof (uintptr_t) == sizeof (size_t),
      55  		  "theoretical maximum object size covers address space");
      56    return dest_length + src_length;
      57  }
      58  libc_hidden_def (__strlcat)
      59  weak_alias (__strlcat, strlcat)