Re: [-Wstringop-overflow=] strncat(3)

Alejandro Colomar
2022-12-14T22:51:18Z

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On 12/14/22 23:45, Alejandro Colomar wrote:
> Hi,
>
> I was rewriting the strncat(3) manual page, and when I tried to compile the
> example program, I got a surprise from the compiler.
>
> Here goes the page:
>
>
>   strncat(3)               Library Functions Manual              strncat(3)
>
>   NAME
>          strncat  -  concatenate  a  null‐padded  character sequence into a
>          string
>
>   LIBRARY
>          Standard C library (libc, -lc)
>
>   SYNOPSIS
>          #include <string.h>
>
>          char *strncat(char *restrict dst, const char src[restrict .sz],
>                         size_t sz);
>
>   DESCRIPTION
>          This function catenates the input character sequence contained  in
>          a  null‐padded  fixed‐width  buffer,  into  a string at the buffer
>          pointed to by dst.  The programmer is responsible for allocating a
>          buffer large enough, that is, strlen(dst) + strnlen(src, sz) + 1.
>
>          An implementation of this function might be:
>
>              char *
>              strncat(char *restrict dst, const char *restrict src, size_t sz)
>              {
>                  int   len;
>                  char  *end;
>
>                  len = strnlen(src, sz);
>                  end = dst + strlen(dst);
>                  end = mempcpy(end, src, len);
>                  *end = '\0';
>
>                  return dst;
>              }
>
>   RETURN VALUE
>          strncat() returns dest.
>
>   ATTRIBUTES
>          [...]
>
>   STANDARDS
>          POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD.
>
>   CAVEATS
>          The  name of this function is confusing.  This function has no re‐
>          lation with strncpy(3).
>
>          If the destination buffer is not large enough, the behavior is un‐
>          defined.  See _FORTIFY_SOURCE in feature_test_macros(7).
>
>   BUGS
>          This function  can  be  very  inefficient.   Read  about  Shlemiel
>          the       painter      ⟨https://www.joelonsoftware.com/2001/12/11/
>          back-to-basics/⟩.
>
>   EXAMPLES
>          #include <stdio.h>
>          #include <stdlib.h>
>          #include <string.h>
>
>          int
>          main(void)
>          {
>              char    buf[BUFSIZ];
>              size_t  len;
>
>              buf[0] = '\0';  // There’s no ’cpy’ function to this ’cat’.
>              strncat(buf, "Hello ", 6);
>              strncat(buf, "world", 42);  // Padding null bytes ignored.
>              strncat(buf, "!", 1);
>              len = strlen(buf);
>              printf("[len = %zu]: <%s>\n", len, buf);
>
>              exit(EXIT_SUCCESS);
>          }
>
>   SEE ALSO
>          string(3), string_copy(3)
>
>   Linux man‐pages (unreleased)      (date)                       strncat(3)
>
>
> And when you compile that, you get:
>
> $ cc -Wall -Wextra ./strncat.c
> ./strncat.c: In function ‘main’:
> ./strncat.c:12:12: warning: ‘strncat’ specified bound 6 equals source length
> [-Wstringop-overflow=]
>    12 |            strncat(buf, "Hello ", 6);
>       |            ^~~~~~~~~~~~~~~~~~~~~~~~~
> ./strncat.c:14:12: warning: ‘strncat’ specified bound 1 equals source length
> [-Wstringop-overflow=]
>    14 |            strncat(buf, "!", 1);
>       |            ^~~~~~~~~~~~~~~~~~~~
>
>
> So, what?  Where's the problem?  This function does exactly that: "take an
> unterminated character sequence and catenate it to an existing string".  Clang
> seems to be fine with the code.

Maybe it's saying that I should be using strncat(buf, "!"); because the length
is useless?

>
> Cheers,
>
> Alex