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

Alejandro Colomar
2022-12-14T22:52:05Z

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On 12/14/22 23:51, Alejandro Colomar wrote:
>
>
> 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

oops; of course, I meant strcat().

> is useless?
>
>>
>> Cheers,
>>
>> Alex
>>
>>