[-Wstringop-overflow=] strncat(3)

Alejandro Colomar
2022-12-14T22:45:28Z

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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.

Cheers,

Alex