Re: [PATCH] strcat.3, strncat.3: RIP strncat(3)

[Alejandro Colomar <alx.manpages@gmail.com>]

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On 12/5/22 16:49, Alejandro Colomar wrote:
> Never use this function.  Really.
> 
> Cc: <pkg-shadow-devel@alioth-lists.debian.net>
> Cc: <libc-alpha@sourceware.org>
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
> ---
> 
> Hi!
> 
> To shadow-utils readers, I've seen there are a few uses of strncat(3) in
> shadow-utils.  I'll review my current PR about string handling to also
> address this issue.
> 
> To glibc readers, please bury this function deep down as if it were
> radioactive waste.
> 
> Cheers,
> 
> Alex
> 

The rendered version of the new manual page for strncpy(3) is:


strncat(3)              Library Functions Manual             strncat(3)

NAME
        strncat - concatenate two strings

LIBRARY
        Standard C library (libc, -lc)

SYNOPSIS
        #include <string.h>

        [[deprecated]]
        char *strncat(char dest[restrict strlen(.dest) + strnlen(.n) + 1],
                      const char src[restrict .n],
                      size_t n);

DESCRIPTION
        Note: Never use this function.

        For  safe  string concatenation, see strlcat(3bsd).  For copying
        or concatenating a string into a fixed‐length buffer with  zero‐
        ing of the rest, see stpncpy(3).

        strncat() appends at most n characters of src to the end of dst.
        It  always terminates with a null character the string placed in
        dest.

        A simple implementation of strncat() might be:

            char *
            strncat(char *dest, const char *src, size_t n)
            {
                return memcpy(dest + strlen(dest), src, strnlen(src, n));
            }

RETURN VALUE
        strncat() returns a pointer to the resulting string dest.

ATTRIBUTES
        For an explanation of the terms used in this  section,  see  at‐
        tributes(7).
        ┌─────────────────────────────────────┬───────────────┬─────────┐
        │Interface                            │ Attribute     │ Value   │
        ├─────────────────────────────────────┼───────────────┼─────────┤
        │strncat()                            │ Thread safety │ MT‐Safe │
        └─────────────────────────────────────┴───────────────┴─────────┘

STANDARDS
        POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD.

BUGS
        All.  Seriously, there’s no use case for this function.

        It  has  a very misleading name.  This function has no relation‐
        ship with strncpy(3).

        Since it doesn’t know the size of the destination  buffer,  this
        function  can  easily  write past the end of the array, being an
        open door to all kinds of crackers.

SEE ALSO
        strcpy(3), string(3)

Linux man‐pages (unreleased)     (date)                      strncat(3)

-- 
<http://www.alejandro-colomar.es/>

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