Re: [PATCH] stpncpy.3, string_copying.7: Clarify that st[rp]ncpy() do NOT produce a string

Alejandro Colomar
2023-11-09T00:24:20Z

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Hi Paul,

On Wed, Nov 08, 2023 at 03:06:40PM -0800, Paul Eggert wrote:
> On 11/8/23 14:17, Alejandro Colomar wrote:
> > These copy*from* a string
>
> Not necessarily. For example, in strncpy (DST, SRC, N), SRC need not be a
> string.

Pedantically, true. But since it's quite rare to copy from a
fixed-width null-padded array into another, I didn't want to waste
space on that and possibly confuse readers. In such a case, the source
buffer must be at least as large as the destination buffer, and will
likely be the same size (because having fixed-width stuff, why make it
different), so memcpy(3) will probably be simpler.

>
> By the way, have you looked at the recent (i.e., this-year) changes to the
> glibc manual's string section? They're relevant.

I hadn't; after your message, I have.
<https://sourceware.org/glibc/manual/2.38/html_mono/libc.html#String-and-Array-Utilities>

I like how it connects all the functions, and it explains the concepts
and gives advice (e.g., avoid truncation as it's usually evil), and
compares the different functions.

However, I think it misses a few things:

- strncpy(3) and strncat(3) are not related at all. They don't have
the same relation that strcpy(3) and strcat(3) have. You can't
write the following code in any case:

strncpy(dst, foo, sizeof(dst));
strncat(dst, bar, sizeof(dst));

as you would with strcpy(3) or strlcpy(3).

strncpy(3) and strncat(3) are opposite functions: the former reads
from a string and writes to a fixed-width null-padded buffer, and the
latter reads from a fixed-width buffer and writes to a string. (You
can use them in other cases, pedantically, as you said above, but
those cases are rather unreal.)

- strncpy(3) is in a section that starts by saying:

> The functions described in this section copy or concatenate the
> possibly-truncated contents of a string or array to another

This may mislead programmers to believe it is useful for producing
strings, when it's not.

In general, I would like the manual to put some more distance between
these functions and the term "string". As DJ mentioned, it might be
useful to mention utmp(5) and tar(1) as niche use cases for
st[rp]ncpy(3).

And now for some typo:

- In the following sentence under "5.2 String and Array Conventions":

> The array arguments and return values for these functions have type
> void * or wchar_t.

I believe it meant `void *` or `wchar_t *`

Cheers,

Alex

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