stpncpy.3, strncpy.3: Document in a single page

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'\" t
.\" Copyright 2022 Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH stpncpy 3 (date) "Linux man-pages (unreleased)"
.SH NAME
stpncpy, strncpy
\- zero a fixed-width buffer and
copy a string into a character sequence with truncation
and zero the rest of it
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.PP
.BI "char *stpncpy(char " dst "[restrict ." sz "], \
const char *restrict " src ,
.BI "               size_t " sz );
.BI "char *strncpy(char " dst "[restrict ." sz "], \
const char *restrict " src ,
.BI "               size_t " sz );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR stpncpy ():
.nf
    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _GNU_SOURCE
.fi
.SH DESCRIPTION
These functions copy the string pointed to by
.I src
into a null-padded character sequence at the fixed-width buffer pointed to by
.IR dst .
If the destination buffer,
limited by its size,
isn't large enough to hold the copy,
the resulting character sequence is truncated.
For the difference between the two functions, see RETURN VALUE.
.PP
An implementation of these functions might be:
.PP
.in +4n
.EX
char *
stpncpy(char *restrict dst, const char *restrict src, size_t sz)
{
    bzero(dst, sz);
    return mempcpy(dst, src, strnlen(src, sz));
}

char *
strncpy(char *restrict dst, const char *restrict src, size_t sz)
{
    stpncpy(dst, src, sz);
    return dst;
}
.EE
.in
.SH RETURN VALUE
.TP
.BR stpncpy ()
returns a pointer to
one after the last character in the destination character sequence.
.TP
.BR strncpy ()
returns
.IR dst .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR stpncpy (),
.BR strncpy ()
T}	Thread safety	MT-Safe
.TE
.hy
.ad
.sp 1
.SH STANDARDS
.TP
.BR stpncpy ()
POSIX.1-2008.
.\" Before that, it was a GNU extension.
.\" It first appeared in glibc 1.07 in 1993.
.TP
.BR strncpy ()
POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
.SH CAVEATS
The name of these functions is confusing.
These functions produce a null-padded character sequence,
not a string (see
.BR string_copying (7)).
.PP
It's impossible to distinguish truncation by the result of the call,
from a character sequence that just fits the destination buffer;
truncation should be detected by
comparing the length of the input string
with the size of the destination buffer.
.PP
If you're going to use this function in chained calls,
it would be useful to develop a similar function that accepts
a pointer to the end (one after the last element) of the destination buffer
instead of its size.
.SH EXAMPLES
.\" SRC BEGIN (stpncpy.c)
.EX
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int
main(void)
{
    char    *p;
    char    buf1[20];
    char    buf2[20];
    size_t  len;

    if (sizeof(buf1) < strlen("Hello world!"))
        warnx("stpncpy: truncating character sequence");
    p = stpncpy(buf1, "Hello world!", sizeof(buf1));
    len = p \- buf1;

    printf("[len = %zu]: ", len);
    printf("%.*s\en", (int) len, buf1);  // "Hello world!"

    if (sizeof(buf2) < strlen("Hello world!"))
        warnx("strncpy: truncating character sequence");
    strncpy(buf2, "Hello world!", sizeof(buf));
    len = strnlen(buf2, sizeof(buf2));

    printf("[len = %zu]: ", len);
    printf("%.*s\en", (int) len, buf2);  // "Hello world!"

    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR wcpncpy (3),
.BR string_copying (7)