'\" t
.\" Copyright 2022 Alejandro Colomar <alx@kernel.org>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH stpncpy 3 2024-02-12 "Linux man-pages 6.7"
.SH NAME
stpncpy, strncpy
\-
fill a fixed-size buffer with non-null bytes from a string,
padding with null bytes as needed
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.P
.BI "char *strncpy(char " dst "[restrict ." dsize "], \
const char *restrict " src ,
.BI "              size_t " dsize );
.BI "char *stpncpy(char " dst "[restrict ." dsize "], \
const char *restrict " src ,
.BI "              size_t " dsize );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR stpncpy ():
.nf
    Since glibc 2.10:
        _POSIX_C_SOURCE >= 200809L
    Before glibc 2.10:
        _GNU_SOURCE
.fi
.SH DESCRIPTION
These functions copy non-null bytes from the string pointed to by
.I src
into the array pointed to by
.IR dst .
If the source has too few non-null bytes to fill the destination,
the functions pad the destination with trailing null bytes.
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.
.P
An implementation of these functions might be:
.P
.in +4n
.EX
char *
strncpy(char *restrict dst, const char *restrict src, size_t dsize)
{
    stpncpy(dst, src, dsize);
    return dst;
}
\&
char *
stpncpy(char *restrict dst, const char *restrict src, size_t dsize)
{
    size_t  dlen;
\&
    dlen = strnlen(src, dsize);
    return memset(mempcpy(dst, src, dlen), 0, dsize \- dlen);
}
.EE
.in
.SH RETURN VALUE
.TP
.BR strncpy ()
returns
.IR dst .
.TP
.BR stpncpy ()
returns a pointer to
one after the last character in the destination character sequence.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR stpncpy (),
.BR strncpy ()
T}	Thread safety	MT-Safe
.TE
.SH STANDARDS
.TP
.BR strncpy ()
C11, POSIX.1-2008.
.TP
.BR stpncpy ()
POSIX.1-2008.
.SH HISTORY
.TP
.BR strncpy ()
C89, POSIX.1-2001, SVr4, 4.3BSD.
.TP
.BR stpncpy ()
glibc 1.07.
POSIX.1-2008.
.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)).
For example:
.P
.in +4n
.EX
strncpy(buf, "1", 5);       // { \[aq]1\[aq],   0,   0,   0,   0 }
strncpy(buf, "1234", 5);    // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq],   0 }
strncpy(buf, "12345", 5);   // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
strncpy(buf, "123456", 5);  // { \[aq]1\[aq], \[aq]2\[aq], \[aq]3\[aq], \[aq]4\[aq], \[aq]5\[aq] }
.EE
.in
.P
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.
.P
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(buf2) < strlen("Hello world!"))
        errx("strncpy: truncating character sequence");
    strncpy(buf2, "Hello world!", sizeof(buf2));
    len = strnlen(buf2, sizeof(buf2));
\&
    printf("[len = %zu]: ", len);
    fwrite(buf2, 1, len, stdout);
    putchar(\[aq]\en\[aq]);
\&
    if (sizeof(buf1) < strlen("Hello world!"))
        errx("stpncpy: truncating character sequence");
    p = stpncpy(buf1, "Hello world!", sizeof(buf1));
    len = p \- buf1;
\&
    printf("[len = %zu]: ", len);
    fwrite(buf1, 1, len, stdout);
    putchar(\[aq]\en\[aq]);
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR wcpncpy (3),
.BR string_copying (7)