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'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH strncat 3 2025-06-28 "Linux man-pages (unreleased)"
.SH NAME
strncat
\-
append non-null bytes from a source array to a string,
and null-terminate the result
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.P
.BR "char *strncat(" "size_t ssize;"
.BI " char *restrict " dst ", const char " src "[restrict " ssize ],
.BI " size_t " ssize );
.fi
.SH DESCRIPTION
This function appends at most
.I ssize
non-null bytes from the array pointed to by
.IR src ,
followed by a null character,
to the end of the string pointed to by
.IR dst .
.I dst
must point to a string contained in a buffer that is large enough,
that is, the buffer size must be at least
.IR "strlen(dst) + strnlen(src, ssize) + 1" .
.P
An implementation of this function might be:
.P
.in +4n
.EX
char *
strncat(char *restrict dst, const char *restrict src, size_t ssize)
{
#define strnul(s) (s + strlen(s))
\&
stpcpy(mempcpy(strnul(dst), src, strnlen(src, ssize)), "");
return dst;
}
.EE
.in
.SH RETURN VALUE
.BR strncat ()
returns
.IR dst .
.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 strncat ()
T} Thread safety MT-Safe
.TE
.SH STANDARDS
C11, POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, C89, SVr4, 4.3BSD.
.SH CAVEATS
The name of this function is confusing;
it has no relation to
.BR strncpy (3).
.P
If the destination buffer does not already contain a string,
or is not large enough,
the behavior is undefined.
See
.B _FORTIFY_SOURCE
in
.BR feature_test_macros (7).
.SH BUGS
This function can be very inefficient.
Read about
.UR https://www.joelonsoftware.com/\:2001/12/11/\:back\-to\-basics/
Shlemiel the painter
.UE .
.SH EXAMPLES
.\" SRC BEGIN (strncat.c)
.EX
#include <err.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
\&
#define nitems(arr) (sizeof((arr)) / sizeof((arr)[0]))
\&
int
main(void)
{
size_t n;
\&
// Null-padded fixed-size character sequences
char pre[4] = "pre.";
char new_post[50] = ".foo.bar";
\&
// Strings
char post[] = ".post";
char src[] = "some_long_body.post";
char *dest;
\&
n = nitems(pre) + strlen(src) \- strlen(post) + nitems(new_post) + 1;
dest = malloc(sizeof(*dest) * n);
if (dest == NULL)
err(EXIT_FAILURE, "malloc()");
\&
dest[0] = \[aq]\[rs]0\[aq]; // There's no 'cpy' function to this 'cat'.
strncat(dest, pre, nitems(pre));
strncat(dest, src, strlen(src) \- strlen(post));
strncat(dest, new_post, nitems(new_post));
\&
puts(dest); // "pre.some_long_body.foo.bar"
free(dest);
exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.in
.SH SEE ALSO
.BR string (3),
.BR string_copying (7)
|
