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+'\" t
+.\" Copyright 2022 Alejandro Colomar <alx@kernel.org>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH strncat 3 2024-05-02 "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
+.BI "char *strncat(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]\e0\[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)